lkmpg.html

  The Linux Kernel Module Programming Guide

Peter Jay Salzman
Michael Burian
Ori Pomerantz

Copyright © 2001 Peter Jay Salzman

2007-05-18 ver 2.6.4

The Linux Kernel Module Programming Guide is a free book; you may
reproduce and/or modify it under the terms of the Open Software License,
version 1.1. You can obtain a copy of this license at
http://opensource.org/licenses/osl.php.

This book is distributed in the hope it will be useful, but without any
warranty, without even the implied warranty of merchantability or
fitness for a particular purpose.

The author encourages wide distribution of this book for personal or
commercial use, provided the above copyright notice remains intact and
the method adheres to the provisions of the Open Software License. In
summary, you may copy and distribute this book free of charge or for a
profit. No explicit permission is required from the author for
reproduction of this book in any medium, physical or electronic.

Derivative works and translations of this document must be placed under
the Open Software License, and the original copyright notice must remain
intact. If you have contributed new material to this book, you must make
the material and source code available for your revisions. Please make
revisions and updates available directly to the document maintainer,
Peter Jay Salzman <p@dirac.org <mailto:p@dirac.org>>. This will allow
for the merging of updates and provide consistent revisions to the Linux
community.

If you publish or distribute this book commercially, donations,
royalties, and/or printed copies are greatly appreciated by the author
and the Linux Documentation Project <http://www.tldp.org> (LDP).
Contributing in this way shows your support for free software and the
LDP. If you have questions or comments, please contact the address above.

------------------------------------------------------------------------

*Table of Contents*
Foreword <#AEN25>

    1. Authorship <#AEN27>
    2. Versioning and Notes <#AEN30>
    3. Acknowledgements <#AEN35>

1. Introduction <#AEN38>

    1.1. What Is A Kernel Module? <#AEN40>
    1.2. How Do Modules Get Into The Kernel? <#AEN44>

2. Hello World <#AEN119>

    2.1. Hello, World (part 1): The Simplest Module <#AEN121>
    2.2. Compiling Kernel Modules <#AEN181>
    2.3. Hello World (part 2) <#HELLO2>
    2.4. Hello World (part 3): The __init and __exit Macros <#AEN245>
    2.5. Hello World (part 4): Licensing and Module Documentation <#AEN279>
    2.6. Passing Command Line Arguments to a Module <#AEN323>
    2.7. Modules Spanning Multiple Files <#AEN351>
    2.8. Building modules for a precompiled kernel <#AEN380>

3. Preliminaries <#AEN425>

    3.1. Modules vs Programs <#AEN427>

4. Character Device Files <#AEN567>

    4.1. Character Device Drivers <#AEN569>

5. The /proc File System <#AEN708>

    5.1. The /proc File System <#AEN710>
    5.2. Read and Write a /proc File <#AEN769>
    5.3. Manage /proc file with standard filesystem <#AEN810>
    5.4. Manage /proc file with seq_file <#AEN861>

6. Using /proc For Input <#AEN885>

    6.1. TODO: Write a chapter about sysfs <#AEN887>

7. Talking To Device Files <#AEN890>

    7.1. Talking to Device Files (writes and IOCTLs) <#AEN892>

8. System Calls <#AEN976>

    8.1. System Calls <#AEN978>

9. Blocking Processes <#AEN1050>

    9.1. Blocking Processes <#AEN1052>

10. Replacing Printks <#AEN1159>

    10.1. Replacing printk <#AEN1161>
    10.2. Flashing keyboard LEDs <#AEN1194>

11. Scheduling Tasks <#AEN1209>

    11.1. Scheduling Tasks <#AEN1211>

12. Interrupt Handlers <#INTERRUPTHANDLERS>

    12.1. Interrupt Handlers <#AEN1256>

13. Symmetric Multi Processing <#AEN1324>

    13.1. Symmetrical Multi-Processing <#AEN1326>

14. Common Pitfalls <#AEN1350>

    14.1. Common Pitfalls <#AEN1352>

A. Changes: 2.0 To 2.2 <#AEN1387>

    A.1. Changes between 2.4 and 2.6 <#AEN1389>

B. Where To Go From Here <#AEN1403>

    B.1. Where From Here? <#AEN1405>

Index <#DOC-INDEX>

*List of Figures*
5-1. How seq_file works <#AEN868>

*List of Examples*
2-1. hello-1.c <#AEN128>
2-2. Makefile for a basic kernel module <#AEN189>
2-3. hello-2.c <#AEN232>
2-4. Makefile for both our modules <#AEN237>
2-5. hello-3.c <#AEN275>
2-6. hello-4.c <#AEN319>
2-7. hello-5.c <#AEN345>
2-8. start.c <#AEN361>
2-9. stop.c <#AEN369>
2-10. Makefile <#AEN374>
4-1. chardev.c <#AEN687>
5-1. procfs1.c <#AEN765>
5-2. procfs2.c <#AEN806>
5-3. procfs3.c <#AEN853>
5-4. procfs4.c <#AEN872>
7-1. chardev.c <#AEN951>
7-2. chardev.h <#AEN959>
7-3. ioctl.c <#AEN972>
8-1. syscall.c <#AEN1046>
9-1. sleep.c <#AEN1151>
9-2. cat_noblock.c <#AEN1155>
10-1. print_string.c <#AEN1190>
10-2. kbleds.c <#AEN1201>
11-1. sched.c <#AEN1250>
12-1. intrpt.c <#AEN1320>

------------------------------------------------------------------------


  Foreword


  1. Authorship

The Linux Kernel Module Programming Guide was originally written for the
2.2 kernels by Ori Pomerantz. Eventually, Ori no longer had time to
maintain the document. After all, the Linux kernel is a fast moving
target. Peter Jay Salzman took over maintenance and updated it for the
2.4 kernels. Eventually, Peter no longer had time to follow developments
with the 2.6 kernel, so Michael Burian became a co-maintainer to update
the document for the 2.6 kernels.

------------------------------------------------------------------------


  2. Versioning and Notes

The Linux kernel is a moving target. There has always been a question
whether the LKMPG should remove deprecated information or keep it around
for historical sake. Michael Burian and I decided to create a new branch
of the LKMPG for each new stable kernel version. So version LKMPG 2.4.x
will address Linux kernel 2.4 and LKMPG 2.6.x will address Linux kernel
2.6. No attempt will be made to archive historical information; a person
wishing this information should read the appropriately versioned LKMPG.

The source code and discussions should apply to most architectures, but
I can't promise anything. One exception is Chapter 12
<#INTERRUPTHANDLERS>, Interrupt Handlers, which should not work on any
architecture except for x86.

------------------------------------------------------------------------


  3. Acknowledgements

The following people have contributed corrections or good suggestions:
Ignacio Martin, David Porter, Daniele Paolo Scarpazza, Dimo Velev,
Francois Audeon and Horst Schirmeier.

------------------------------------------------------------------------


  Chapter 1. Introduction


  1.1. What Is A Kernel Module?

So, you want to write a kernel module. You know C, you've written a few
normal programs to run as processes, and now you want to get to where
the real action is, to where a single wild pointer can wipe out your
file system and a core dump means a reboot.

What exactly is a kernel module? Modules are pieces of code that can be
loaded and unloaded into the kernel upon demand. They extend the
functionality of the kernel without the need to reboot the system. For
example, one type of module is the device driver, which allows the
kernel to access hardware connected to the system. Without modules, we
would have to build monolithic kernels and add new functionality
directly into the kernel image. Besides having larger kernels, this has
the disadvantage of requiring us to rebuild and reboot the kernel every
time we want new functionality.

------------------------------------------------------------------------


  1.2. How Do Modules Get Into The Kernel?

You can see what modules are already loaded into the kernel by running
*lsmod*, which gets its information by reading the file /proc/modules.

How do these modules find their way into the kernel? When the kernel
needs a feature that is not resident in the kernel, the kernel module
daemon kmod[1] <#FTN.AEN62> execs modprobe to load the module in.
modprobe is passed a string in one of two forms:

  *

    A module name like softdog or ppp.

  *

    A more generic identifier like char-major-10-30.

If modprobe is handed a generic identifier, it first looks for that
string in the file /etc/modprobe.conf.[2] <#FTN.AEN74> If it finds an
alias line like:

alias char-major-10-30 softdog
	

it knows that the generic identifier refers to the module softdog.ko.

Next, modprobe looks through the file /lib/modules/version/modules.dep,
to see if other modules must be loaded before the requested module may
be loaded. This file is created by *depmod -a* and contains module
dependencies. For example, msdos.ko requires the fat.ko module to be
already loaded into the kernel. The requested module has a dependency on
another module if the other module defines symbols (variables or
functions) that the requested module uses.

Lastly, modprobe uses insmod to first load any prerequisite modules into
the kernel, and then the requested module. modprobe directs insmod to
/lib/modules/version/[3] <#FTN.AEN87>, the standard directory for
modules. insmod is intended to be fairly dumb about the location of
modules, whereas modprobe is aware of the default location of modules,
knows how to figure out the dependencies and load the modules in the
right order. So for example, if you wanted to load the msdos module,
you'd have to either run:

insmod /lib/modules/2.6.11/kernel/fs/fat/fat.ko
insmod /lib/modules/2.6.11/kernel/fs/msdos/msdos.ko
	

or:

modprobe msdos
	

What we've seen here is: *insmod * requires you to pass it the full
pathname and to insert the modules in the right order, while *modprobe *
just takes the name, without any extension, and figures out all it needs
to know by parsing /lib/modules/version/modules.dep.

Linux distros provide modprobe, insmod and depmod as a package called
module-init-tools. In previous versions that package was called
modutils. Some distros also set up some wrappers that allow both
packages to be installed in parallel and do the right thing in order to
be able to deal with 2.4 and 2.6 kernels. Users should not need to care
about the details, as long as they're running recent versions of those
tools.

Now you know how modules get into the kernel. There's a bit more to the
story if you want to write your own modules which depend on other
modules (we calling this `stacking modules'). But this will have to wait
for a future chapter. We have a lot to cover before addressing this
relatively high-level issue.

------------------------------------------------------------------------


    1.2.1. Before We Begin

Before we delve into code, there are a few issues we need to cover.
Everyone's system is different and everyone has their own groove.
Getting your first "hello world" program to compile and load correctly
can sometimes be a trick. Rest assured, after you get over the initial
hurdle of doing it for the first time, it will be smooth sailing thereafter.

------------------------------------------------------------------------


      1.2.1.1. Modversioning

A module compiled for one kernel won't load if you boot a different
kernel unless you enable CONFIG_MODVERSIONS in the kernel. We won't go
into module versioning until later in this guide. Until we cover
modversions, the examples in the guide may not work if you're running a
kernel with modversioning turned on. However, most stock Linux distro
kernels come with it turned on. If you're having trouble loading the
modules because of versioning errors, compile a kernel with
modversioning turned off.

------------------------------------------------------------------------


      1.2.1.2. Using X

It is highly recommended that you type in, compile and load all the
examples this guide discusses. It's also highly recommended you do this
from a console. You should not be working on this stuff in X.

Modules can't print to the screen like printf() can, but they can log
information and warnings, which ends up being printed on your screen,
but only on a console. If you insmod a module from an xterm, the
information and warnings will be logged, but only to your log files. You
won't see it unless you look through your log files. To have immediate
access to this information, do all your work from the console.

------------------------------------------------------------------------


      1.2.1.3. Compiling Issues and Kernel Version

Very often, Linux distros will distribute kernel source that has been
patched in various non-standard ways, which may cause trouble.

A more common problem is that some Linux distros distribute incomplete
kernel headers. You'll need to compile your code using various header
files from the Linux kernel. Murphy's Law states that the headers that
are missing are exactly the ones that you'll need for your module work.

To avoid these two problems, I highly recommend that you download,
compile and boot into a fresh, stock Linux kernel which can be
downloaded from any of the Linux kernel mirror sites. See the Linux
Kernel HOWTO for more details.

Ironically, this can also cause a problem. By default, gcc on your
system may look for the kernel headers in their default location rather
than where you installed the new copy of the kernel (usually in
/usr/src/. This can be fixed by using gcc's -I switch.

------------------------------------------------------------------------


  Chapter 2. Hello World


  2.1. Hello, World (part 1): The Simplest Module

When the first caveman programmer chiseled the first program on the
walls of the first cave computer, it was a program to paint the string
`Hello, world' in Antelope pictures. Roman programming textbooks began
with the `Salut, Mundi' program. I don't know what happens to people who
break with this tradition, but I think it's safer not to find out. We'll
start with a series of hello world programs that demonstrate the
different aspects of the basics of writing a kernel module.

Here's the simplest module possible. Don't compile it yet; we'll cover
module compilation in the next section.

*Example 2-1. hello-1.c*

/*  
 *  hello-1.c - The simplest kernel module.
 */
#include <linux/module.h>	/* Needed by all modules */
#include <linux/kernel.h>	/* Needed for KERN_INFO */

int init_module(void)
{
	printk(KERN_INFO "Hello world 1.\n");

	/* 
	 * A non 0 return means init_module failed; module can't be loaded. 
	 */
	return 0;
}

void cleanup_module(void)
{
	printk(KERN_INFO "Goodbye world 1.\n");
}

Kernel modules must have at least two functions: a "start"
(initialization) function called init_module() which is called when the
module is insmoded into the kernel, and an "end" (cleanup) function
called cleanup_module() which is called just before it is rmmoded.
Actually, things have changed starting with kernel 2.3.13. You can now
use whatever name you like for the start and end functions of a module,
and you'll learn how to do this in Section 2.3 <#HELLO2>. In fact, the
new method is the preferred method. However, many people still use
init_module() and cleanup_module() for their start and end functions.

Typically, init_module() either registers a handler for something with
the kernel, or it replaces one of the kernel functions with its own code
(usually code to do something and then call the original function). The
cleanup_module() function is supposed to undo whatever init_module()
did, so the module can be unloaded safely.

Lastly, every kernel module needs to include linux/module.h. We needed
to include linux/kernel.h only for the macro expansion for the printk()
log level, KERN_ALERT, which you'll learn about in Section 2.1.1
<#INTRODUCINGPRINTK>.

------------------------------------------------------------------------


    2.1.1. Introducing printk()

Despite what you might think, printk() was not meant to communicate
information to the user, even though we used it for exactly this purpose
in hello-1! It happens to be a logging mechanism for the kernel, and is
used to log information or give warnings. Therefore, each printk()
statement comes with a priority, which is the <1> and KERN_ALERT you
see. There are 8 priorities and the kernel has macros for them, so you
don't have to use cryptic numbers, and you can view them (and their
meanings) in linux/kernel.h. If you don't specify a priority level, the
default priority, DEFAULT_MESSAGE_LOGLEVEL, will be used.

Take time to read through the priority macros. The header file also
describes what each priority means. In practise, don't use number, like
<4>. Always use the macro, like KERN_WARNING.

If the priority is less than int console_loglevel, the message is
printed on your current terminal. If both *syslogd* and klogd are
running, then the message will also get appended to /var/log/messages,
whether it got printed to the console or not. We use a high priority,
like KERN_ALERT, to make sure the printk() messages get printed to your
console rather than just logged to your logfile. When you write real
modules, you'll want to use priorities that are meaningful for the
situation at hand.

------------------------------------------------------------------------


  2.2. Compiling Kernel Modules

Kernel modules need to be compiled a bit differently from regular
userspace apps. Former kernel versions required us to care much about
these settings, which are usually stored in Makefiles. Although
hierarchically organized, many redundant settings accumulated in
sublevel Makefiles and made them large and rather difficult to maintain.
Fortunately, there is a new way of doing these things, called kbuild,
and the build process for external loadable modules is now fully
integrated into the standard kernel build mechanism. To learn more on
how to compile modules which are not part of the official kernel (such
as all the examples you'll find in this guide), see file
linux/Documentation/kbuild/modules.txt.

So, let's look at a simple Makefile for compiling a module named hello-1.c:

*Example 2-2. Makefile for a basic kernel module*

obj-m += hello-1.o

all:
	make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules

clean:
	make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clean

From a technical point of view just the first line is really necessary,
the "all" and "clean" targets were added for pure convenience.

Now you can compile the module by issuing the command *make *. You
should obtain an output which resembles the following:

hostname:~/lkmpg-examples/02-HelloWorld# make
make -C /lib/modules/2.6.11/build M=/root/lkmpg-examples/02-HelloWorld modules
make[1]: Entering directory `/usr/src/linux-2.6.11'
  CC [M]  /root/lkmpg-examples/02-HelloWorld/hello-1.o
 Building modules, stage 2.
  MODPOST
  CC      /root/lkmpg-examples/02-HelloWorld/hello-1.mod.o
  LD [M]  /root/lkmpg-examples/02-HelloWorld/hello-1.ko
make[1]: Leaving directory `/usr/src/linux-2.6.11'
hostname:~/lkmpg-examples/02-HelloWorld#
	

Note that kernel 2.6 introduces a new file naming convention: kernel
modules now have a .ko extension (in place of the old .o extension)
which easily distinguishes them from conventional object files. The
reason for this is that they contain an additional .modinfo section that
where additional information about the module is kept. We'll soon see
what this information is good for.

Use *modinfo hello-*.ko * to see what kind of information it is.

hostname:~/lkmpg-examples/02-HelloWorld# modinfo hello-1.ko
filename:       hello-1.ko
vermagic:       2.6.11 preempt PENTIUMII 4KSTACKS gcc-3.3
depends:

Nothing spectacular, so far. That changes once we're using modinfo on
one of our the later examples, hello-5.ko .

hostname:~/lkmpg-examples/02-HelloWorld# modinfo hello-5.ko
filename:       hello-5.ko
license:        GPL
author:         Peter Jay Salzman
vermagic:       2.6.11 preempt PENTIUMII 4KSTACKS gcc-3.3
depends:
parm:           myintArray:An array of integers (array of int)
parm:           mystring:A character string (charp)
parm:           mylong:A long integer (long)
parm:           myint:An integer (int)
parm:           myshort:A short integer (short)
hostname:~/lkmpg-examples/02-HelloWorld# 

Lot's of useful information to see here. An author string for
bugreports, license information, even a short description of the
parameters it accepts.

Additional details about Makefiles for kernel modules are available in
linux/Documentation/kbuild/makefiles.txt. Be sure to read this and the
related files before starting to hack Makefiles. It'll probably save you
lots of work.

Now it is time to insert your freshly-compiled module it into the kernel
with *insmod ./hello-1.ko* (ignore anything you see about tainted
kernels; we'll cover that shortly).

All modules loaded into the kernel are listed in /proc/modules. Go ahead
and cat that file to see that your module is really a part of the
kernel. Congratulations, you are now the author of Linux kernel code!
When the novelty wears off, remove your module from the kernel by using
*rmmod hello-1*. Take a look at /var/log/messages just to see that it
got logged to your system logfile.

Here's another exercise for the reader. See that comment above the
return statement in init_module()? Change the return value to something
negative, recompile and load the module again. What happens?

------------------------------------------------------------------------


  2.3. Hello World (part 2)

As of Linux 2.4, you can rename the init and cleanup functions of your
modules; they no longer have to be called init_module() and
cleanup_module() respectively. This is done with the module_init() and
module_exit() macros. These macros are defined in linux/init.h. The only
caveat is that your init and cleanup functions must be defined before
calling the macros, otherwise you'll get compilation errors. Here's an
example of this technique:

*Example 2-3. hello-2.c*

/*  
 *  hello-2.c - Demonstrating the module_init() and module_exit() macros.
 *  This is preferred over using init_module() and cleanup_module().
 */
#include <linux/module.h>	/* Needed by all modules */
#include <linux/kernel.h>	/* Needed for KERN_INFO */
#include <linux/init.h>		/* Needed for the macros */

static int __init hello_2_init(void)
{
	printk(KERN_INFO "Hello, world 2\n");
	return 0;
}

static void __exit hello_2_exit(void)
{
	printk(KERN_INFO "Goodbye, world 2\n");
}

module_init(hello_2_init);
module_exit(hello_2_exit);

So now we have two real kernel modules under our belt. Adding another
module is as simple as this:

*Example 2-4. Makefile for both our modules*

obj-m += hello-1.o
obj-m += hello-2.o

all:
	make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules

clean:
	make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clean

Now have a look at linux/drivers/char/Makefile for a real world example.
As you can see, some things get hardwired into the kernel (obj-y) but
where are all those obj-m gone? Those familiar with shell scripts will
easily be able to spot them. For those not, the obj-$(CONFIG_FOO)
entries you see everywhere expand into obj-y or obj-m, depending on
whether the CONFIG_FOO variable has been set to y or m. While we are at
it, those were exactly the kind of variables that you have set in the
linux/.config file, the last time when you said *make menuconfig* or
something like that.

------------------------------------------------------------------------


  2.4. Hello World (part 3): The __init and __exit Macros

This demonstrates a feature of kernel 2.2 and later. Notice the change
in the definitions of the init and cleanup functions. The __init macro
causes the init function to be discarded and its memory freed once the
init function finishes for built-in drivers, but not loadable modules.
If you think about when the init function is invoked, this makes perfect
sense.

There is also an __initdata which works similarly to __init but for init
variables rather than functions.

The __exit macro causes the omission of the function when the module is
built into the kernel, and like __exit, has no effect for loadable
modules. Again, if you consider when the cleanup function runs, this
makes complete sense; built-in drivers don't need a cleanup function,
while loadable modules do.

These macros are defined in linux/init.h and serve to free up kernel
memory. When you boot your kernel and see something like Freeing unused
kernel memory: 236k freed, this is precisely what the kernel is freeing.

*Example 2-5. hello-3.c*

/*  
 *  hello-3.c - Illustrating the __init, __initdata and __exit macros.
 */
#include <linux/module.h>	/* Needed by all modules */
#include <linux/kernel.h>	/* Needed for KERN_INFO */
#include <linux/init.h>		/* Needed for the macros */

static int hello3_data __initdata = 3;

static int __init hello_3_init(void)
{
	printk(KERN_INFO "Hello, world %d\n", hello3_data);
	return 0;
}

static void __exit hello_3_exit(void)
{
	printk(KERN_INFO "Goodbye, world 3\n");
}

module_init(hello_3_init);
module_exit(hello_3_exit);

------------------------------------------------------------------------


  2.5. Hello World (part 4): Licensing and Module Documentation

If you're running kernel 2.4 or later, you might have noticed something
like this when you loaded proprietary modules:

# insmod xxxxxx.o
Warning: loading xxxxxx.ko will taint the kernel: no license
  See http://www.tux.org/lkml/#export-tainted for information about tainted modules
Module xxxxxx loaded, with warnings
	

In kernel 2.4 and later, a mechanism was devised to identify code
licensed under the GPL (and friends) so people can be warned that the
code is non open-source. This is accomplished by the MODULE_LICENSE()
macro which is demonstrated in the next piece of code. By setting the
license to GPL, you can keep the warning from being printed. This
license mechanism is defined and documented in linux/module.h:

/*
 * The following license idents are currently accepted as indicating free
 * software modules
 *
 *	"GPL"				[GNU Public License v2 or later]
 *	"GPL v2"			[GNU Public License v2]
 *	"GPL and additional rights"	[GNU Public License v2 rights and more]
 *	"Dual BSD/GPL"			[GNU Public License v2
 *					 or BSD license choice]
 *	"Dual MIT/GPL"			[GNU Public License v2
 *					 or MIT license choice]
 *	"Dual MPL/GPL"			[GNU Public License v2
 *					 or Mozilla license choice]
 *
 * The following other idents are available
 *
 *	"Proprietary"			[Non free products]
 *
 * There are dual licensed components, but when running with Linux it is the
 * GPL that is relevant so this is a non issue. Similarly LGPL linked with GPL
 * is a GPL combined work.
 *
 * This exists for several reasons
 * 1.	So modinfo can show license info for users wanting to vet their setup 
 *	is free
 * 2.	So the community can ignore bug reports including proprietary modules
 * 3.	So vendors can do likewise based on their own policies
 */

Similarly, MODULE_DESCRIPTION() is used to describe what the module
does, MODULE_AUTHOR() declares the module's author, and
MODULE_SUPPORTED_DEVICE() declares what types of devices the module
supports.

These macros are all defined in linux/module.h and aren't used by the
kernel itself. They're simply for documentation and can be viewed by a
tool like objdump. As an exercise to the reader, try and search fo these
macros in linux/drivers to see how module authors use these macros to
document their modules.

I'd recommend to use something like *grep -inr MODULE_AUTHOR * * in
/usr/src/linux-2.6.x/ . People unfamiliar with command line tools will
probably like some web base solution, search for sites that offer kernel
trees that got indexed with LXR. (or setup it up on your local machine).

Users of traditional Unix editors, like *emacs * or *vi * will also find
tag files useful. They can be generated by *make tags * or *make TAGS *
in /usr/src/linux-2.6.x/ . Once you've got such a tagfile in your
kerneltree you can put the cursor on some function call and use some key
combination to directly jump to the definition function.

*Example 2-6. hello-4.c*

/*  
 *  hello-4.c - Demonstrates module documentation.
 */
#include <linux/module.h>	/* Needed by all modules */
#include <linux/kernel.h>	/* Needed for KERN_INFO */
#include <linux/init.h>		/* Needed for the macros */
#define DRIVER_AUTHOR "Peter Jay Salzman <p@dirac.org>"
#define DRIVER_DESC   "A sample driver"

static int __init init_hello_4(void)
{
	printk(KERN_INFO "Hello, world 4\n");
	return 0;
}

static void __exit cleanup_hello_4(void)
{
	printk(KERN_INFO "Goodbye, world 4\n");
}

module_init(init_hello_4);
module_exit(cleanup_hello_4);

/*  
 *  You can use strings, like this:
 */

/* 
 * Get rid of taint message by declaring code as GPL. 
 */
MODULE_LICENSE("GPL");

/*
 * Or with defines, like this:
 */
MODULE_AUTHOR(DRIVER_AUTHOR);	/* Who wrote this module? */
MODULE_DESCRIPTION(DRIVER_DESC);	/* What does this module do */

/*  
 *  This module uses /dev/testdevice.  The MODULE_SUPPORTED_DEVICE macro might
 *  be used in the future to help automatic configuration of modules, but is 
 *  currently unused other than for documentation purposes.
 */
MODULE_SUPPORTED_DEVICE("testdevice");

------------------------------------------------------------------------


  2.6. Passing Command Line Arguments to a Module

Modules can take command line arguments, but not with the argc/argv you
might be used to.

To allow arguments to be passed to your module, declare the variables
that will take the values of the command line arguments as global and
then use the module_param() macro, (defined in linux/moduleparam.h) to
set the mechanism up. At runtime, insmod will fill the variables with
any command line arguments that are given, like *./insmod mymodule.ko
myvariable=5*. The variable declarations and macros should be placed at
the beginning of the module for clarity. The example code should clear
up my admittedly lousy explanation.

The module_param() macro takes 3 arguments: the name of the variable,
its type and permissions for the corresponding file in sysfs. Integer
types can be signed as usual or unsigned. If you'd like to use arrays of
integers or strings see module_param_array() and module_param_string().

int myint = 3;
module_param(myint, int, 0);
	

Arrays are supported too, but things are a bit different now than they
were in the 2.4. days. To keep track of the number of parameters you
need to pass a pointer to a count variable as third parameter. At your
option, you could also ignore the count and pass NULL instead. We show
both possibilities here:

int myintarray[2];
module_param_array(myintarray, int, NULL, 0); /* not interested in count */

int myshortarray[4];
int count;
module_parm_array(myshortarray, short, , 0); /* put count into "count" variable */
	

A good use for this is to have the module variable's default values set,
like an port or IO address. If the variables contain the default values,
then perform autodetection (explained elsewhere). Otherwise, keep the
current value. This will be made clear later on.

Lastly, there's a macro function, MODULE_PARM_DESC(), that is used to
document arguments that the module can take. It takes two parameters: a
variable name and a free form string describing that variable.

*Example 2-7. hello-5.c*

/*
 *  hello-5.c - Demonstrates command line argument passing to a module.
 */
#include <linux/module.h>
#include <linux/moduleparam.h>
#include <linux/kernel.h>
#include <linux/init.h>
#include <linux/stat.h>

MODULE_LICENSE("GPL");
MODULE_AUTHOR("Peter Jay Salzman");

static short int myshort = 1;
static int myint = 420;
static long int mylong = 9999;
static char *mystring = "blah";
static int myintArray[2] = { -1, -1 };
static int arr_argc = 0;

/* 
 * module_param(foo, int, 0000)
 * The first param is the parameters name
 * The second param is it's data type
 * The final argument is the permissions bits, 
 * for exposing parameters in sysfs (if non-zero) at a later stage.
 */

module_param(myshort, short, S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP);
MODULE_PARM_DESC(myshort, "A short integer");
module_param(myint, int, S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH);
MODULE_PARM_DESC(myint, "An integer");
module_param(mylong, long, S_IRUSR);
MODULE_PARM_DESC(mylong, "A long integer");
module_param(mystring, charp, 0000);
MODULE_PARM_DESC(mystring, "A character string");

/*
 * module_param_array(name, type, num, perm);
 * The first param is the parameter's (in this case the array's) name
 * The second param is the data type of the elements of the array
 * The third argument is a pointer to the variable that will store the number 
 * of elements of the array initialized by the user at module loading time
 * The fourth argument is the permission bits
 */
module_param_array(myintArray, int, &arr_argc, 0000);
MODULE_PARM_DESC(myintArray, "An array of integers");

static int __init hello_5_init(void)
{
	int i;
	printk(KERN_INFO "Hello, world 5\n=============\n");
	printk(KERN_INFO "myshort is a short integer: %hd\n", myshort);
	printk(KERN_INFO "myint is an integer: %d\n", myint);
	printk(KERN_INFO "mylong is a long integer: %ld\n", mylong);
	printk(KERN_INFO "mystring is a string: %s\n", mystring);
	for (i = 0; i < (sizeof myintArray / sizeof (int)); i++)
	{
		printk(KERN_INFO "myintArray[%d] = %d\n", i, myintArray[i]);
	}
	printk(KERN_INFO "got %d arguments for myintArray.\n", arr_argc);
	return 0;
}

static void __exit hello_5_exit(void)
{
	printk(KERN_INFO "Goodbye, world 5\n");
}

module_init(hello_5_init);
module_exit(hello_5_exit);

I would recommend playing around with this code:

satan# insmod hello-5.ko mystring="bebop" mybyte=255 myintArray=-1
mybyte is an 8 bit integer: 255
myshort is a short integer: 1
myint is an integer: 20
mylong is a long integer: 9999
mystring is a string: bebop
myintArray is -1 and 420

satan# rmmod hello-5
Goodbye, world 5

satan# insmod hello-5.ko mystring="supercalifragilisticexpialidocious" \
> mybyte=256 myintArray=-1,-1
mybyte is an 8 bit integer: 0
myshort is a short integer: 1
myint is an integer: 20
mylong is a long integer: 9999
mystring is a string: supercalifragilisticexpialidocious
myintArray is -1 and -1

satan# rmmod hello-5
Goodbye, world 5

satan# insmod hello-5.ko mylong=hello
hello-5.o: invalid argument syntax for mylong: 'h'

------------------------------------------------------------------------


  2.7. Modules Spanning Multiple Files

Sometimes it makes sense to divide a kernel module between several
source files.

Here's an example of such a kernel module.

*Example 2-8. start.c*

/*
 *  start.c - Illustration of multi filed modules
 */

#include <linux/kernel.h>	/* We're doing kernel work */
#include <linux/module.h>	/* Specifically, a module */

int init_module(void)
{
	printk(KERN_INFO "Hello, world - this is the kernel speaking\n");
	return 0;
}

The next file:

*Example 2-9. stop.c*

/*
 *  stop.c - Illustration of multi filed modules
 */

#include <linux/kernel.h>	/* We're doing kernel work */
#include <linux/module.h>	/* Specifically, a module  */

void cleanup_module()
{
	printk(KERN_INFO "Short is the life of a kernel module\n");
}

And finally, the makefile:

*Example 2-10. Makefile*

obj-m += hello-1.o
obj-m += hello-2.o
obj-m += hello-3.o
obj-m += hello-4.o
obj-m += hello-5.o
obj-m += startstop.o
startstop-objs := start.o stop.o

all:
	make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules

clean:
	make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clean

This is the complete makefile for all the examples we've seen so far.
The first five lines are nothing special, but for the last example we'll
need two lines. First we invent an object name for our combined module,
second we tell *make * what object files are part of that module.

------------------------------------------------------------------------


  2.8. Building modules for a precompiled kernel

Obviously, we strongly suggest you to recompile your kernel, so that you
can enable a number of useful debugging features, such as forced module
unloading (MODULE_FORCE_UNLOAD): when this option is enabled, you can
force the kernel to unload a module even when it believes it is unsafe,
via a *rmmod -f module* command. This option can save you a lot of time
and a number of reboots during the development of a module.

Nevertheless, there is a number of cases in which you may want to load
your module into a precompiled running kernel, such as the ones shipped
with common Linux distributions, or a kernel you have compiled in the
past. In certain circumstances you could require to compile and insert a
module into a running kernel which you are not allowed to recompile, or
on a machine that you prefer not to reboot. If you can't think of a case
that will force you to use modules for a precompiled kernel you might
want to skip this and treat the rest of this chapter as a big footnote.

Now, if you just install a kernel source tree, use it to compile your
kernel module and you try to insert your module into the kernel, in most
cases you would obtain an error as follows:

insmod: error inserting 'poet_atkm.ko': -1 Invalid module format
	

Less cryptical information are logged to /var/log/messages:

Jun  4 22:07:54 localhost kernel: poet_atkm: version magic '2.6.5-1.358custom 686 
REGPARM 4KSTACKS gcc-3.3' should be '2.6.5-1.358 686 REGPARM 4KSTACKS gcc-3.3'
	

In other words, your kernel refuses to accept your module because
version strings (more precisely, version magics) do not match.
Incidentally, version magics are stored in the module object in the form
of a static string, starting with vermagic:. Version data are inserted
in your module when it is linked against the init/vermagic.o file. To
inspect version magics and other strings stored in a given module, issue
the *modinfo module.ko* command:

[root@pcsenonsrv 02-HelloWorld]# modinfo hello-4.ko 
license:        GPL
author:         Peter Jay Salzman <p@dirac.org>
description:    A sample driver
vermagic:       2.6.5-1.358 686 REGPARM 4KSTACKS gcc-3.3
depends:        
	

To overcome this problem we could resort to the *--force-vermagic*
option, but this solution is potentially unsafe, and unquestionably
inacceptable in production modules. Consequently, we want to compile our
module in an environment which was identical to the one in which our
precompiled kernel was built. How to do this, is the subject of the
remainder of this chapter.

First of all, make sure that a kernel source tree is available, having
exactly the same version as your current kernel. Then, find the
configuration file which was used to compile your precompiled kernel.
Usually, this is available in your current /boot directory, under a name
like config-2.6.x. You may just want to copy it to your kernel source
tree: *cp /boot/config-`uname -r` /usr/src/linux-`uname -r`/.config*.

Let's focus again on the previous error message: a closer look at the
version magic strings suggests that, even with two configuration files
which are exactly the same, a slight difference in the version magic
could be possible, and it is sufficient to prevent insertion of the
module into the kernel. That slight difference, namely the custom string
which appears in the module's version magic and not in the kernel's one,
is due to a modification with respect to the original, in the makefile
that some distribution include. Then, examine your
/usr/src/linux/Makefile, and make sure that the specified version
information matches exactly the one used for your current kernel. For
example, you makefile could start as follows:

VERSION = 2
PATCHLEVEL = 6
SUBLEVEL = 5
EXTRAVERSION = -1.358custom
...
	

In this case, you need to restore the value of symbol EXTRAVERSION to
-1.358. We suggest to keep a backup copy of the makefile used to compile
your kernel available in /lib/modules/2.6.5-1.358/build. A simple *cp
/lib/modules/`uname -r`/build/Makefile /usr/src/linux-`uname -r`* should
suffice. Additionally, if you already started a kernel build with the
previous (wrong) Makefile, you should also rerun *make*, or directly
modify symbol UTS_RELEASE in file
/usr/src/linux-2.6.x/include/linux/version.h according to contents of
file /lib/modules/2.6.x/build/include/linux/version.h, or overwrite the
latter with the first.

Now, please run *make* to update configuration and version headers and
objects:

[root@pcsenonsrv linux-2.6.x]# make
CHK     include/linux/version.h
UPD     include/linux/version.h
SYMLINK include/asm -> include/asm-i386
SPLIT   include/linux/autoconf.h -> include/config/*
HOSTCC  scripts/basic/fixdep
HOSTCC  scripts/basic/split-include
HOSTCC  scripts/basic/docproc
HOSTCC  scripts/conmakehash
HOSTCC  scripts/kallsyms
CC      scripts/empty.o
...
	

If you do not desire to actually compile the kernel, you can interrupt
the build process (*CTRL-C*) just after the SPLIT line, because at that
time, the files you need will be are ready. Now you can turn back to the
directory of your module and compile it: It will be built exactly
according your current kernel settings, and it will load into it without
any errors.

------------------------------------------------------------------------


  Chapter 3. Preliminaries


  3.1. Modules vs Programs


    3.1.1. How modules begin and end

A program usually begins with a main() function, executes a bunch of
instructions and terminates upon completion of those instructions.
Kernel modules work a bit differently. A module always begin with either
the init_module or the function you specify with module_init call. This
is the entry function for modules; it tells the kernel what
functionality the module provides and sets up the kernel to run the
module's functions when they're needed. Once it does this, entry
function returns and the module does nothing until the kernel wants to
do something with the code that the module provides.

All modules end by calling either cleanup_module or the function you
specify with the module_exit call. This is the exit function for
modules; it undoes whatever entry function did. It unregisters the
functionality that the entry function registered.

Every module must have an entry function and an exit function. Since
there's more than one way to specify entry and exit functions, I'll try
my best to use the terms `entry function' and `exit function', but if I
slip and simply refer to them as init_module and cleanup_module, I think
you'll know what I mean.

------------------------------------------------------------------------


    3.1.2. Functions available to modules

Programmers use functions they don't define all the time. A prime
example of this is printf(). You use these library functions which are
provided by the standard C library, libc. The definitions for these
functions don't actually enter your program until the linking stage,
which insures that the code (for printf() for example) is available, and
fixes the call instruction to point to that code.

Kernel modules are different here, too. In the hello world example, you
might have noticed that we used a function, printk() but didn't include
a standard I/O library. That's because modules are object files whose
symbols get resolved upon insmod'ing. The definition for the symbols
comes from the kernel itself; the only external functions you can use
are the ones provided by the kernel. If you're curious about what
symbols have been exported by your kernel, take a look at /proc/kallsyms.

One point to keep in mind is the difference between library functions
and system calls. Library functions are higher level, run completely in
user space and provide a more convenient interface for the programmer to
the functions that do the real work---system calls. System calls run in
kernel mode on the user's behalf and are provided by the kernel itself.
The library function printf() may look like a very general printing
function, but all it really does is format the data into strings and
write the string data using the low-level system call write(), which
then sends the data to standard output.

Would you like to see what system calls are made by printf()? It's easy!
Compile the following program:

#include <stdio.h>
int main(void)
{ printf("hello"); return 0; }
				

with *gcc -Wall -o hello hello.c*. Run the exectable with *strace
./hello*. Are you impressed? Every line you see corresponds to a system
call. strace[4] <#FTN.AEN467> is a handy program that gives you details
about what system calls a program is making, including which call is
made, what its arguments are what it returns. It's an invaluable tool
for figuring out things like what files a program is trying to access.
Towards the end, you'll see a line which looks like write(1, "hello",
5hello). There it is. The face behind the printf() mask. You may not be
familiar with write, since most people use library functions for file
I/O (like fopen, fputs, fclose). If that's the case, try looking at *man
2 write*. The 2nd man section is devoted to system calls (like kill()
and read(). The 3rd man section is devoted to library calls, which you
would probably be more familiar with (like cosh() and random()).

You can even write modules to replace the kernel's system calls, which
we'll do shortly. Crackers often make use of this sort of thing for
backdoors or trojans, but you can write your own modules to do more
benign things, like have the kernel write /Tee hee, that tickles!/
everytime someone tries to delete a file on your system.

------------------------------------------------------------------------


    3.1.3. User Space vs Kernel Space

A kernel is all about access to resources, whether the resource in
question happens to be a video card, a hard drive or even memory.
Programs often compete for the same resource. As I just saved this
document, updatedb started updating the locate database. My vim session
and updatedb are both using the hard drive concurrently. The kernel
needs to keep things orderly, and not give users access to resources
whenever they feel like it. To this end, a CPU can run in different
modes. Each mode gives a different level of freedom to do what you want
on the system. The Intel 80386 architecture has 4 of these modes, which
are called rings. Unix uses only two rings; the highest ring (ring 0,
also known as `supervisor mode' where everything is allowed to happen)
and the lowest ring, which is called `user mode'.

Recall the discussion about library functions vs system calls.
Typically, you use a library function in user mode. The library function
calls one or more system calls, and these system calls execute on the
library function's behalf, but do so in supervisor mode since they are
part of the kernel itself. Once the system call completes its task, it
returns and execution gets transfered back to user mode.

------------------------------------------------------------------------


    3.1.4. Name Space

When you write a small C program, you use variables which are convenient
and make sense to the reader. If, on the other hand, you're writing
routines which will be part of a bigger problem, any global variables
you have are part of a community of other peoples' global variables;
some of the variable names can clash. When a program has lots of global
variables which aren't meaningful enough to be distinguished, you get
/namespace pollution/. In large projects, effort must be made to
remember reserved names, and to find ways to develop a scheme for naming
unique variable names and symbols.

When writing kernel code, even the smallest module will be linked
against the entire kernel, so this is definitely an issue. The best way
to deal with this is to declare all your variables as static and to use
a well-defined prefix for your symbols. By convention, all kernel
prefixes are lowercase. If you don't want to declare everything as
static, another option is to declare a symbol table and register it with
a kernel. We'll get to this later.

The file /proc/kallsyms holds all the symbols that the kernel knows
about and which are therefore accessible to your modules since they
share the kernel's codespace.

------------------------------------------------------------------------


    3.1.5. Code space

Memory management is a very complicated subject---the majority of
O'Reilly's `Understanding The Linux Kernel' is just on memory
management! We're not setting out to be experts on memory managements,
but we do need to know a couple of facts to even begin worrying about
writing real modules.

If you haven't thought about what a segfault really means, you may be
surprised to hear that pointers don't actually point to memory
locations. Not real ones, anyway. When a process is created, the kernel
sets aside a portion of real physical memory and hands it to the process
to use for its executing code, variables, stack, heap and other things
which a computer scientist would know about[5] <#FTN.AEN514>. This
memory begins with 0x00000000 and extends up to whatever it needs to be.
Since the memory space for any two processes don't overlap, every
process that can access a memory address, say 0xbffff978, would be
accessing a different location in real physical memory! The processes
would be accessing an index named 0xbffff978 which points to some kind
of offset into the region of memory set aside for that particular
process. For the most part, a process like our Hello, World program
can't access the space of another process, although there are ways which
we'll talk about later.

The kernel has its own space of memory as well. Since a module is code
which can be dynamically inserted and removed in the kernel (as opposed
to a semi-autonomous object), it shares the kernel's codespace rather
than having its own. Therefore, if your module segfaults, the kernel
segfaults. And if you start writing over data because of an off-by-one
error, then you're trampling on kernel data (or code). This is even
worse than it sounds, so try your best to be careful.

By the way, I would like to point out that the above discussion is true
for any operating system which uses a monolithic kernel[6]
<#FTN.AEN520>. There are things called microkernels which have modules
which get their own codespace. The GNU Hurd and QNX Neutrino are two
examples of a microkernel.

------------------------------------------------------------------------


    3.1.6. Device Drivers

One class of module is the device driver, which provides functionality
for hardware like a TV card or a serial port. On unix, each piece of
hardware is represented by a file located in /dev named a device file
which provides the means to communicate with the hardware. The device
driver provides the communication on behalf of a user program. So the
es1370.o sound card device driver might connect the /dev/sound device
file to the Ensoniq IS1370 sound card. A userspace program like
mp3blaster can use /dev/sound without ever knowing what kind of sound
card is installed.

------------------------------------------------------------------------


      3.1.6.1. Major and Minor Numbers

Let's look at some device files. Here are device files which represent
the first three partitions on the primary master IDE hard drive:

# ls -l /dev/hda[1-3]
brw-rw----  1 root  disk  3, 1 Jul  5  2000 /dev/hda1
brw-rw----  1 root  disk  3, 2 Jul  5  2000 /dev/hda2
brw-rw----  1 root  disk  3, 3 Jul  5  2000 /dev/hda3
					

Notice the column of numbers separated by a comma? The first number is
called the device's major number. The second number is the minor number.
The major number tells you which driver is used to access the hardware.
Each driver is assigned a unique major number; all device files with the
same major number are controlled by the same driver. All the above major
numbers are 3, because they're all controlled by the same driver.

The minor number is used by the driver to distinguish between the
various hardware it controls. Returning to the example above, although
all three devices are handled by the same driver they have unique minor
numbers because the driver sees them as being different pieces of hardware.

Devices are divided into two types: character devices and block devices.
The difference is that block devices have a buffer for requests, so they
can choose the best order in which to respond to the requests. This is
important in the case of storage devices, where it's faster to read or
write sectors which are close to each other, rather than those which are
further apart. Another difference is that block devices can only accept
input and return output in blocks (whose size can vary according to the
device), whereas character devices are allowed to use as many or as few
bytes as they like. Most devices in the world are character, because
they don't need this type of buffering, and they don't operate with a
fixed block size. You can tell whether a device file is for a block
device or a character device by looking at the first character in the
output of *ls -l*. If it's `b' then it's a block device, and if it's `c'
then it's a character device. The devices you see above are block
devices. Here are some character devices (the serial ports):

crw-rw----  1 root  dial 4, 64 Feb 18 23:34 /dev/ttyS0
crw-r-----  1 root  dial 4, 65 Nov 17 10:26 /dev/ttyS1
crw-rw----  1 root  dial 4, 66 Jul  5  2000 /dev/ttyS2
crw-rw----  1 root  dial 4, 67 Jul  5  2000 /dev/ttyS3
					

If you want to see which major numbers have been assigned, you can look
at /usr/src/linux/Documentation/devices.txt.

When the system was installed, all of those device files were created by
the *mknod* command. To create a new char device named `coffee' with
major/minor number 12 and 2, simply do *mknod /dev/coffee c 12 2*. You
don't /have/ to put your device files into /dev, but it's done by
convention. Linus put his device files in /dev, and so should you.
However, when creating a device file for testing purposes, it's probably
OK to place it in your working directory where you compile the kernel
module. Just be sure to put it in the right place when you're done
writing the device driver.

I would like to make a few last points which are implicit from the above
discussion, but I'd like to make them explicit just in case. When a
device file is accessed, the kernel uses the major number of the file to
determine which driver should be used to handle the access. This means
that the kernel doesn't really need to use or even know about the minor
number. The driver itself is the only thing that cares about the minor
number. It uses the minor number to distinguish between different pieces
of hardware.

By the way, when I say `hardware', I mean something a bit more abstract
than a PCI card that you can hold in your hand. Look at these two device
files:

% ls -l /dev/fd0 /dev/fd0u1680
brwxrwxrwx   1 root  floppy   2,  0 Jul  5  2000 /dev/fd0
brw-rw----   1 root  floppy   2, 44 Jul  5  2000 /dev/fd0u1680
					

By now you can look at these two device files and know instantly that
they are block devices and are handled by same driver (block major 2).
You might even be aware that these both represent your floppy drive,
even if you only have one floppy drive. Why two files? One represents
the floppy drive with 1.44 MB of storage. The other is the /same/ floppy
drive with 1.68 MB of storage, and corresponds to what some people call
a `superformatted' disk. One that holds more data than a standard
formatted floppy. So here's a case where two device files with different
minor number actually represent the same piece of physical hardware. So
just be aware that the word `hardware' in our discussion can mean
something very abstract.

------------------------------------------------------------------------


  Chapter 4. Character Device Files


  4.1. Character Device Drivers

------------------------------------------------------------------------


    4.1.1. The file_operations Structure

The file_operations structure is defined in linux/fs.h, and holds
pointers to functions defined by the driver that perform various
operations on the device. Each field of the structure corresponds to the
address of some function defined by the driver to handle a requested
operation.

For example, every character driver needs to define a function that
reads from the device. The file_operations structure holds the address
of the module's function that performs that operation. Here is what the
definition looks like for kernel 2.6.5:

struct file_operations {
	struct module *owner;
	 loff_t(*llseek) (struct file *, loff_t, int);
	 ssize_t(*read) (struct file *, char __user *, size_t, loff_t *);
	 ssize_t(*aio_read) (struct kiocb *, char __user *, size_t, loff_t);
	 ssize_t(*write) (struct file *, const char __user *, size_t, loff_t *);
	 ssize_t(*aio_write) (struct kiocb *, const char __user *, size_t,
			      loff_t);
	int (*readdir) (struct file *, void *, filldir_t);
	unsigned int (*poll) (struct file *, struct poll_table_struct *);
	int (*ioctl) (struct inode *, struct file *, unsigned int,
		      unsigned long);
	int (*mmap) (struct file *, struct vm_area_struct *);
	int (*open) (struct inode *, struct file *);
	int (*flush) (struct file *);
	int (*release) (struct inode *, struct file *);
	int (*fsync) (struct file *, struct dentry *, int datasync);
	int (*aio_fsync) (struct kiocb *, int datasync);
	int (*fasync) (int, struct file *, int);
	int (*lock) (struct file *, int, struct file_lock *);
	 ssize_t(*readv) (struct file *, const struct iovec *, unsigned long,
			  loff_t *);
	 ssize_t(*writev) (struct file *, const struct iovec *, unsigned long,
			   loff_t *);
	 ssize_t(*sendfile) (struct file *, loff_t *, size_t, read_actor_t,
			     void __user *);
	 ssize_t(*sendpage) (struct file *, struct page *, int, size_t,
			     loff_t *, int);
	unsigned long (*get_unmapped_area) (struct file *, unsigned long,
					    unsigned long, unsigned long,
					    unsigned long);
};
			

Some operations are not implemented by a driver. For example, a driver
that handles a video card won't need to read from a directory structure.
The corresponding entries in the file_operations structure should be set
to NULL.

There is a gcc extension that makes assigning to this structure more
convenient. You'll see it in modern drivers, and may catch you by
surprise. This is what the new way of assigning to the structure looks like:

struct file_operations fops = {
	read: device_read,
	write: device_write,
	open: device_open,
	release: device_release
};
			

However, there's also a C99 way of assigning to elements of a structure,
and this is definitely preferred over using the GNU extension. The
version of gcc the author used when writing this, 2.95, supports the new
C99 syntax. You should use this syntax in case someone wants to port
your driver. It will help with compatibility:

struct file_operations fops = {
	.read = device_read,
	.write = device_write,
	.open = device_open,
	.release = device_release
};
			

The meaning is clear, and you should be aware that any member of the
structure which you don't explicitly assign will be initialized to NULL
by gcc.

An instance of struct file_operations containing pointers to functions
that are used to implement read, write, open, ... syscalls is commonly
named fops.

------------------------------------------------------------------------


    4.1.2. The file structure

Each device is represented in the kernel by a file structure, which is
defined in linux/fs.h. Be aware that a file is a kernel level structure
and never appears in a user space program. It's not the same thing as a
FILE, which is defined by glibc and would never appear in a kernel space
function. Also, its name is a bit misleading; it represents an abstract
open `file', not a file on a disk, which is represented by a structure
named inode.

An instance of struct file is commonly named filp. You'll also see it
refered to as struct file file. Resist the temptation.

Go ahead and look at the definition of file. Most of the entries you
see, like struct dentry aren't used by device drivers, and you can
ignore them. This is because drivers don't fill file directly; they only
use structures contained in file which are created elsewhere.

------------------------------------------------------------------------


    4.1.3. Registering A Device

As discussed earlier, char devices are accessed through device files,
usually located in /dev[7] <#FTN.AEN630>. The major number tells you
which driver handles which device file. The minor number is used only by
the driver itself to differentiate which device it's operating on, just
in case the driver handles more than one device.

Adding a driver to your system means registering it with the kernel.
This is synonymous with assigning it a major number during the module's
initialization. You do this by using the register_chrdev function,
defined by linux/fs.h.

int register_chrdev(unsigned int major, const char *name, struct file_operations *fops);
			

where unsigned int major is the major number you want to request, const
char *name is the name of the device as it'll appear in /proc/devices
and struct file_operations *fops is a pointer to the file_operations
table for your driver. A negative return value means the registration
failed. Note that we didn't pass the minor number to register_chrdev.
That's because the kernel doesn't care about the minor number; only our
driver uses it.

Now the question is, how do you get a major number without hijacking one
that's already in use? The easiest way would be to look through
Documentation/devices.txt and pick an unused one. That's a bad way of
doing things because you'll never be sure if the number you picked will
be assigned later. The answer is that you can ask the kernel to assign
you a dynamic major number.

If you pass a major number of 0 to register_chrdev, the return value
will be the dynamically allocated major number. The downside is that you
can't make a device file in advance, since you don't know what the major
number will be. There are a couple of ways to do this. First, the driver
itself can print the newly assigned number and we can make the device
file by hand. Second, the newly registered device will have an entry in
/proc/devices, and we can either make the device file by hand or write a
shell script to read the file in and make the device file. The third
method is we can have our driver make the the device file using the
mknod system call after a successful registration and rm during the call
to cleanup_module.

------------------------------------------------------------------------


    4.1.4. Unregistering A Device

We can't allow the kernel module to be rmmod'ed whenever root feels like
it. If the device file is opened by a process and then we remove the
kernel module, using the file would cause a call to the memory location
where the appropriate function (read/write) used to be. If we're lucky,
no other code was loaded there, and we'll get an ugly error message. If
we're unlucky, another kernel module was loaded into the same location,
which means a jump into the middle of another function within the
kernel. The results of this would be impossible to predict, but they
can't be very positive.

Normally, when you don't want to allow something, you return an error
code (a negative number) from the function which is supposed to do it.
With cleanup_module that's impossible because it's a void function.
However, there's a counter which keeps track of how many processes are
using your module. You can see what it's value is by looking at the 3rd
field of /proc/modules. If this number isn't zero, rmmod will fail. Note
that you don't have to check the counter from within cleanup_module
because the check will be performed for you by the system call
sys_delete_module, defined in linux/module.c. You shouldn't use this
counter directly, but there are functions defined in linux/module.h
which let you increase, decrease and display this counter:

  *

    try_module_get(THIS_MODULE): Increment the use count.

  *

    module_put(THIS_MODULE): Decrement the use count.

It's important to keep the counter accurate; if you ever do lose track
of the correct usage count, you'll never be able to unload the module;
it's now reboot time, boys and girls. This is bound to happen to you
sooner or later during a module's development.

------------------------------------------------------------------------


    4.1.5. chardev.c

The next code sample creates a char driver named chardev. You can cat
its device file (or open the file with a program) and the driver will
put the number of times the device file has been read from into the
file. We don't support writing to the file (like *echo "hi" >
/dev/hello*), but catch these attempts and tell the user that the
operation isn't supported. Don't worry if you don't see what we do with
the data we read into the buffer; we don't do much with it. We simply
read in the data and print a message acknowledging that we received it.

*Example 4-1. chardev.c*

/*
 *  chardev.c: Creates a read-only char device that says how many times
 *  you've read from the dev file
 */

#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/fs.h>
#include <asm/uaccess.h>	/* for put_user */

/*  
 *  Prototypes - this would normally go in a .h file
 */
int init_module(void);
void cleanup_module(void);
static int device_open(struct inode *, struct file *);
static int device_release(struct inode *, struct file *);
static ssize_t device_read(struct file *, char *, size_t, loff_t *);
static ssize_t device_write(struct file *, const char *, size_t, loff_t *);

#define SUCCESS 0
#define DEVICE_NAME "chardev"	/* Dev name as it appears in /proc/devices   */
#define BUF_LEN 80		/* Max length of the message from the device */

/* 
 * Global variables are declared as static, so are global within the file. 
 */

static int Major;		/* Major number assigned to our device driver */
static int Device_Open = 0;	/* Is device open?  
				 * Used to prevent multiple access to device */
static char msg[BUF_LEN];	/* The msg the device will give when asked */
static char *msg_Ptr;

static struct file_operations fops = {
	.read = device_read,
	.write = device_write,
	.open = device_open,
	.release = device_release
};

/*
 * This function is called when the module is loaded
 */
int init_module(void)
{
        Major = register_chrdev(0, DEVICE_NAME, &fops);

	if (Major < 0) {
	  printk(KERN_ALERT "Registering char device failed with %d\n", Major);
	  return Major;
	}

	printk(KERN_INFO "I was assigned major number %d. To talk to\n", Major);
	printk(KERN_INFO "the driver, create a dev file with\n");
	printk(KERN_INFO "'mknod /dev/%s c %d 0'.\n", DEVICE_NAME, Major);
	printk(KERN_INFO "Try various minor numbers. Try to cat and echo to\n");
	printk(KERN_INFO "the device file.\n");
	printk(KERN_INFO "Remove the device file and module when done.\n");

	return SUCCESS;
}

/*
 * This function is called when the module is unloaded
 */
void cleanup_module(void)
{
	/* 
	 * Unregister the device 
	 */
	int ret = unregister_chrdev(Major, DEVICE_NAME);
	if (ret < 0)
		printk(KERN_ALERT "Error in unregister_chrdev: %d\n", ret);
}

/*
 * Methods
 */

/* 
 * Called when a process tries to open the device file, like
 * "cat /dev/mycharfile"
 */
static int device_open(struct inode *inode, struct file *file)
{
	static int counter = 0;

	if (Device_Open)
		return -EBUSY;

	Device_Open++;
	sprintf(msg, "I already told you %d times Hello world!\n", counter++);
	msg_Ptr = msg;
	try_module_get(THIS_MODULE);

	return SUCCESS;
}

/* 
 * Called when a process closes the device file.
 */
static int device_release(struct inode *inode, struct file *file)
{
	Device_Open--;		/* We're now ready for our next caller */

	/* 
	 * Decrement the usage count, or else once you opened the file, you'll
	 * never get get rid of the module. 
	 */
	module_put(THIS_MODULE);

	return 0;
}

/* 
 * Called when a process, which already opened the dev file, attempts to
 * read from it.
 */
static ssize_t device_read(struct file *filp,	/* see include/linux/fs.h   */
			   char *buffer,	/* buffer to fill with data */
			   size_t length,	/* length of the buffer     */
			   loff_t * offset)
{
	/*
	 * Number of bytes actually written to the buffer 
	 */
	int bytes_read = 0;

	/*
	 * If we're at the end of the message, 
	 * return 0 signifying end of file 
	 */
	if (*msg_Ptr == 0)
		return 0;

	/* 
	 * Actually put the data into the buffer 
	 */
	while (length && *msg_Ptr) {

		/* 
		 * The buffer is in the user data segment, not the kernel 
		 * segment so "*" assignment won't work.  We have to use 
		 * put_user which copies data from the kernel data segment to
		 * the user data segment. 
		 */
		put_user(*(msg_Ptr++), buffer++);

		length--;
		bytes_read++;
	}

	/* 
	 * Most read functions return the number of bytes put into the buffer
	 */
	return bytes_read;
}

/*  
 * Called when a process writes to dev file: echo "hi" > /dev/hello 
 */
static ssize_t
device_write(struct file *filp, const char *buff, size_t len, loff_t * off)
{
	printk(KERN_ALERT "Sorry, this operation isn't supported.\n");
	return -EINVAL;
}

------------------------------------------------------------------------


    4.1.6. Writing Modules for Multiple Kernel Versions

The system calls, which are the major interface the kernel shows to the
processes, generally stay the same across versions. A new system call
may be added, but usually the old ones will behave exactly like they
used to. This is necessary for backward compatibility -- a new kernel
version is not supposed to break regular processes. In most cases, the
device files will also remain the same. On the other hand, the internal
interfaces within the kernel can and do change between versions.

The Linux kernel versions are divided between the stable versions
(n.$<$even number$>$.m) and the development versions (n.$<$odd
number$>$.m). The development versions include all the cool new ideas,
including those which will be considered a mistake, or reimplemented, in
the next version. As a result, you can't trust the interface to remain
the same in those versions (which is why I don't bother to support them
in this book, it's too much work and it would become dated too quickly).
In the stable versions, on the other hand, we can expect the interface
to remain the same regardless of the bug fix version (the m number).

There are differences between different kernel versions, and if you want
to support multiple kernel versions, you'll find yourself having to code
conditional compilation directives. The way to do this to compare the
macro LINUX_VERSION_CODE to the macro KERNEL_VERSION. In version a.b.c
of the kernel, the value of this macro would be $2^{16}a+2^{8}b+c$.

While previous versions of this guide showed how you can write backward
compatible code with such constructs in great detail, we decided to
break with this tradition for the better. People interested in doing
such might now use a LKMPG with a version matching to their kernel. We
decided to version the LKMPG like the kernel, at least as far as major
and minor number are concerned. We use the patchlevel for our own
versioning so use LKMPG version 2.4.x for kernels 2.4.x, use LKMPG
version 2.6.x for kernels 2.6.x and so on. Also make sure that you
always use current, up to date versions of both, kernel and guide.

Update: What we've said above was true for kernels up to and including
2.6.10. You might already have noticed that recent kernels look
different. In case you haven't they look like 2.6.x.y now. The meaning
of the first three items basically stays the same, but a subpatchlevel
has been added and will indicate security fixes till the next stable
patchlevel is out. So people can choose between a stable tree with
security updates /and / use the latest kernel as developer tree. Search
the kernel mailing list archives if you're interested in the full story.

------------------------------------------------------------------------


  Chapter 5. The /proc File System


  5.1. The /proc File System

In Linux, there is an additional mechanism for the kernel and kernel
modules to send information to processes --- the /proc file system.
Originally designed to allow easy access to information about processes
(hence the name), it is now used by every bit of the kernel which has
something interesting to report, such as /proc/modules which provides
the list of modules and /proc/meminfo which stats memory usage statistics.

The method to use the proc file system is very similar to the one used
with device drivers --- a structure is created with all the information
needed for the /proc file, including pointers to any handler functions
(in our case there is only one, the one called when somebody attempts to
read from the /proc file). Then, init_module registers the structure
with the kernel and cleanup_module unregisters it.

The reason we use proc_register_dynamic[8] <#FTN.AEN736> is because we
don't want to determine the inode number used for our file in advance,
but to allow the kernel to determine it to prevent clashes. Normal file
systems are located on a disk, rather than just in memory (which is
where /proc is), and in that case the inode number is a pointer to a
disk location where the file's index-node (inode for short) is located.
The inode contains information about the file, for example the file's
permissions, together with a pointer to the disk location or locations
where the file's data can be found.

Because we don't get called when the file is opened or closed, there's
nowhere for us to put try_module_get and try_module_put in this module,
and if the file is opened and then the module is removed, there's no way
to avoid the consequences.

Here a simple example showing how to use a /proc file. This is the
HelloWorld for the /proc filesystem. There are three parts: create the
file /proc/helloworld in the function init_module, return a value (and a
buffer) when the file /proc/helloworld is read in the callback function
procfs_read, and delete the file /proc/helloworld in the function
cleanup_module.

The /proc/helloworld is created when the module is loaded with the
function create_proc_entry. The return value is a 'struct proc_dir_entry
*', and it will be used to configure the file /proc/helloworld (for
example, the owner of this file). A null return value means that the
creation has failed.

Each time, everytime the file /proc/helloworld is read, the function
procfs_read is called. Two parameters of this function are very
important: the buffer (the first parameter) and the offset (the third
one). The content of the buffer will be returned to the application
which read it (for example the cat command). The offset is the current
position in the file. If the return value of the function isn't null,
then this function is called again. So be careful with this function, if
it never returns zero, the read function is called endlessly.

% cat /proc/helloworld
HelloWorld!
        

*Example 5-1. procfs1.c*

/*
 *  procfs1.c -  create a "file" in /proc
 *
 */

#include <linux/module.h>	/* Specifically, a module */
#include <linux/kernel.h>	/* We're doing kernel work */
#include <linux/proc_fs.h>	/* Necessary because we use the proc fs */

#define procfs_name "helloworld"

/**
 * This structure hold information about the /proc file
 *
 */
struct proc_dir_entry *Our_Proc_File;

/* Put data into the proc fs file.
 * 
 * Arguments
 * =========
 * 1. The buffer where the data is to be inserted, if
 *    you decide to use it.
 * 2. A pointer to a pointer to characters. This is
 *    useful if you don't want to use the buffer
 *    allocated by the kernel.
 * 3. The current position in the file
 * 4. The size of the buffer in the first argument.
 * 5. Write a "1" here to indicate EOF.
 * 6. A pointer to data (useful in case one common 
 *    read for multiple /proc/... entries)
 *
 * Usage and Return Value
 * ======================
 * A return value of zero means you have no further
 * information at this time (end of file). A negative
 * return value is an error condition.
 *
 * For More Information
 * ====================
 * The way I discovered what to do with this function
 * wasn't by reading documentation, but by reading the
 * code which used it. I just looked to see what uses
 * the get_info field of proc_dir_entry struct (I used a
 * combination of find and grep, if you're interested),
 * and I saw that  it is used in <kernel source
 * directory>/fs/proc/array.c.
 *
 * If something is unknown about the kernel, this is
 * usually the way to go. In Linux we have the great
 * advantage of having the kernel source code for
 * free - use it.
 */
int
procfile_read(char *buffer,
	      char **buffer_location,
	      off_t offset, int buffer_length, int *eof, void *data)
{
	int ret;
	
	printk(KERN_INFO "procfile_read (/proc/%s) called\n", procfs_name);
	
	/* 
	 * We give all of our information in one go, so if the
	 * user asks us if we have more information the
	 * answer should always be no.
	 *
	 * This is important because the standard read
	 * function from the library would continue to issue
	 * the read system call until the kernel replies
	 * that it has no more information, or until its
	 * buffer is filled.
	 */
	if (offset > 0) {
		/* we have finished to read, return 0 */
		ret  = 0;
	} else {
		/* fill the buffer, return the buffer size */
		ret = sprintf(buffer, "HelloWorld!\n");
	}

	return ret;
}

int init_module()
{
	Our_Proc_File = create_proc_entry(procfs_name, 0644, NULL);
	
	if (Our_Proc_File == NULL) {
		remove_proc_entry(procfs_name, &proc_root);
		printk(KERN_ALERT "Error: Could not initialize /proc/%s\n",
		       procfs_name);
		return -ENOMEM;
	}

	Our_Proc_File->read_proc = procfile_read;
	Our_Proc_File->owner 	 = THIS_MODULE;
	Our_Proc_File->mode 	 = S_IFREG | S_IRUGO;
	Our_Proc_File->uid 	 = 0;
	Our_Proc_File->gid 	 = 0;
	Our_Proc_File->size 	 = 37;

	printk(KERN_INFO "/proc/%s created\n", procfs_name);	
	return 0;	/* everything is ok */
}

void cleanup_module()
{
	remove_proc_entry(procfs_name, &proc_root);
	printk(KERN_INFO "/proc/%s removed\n", procfs_name);
}

------------------------------------------------------------------------


  5.2. Read and Write a /proc File

We have seen a very simple example for a /proc file where we only read
the file /proc/helloworld. It's also possible to write in a /proc file.
It works the same way as read, a function is called when the /proc file
is written. But there is a little difference with read, data comes from
user, so you have to import data from user space to kernel space (with
copy_from_user or get_user)

The reason for copy_from_user or get_user is that Linux memory (on Intel
architecture, it may be different under some other processors) is
segmented. This means that a pointer, by itself, does not reference a
unique location in memory, only a location in a memory segment, and you
need to know which memory segment it is to be able to use it. There is
one memory segment for the kernel, and one for each of the processes.

The only memory segment accessible to a process is its own, so when
writing regular programs to run as processes, there's no need to worry
about segments. When you write a kernel module, normally you want to
access the kernel memory segment, which is handled automatically by the
system. However, when the content of a memory buffer needs to be passed
between the currently running process and the kernel, the kernel
function receives a pointer to the memory buffer which is in the process
segment. The put_user and get_user macros allow you to access that
memory. These functions handle only one caracter, you can handle several
caracters with copy_to_user and copy_from_user. As the buffer (in read
or write function) is in kernel space, for write function you need to
import data because it comes from user space, but not for the read
function because data is already in kernel space.

*Example 5-2. procfs2.c*

/**
 *  procfs2.c -  create a "file" in /proc
 *
 */

#include <linux/module.h>	/* Specifically, a module */
#include <linux/kernel.h>	/* We're doing kernel work */
#include <linux/proc_fs.h>	/* Necessary because we use the proc fs */
#include <asm/uaccess.h>	/* for copy_from_user */

#define PROCFS_MAX_SIZE		1024
#define PROCFS_NAME 		"buffer1k"

/**
 * This structure hold information about the /proc file
 *
 */
static struct proc_dir_entry *Our_Proc_File;

/**
 * The buffer used to store character for this module
 *
 */
static char procfs_buffer[PROCFS_MAX_SIZE];

/**
 * The size of the buffer
 *
 */
static unsigned long procfs_buffer_size = 0;

/** 
 * This function is called then the /proc file is read
 *
 */
int 
procfile_read(char *buffer,
	      char **buffer_location,
	      off_t offset, int buffer_length, int *eof, void *data)
{
	int ret;
	
	printk(KERN_INFO "procfile_read (/proc/%s) called\n", PROCFS_NAME);
	
	if (offset > 0) {
		/* we have finished to read, return 0 */
		ret  = 0;
	} else {
		/* fill the buffer, return the buffer size */
		memcpy(buffer, procfs_buffer, procfs_buffer_size);
		ret = procfs_buffer_size;
	}

	return ret;
}

/**
 * This function is called with the /proc file is written
 *
 */
int procfile_write(struct file *file, const char *buffer, unsigned long count,
		   void *data)
{
	/* get buffer size */
	procfs_buffer_size = count;
	if (procfs_buffer_size > PROCFS_MAX_SIZE ) {
		procfs_buffer_size = PROCFS_MAX_SIZE;
	}
	
	/* write data to the buffer */
	if ( copy_from_user(procfs_buffer, buffer, procfs_buffer_size) ) {
		return -EFAULT;
	}
	
	return procfs_buffer_size;
}

/**
 *This function is called when the module is loaded
 *
 */
int init_module()
{
	/* create the /proc file */
	Our_Proc_File = create_proc_entry(PROCFS_NAME, 0644, NULL);
	
	if (Our_Proc_File == NULL) {
		remove_proc_entry(PROCFS_NAME, &proc_root);
		printk(KERN_ALERT "Error: Could not initialize /proc/%s\n",
			PROCFS_NAME);
		return -ENOMEM;
	}

	Our_Proc_File->read_proc  = procfile_read;
	Our_Proc_File->write_proc = procfile_write;
	Our_Proc_File->owner 	  = THIS_MODULE;
	Our_Proc_File->mode 	  = S_IFREG | S_IRUGO;
	Our_Proc_File->uid 	  = 0;
	Our_Proc_File->gid 	  = 0;
	Our_Proc_File->size 	  = 37;

	printk(KERN_INFO "/proc/%s created\n", PROCFS_NAME);	
	return 0;	/* everything is ok */
}

/**
 *This function is called when the module is unloaded
 *
 */
void cleanup_module()
{
	remove_proc_entry(PROCFS_NAME, &proc_root);
	printk(KERN_INFO "/proc/%s removed\n", PROCFS_NAME);
}

------------------------------------------------------------------------


  5.3. Manage /proc file with standard filesystem

We have seen how to read and write a /proc file with the /proc
interface. But it's also possible to manage /proc file with inodes. The
main interest is to use advanced function, like permissions.

In Linux, there is a standard mechanism for file system registration.
Since every file system has to have its own functions to handle inode
and file operations[9] <#FTN.AEN814>, there is a special structure to
hold pointers to all those functions, struct inode_operations, which
includes a pointer to struct file_operations. In /proc, whenever we
register a new file, we're allowed to specify which struct
inode_operations will be used to access to it. This is the mechanism we
use, a struct inode_operations which includes a pointer to a struct
file_operations which includes pointers to our procfs_read and
procfs_write functions.

Another interesting point here is the module_permission function. This
function is called whenever a process tries to do something with the
/proc file, and it can decide whether to allow access or not. Right now
it is only based on the operation and the uid of the current user (as
available in current, a pointer to a structure which includes
information on the currently running process), but it could be based on
anything we like, such as what other processes are doing with the same
file, the time of day, or the last input we received.

It's important to note that the standard roles of read and write are
reversed in the kernel. Read functions are used for output, whereas
write functions are used for input. The reason for that is that read and
write refer to the user's point of view --- if a process reads something
from the kernel, then the kernel needs to output it, and if a process
writes something to the kernel, then the kernel receives it as input.

*Example 5-3. procfs3.c*

/* 
 *  procfs3.c -  create a "file" in /proc, use the file_operation way
 *  		to manage the file.
 */
 
#include <linux/kernel.h>	/* We're doing kernel work */
#include <linux/module.h>	/* Specifically, a module */
#include <linux/proc_fs.h>	/* Necessary because we use proc fs */
#include <asm/uaccess.h>	/* for copy_*_user */

#define PROC_ENTRY_FILENAME 	"buffer2k"
#define PROCFS_MAX_SIZE 	2048

/**
 * The buffer (2k) for this module
 *
 */
static char procfs_buffer[PROCFS_MAX_SIZE];

/**
 * The size of the data hold in the buffer
 *
 */
static unsigned long procfs_buffer_size = 0;

/**
 * The structure keeping information about the /proc file
 *
 */
static struct proc_dir_entry *Our_Proc_File;

/**
 * This funtion is called when the /proc file is read
 *
 */
static ssize_t procfs_read(struct file *filp,	/* see include/linux/fs.h   */
			     char *buffer,	/* buffer to fill with data */
			     size_t length,	/* length of the buffer     */
			     loff_t * offset)
{
	static int finished = 0;

	/* 
	 * We return 0 to indicate end of file, that we have
	 * no more information. Otherwise, processes will
	 * continue to read from us in an endless loop. 
	 */
	if ( finished ) {
		printk(KERN_INFO "procfs_read: END\n");
		finished = 0;
		return 0;
	}
	
	finished = 1;
		
	/* 
	 * We use put_to_user to copy the string from the kernel's
	 * memory segment to the memory segment of the process
	 * that called us. get_from_user, BTW, is
	 * used for the reverse. 
	 */
	if ( copy_to_user(buffer, procfs_buffer, procfs_buffer_size) ) {
		return -EFAULT;
	}

	printk(KERN_INFO "procfs_read: read %lu bytes\n", procfs_buffer_size);

	return procfs_buffer_size;	/* Return the number of bytes "read" */
}

/*
 * This function is called when /proc is written
 */
static ssize_t
procfs_write(struct file *file, const char *buffer, size_t len, loff_t * off)
{
	if ( len > PROCFS_MAX_SIZE )	{
		procfs_buffer_size = PROCFS_MAX_SIZE;
	}
	else	{
		procfs_buffer_size = len;
	}
	
	if ( copy_from_user(procfs_buffer, buffer, procfs_buffer_size) ) {
		return -EFAULT;
	}

	printk(KERN_INFO "procfs_write: write %lu bytes\n", procfs_buffer_size);
	
	return procfs_buffer_size;
}

/* 
 * This function decides whether to allow an operation
 * (return zero) or not allow it (return a non-zero
 * which indicates why it is not allowed).
 *
 * The operation can be one of the following values:
 * 0 - Execute (run the "file" - meaningless in our case)
 * 2 - Write (input to the kernel module)
 * 4 - Read (output from the kernel module)
 *
 * This is the real function that checks file
 * permissions. The permissions returned by ls -l are
 * for referece only, and can be overridden here.
 */

static int module_permission(struct inode *inode, int op, struct nameidata *foo)
{
	/* 
	 * We allow everybody to read from our module, but
	 * only root (uid 0) may write to it 
	 */
	if (op == 4 || (op == 2 && current->euid == 0))
		return 0;

	/* 
	 * If it's anything else, access is denied 
	 */
	return -EACCES;
}

/* 
 * The file is opened - we don't really care about
 * that, but it does mean we need to increment the
 * module's reference count. 
 */
int procfs_open(struct inode *inode, struct file *file)
{
	try_module_get(THIS_MODULE);
	return 0;
}

/* 
 * The file is closed - again, interesting only because
 * of the reference count. 
 */
int procfs_close(struct inode *inode, struct file *file)
{
	module_put(THIS_MODULE);
	return 0;		/* success */
}

static struct file_operations File_Ops_4_Our_Proc_File = {
	.read 	 = procfs_read,
	.write 	 = procfs_write,
	.open 	 = procfs_open,
	.release = procfs_close,
};

/* 
 * Inode operations for our proc file. We need it so
 * we'll have some place to specify the file operations
 * structure we want to use, and the function we use for
 * permissions. It's also possible to specify functions
 * to be called for anything else which could be done to
 * an inode (although we don't bother, we just put
 * NULL). 
 */

static struct inode_operations Inode_Ops_4_Our_Proc_File = {
	.permission = module_permission,	/* check for permissions */
};

/* 
 * Module initialization and cleanup 
 */
int init_module()
{
	/* create the /proc file */
	Our_Proc_File = create_proc_entry(PROC_ENTRY_FILENAME, 0644, NULL);
	
	/* check if the /proc file was created successfuly */
	if (Our_Proc_File == NULL){
		printk(KERN_ALERT "Error: Could not initialize /proc/%s\n",
		       PROC_ENTRY_FILENAME);
		return -ENOMEM;
	}
	
	Our_Proc_File->owner = THIS_MODULE;
	Our_Proc_File->proc_iops = &Inode_Ops_4_Our_Proc_File;
	Our_Proc_File->proc_fops = &File_Ops_4_Our_Proc_File;
	Our_Proc_File->mode = S_IFREG | S_IRUGO | S_IWUSR;
	Our_Proc_File->uid = 0;
	Our_Proc_File->gid = 0;
	Our_Proc_File->size = 80;

	printk(KERN_INFO "/proc/%s created\n", PROC_ENTRY_FILENAME);

	return 0;	/* success */
}

void cleanup_module()
{
	remove_proc_entry(PROC_ENTRY_FILENAME, &proc_root);
	printk(KERN_INFO "/proc/%s removed\n", PROC_ENTRY_FILENAME);
}

Still hungry for procfs examples? Well, first of all keep in mind, there
are rumors around, claiming that procfs is on it's way out, consider
using sysfs instead. Second, if you really can't get enough, there's a
highly recommendable bonus level for procfs below
linux/Documentation/DocBook/ . Use *make help * in your toplevel kernel
directory for instructions about how to convert it into your favourite
format. Example: *make htmldocs *. Consider using this mechanism, in
case you want to document something kernel related yourself.

------------------------------------------------------------------------


  5.4. Manage /proc file with seq_file

As we have seen, writing a /proc file may be quite "complex". So to help
people writting /proc file, there is an API named seq_file that helps
formating a /proc file for output. It's based on sequence, which is
composed of 3 functions: start(), next(), and stop(). The seq_file API
starts a sequence when a user read the /proc file.

A sequence begins with the call of the function start(). If the return
is a non NULL value, the function next() is called. This function is an
iterator, the goal is to go thought all the data. Each time next() is
called, the function show() is also called. It writes data values in the
buffer read by the user. The function next() is called until it returns
NULL. The sequence ends when next() returns NULL, then the function
stop() is called.

BE CARREFUL: when a sequence is finished, another one starts. That means
that at the end of function stop(), the function start() is called
again. This loop finishes when the function start() returns NULL. You
can see a scheme of this in the figure "How seq_file works".

*Figure 5-1. How seq_file works*

Seq_file provides basic functions for file_operations, as seq_read,
seq_lseek, and some others. But nothing to write in the /proc file. Of
course, you can still use the same way as in the previous example.

*Example 5-4. procfs4.c*

/**
 *  procfs4.c -  create a "file" in /proc
 * 	This program uses the seq_file library to manage the /proc file.
 *
 */

#include <linux/kernel.h>	/* We're doing kernel work */
#include <linux/module.h>	/* Specifically, a module */
#include <linux/proc_fs.h>	/* Necessary because we use proc fs */
#include <linux/seq_file.h>	/* for seq_file */

#define PROC_NAME	"iter"

MODULE_AUTHOR("Philippe Reynes");
MODULE_LICENSE("GPL");

/**
 * This function is called at the beginning of a sequence.
 * ie, when:
 *	- the /proc file is read (first time)
 *	- after the function stop (end of sequence)
 *
 */
static void *my_seq_start(struct seq_file *s, loff_t *pos)
{
	static unsigned long counter = 0;

	/* beginning a new sequence ? */	
	if ( *pos == 0 )
	{	
		/* yes => return a non null value to begin the sequence */
		return &counter;
	}
	else
	{
		/* no => it's the end of the sequence, return end to stop reading */
		*pos = 0;
		return NULL;
	}
}

/**
 * This function is called after the beginning of a sequence.
 * It's called untill the return is NULL (this ends the sequence).
 *
 */
static void *my_seq_next(struct seq_file *s, void *v, loff_t *pos)
{
	unsigned long *tmp_v = (unsigned long *)v;
	(*tmp_v)++;
	(*pos)++;
	return NULL;
}

/**
 * This function is called at the end of a sequence
 * 
 */
static void my_seq_stop(struct seq_file *s, void *v)
{
	/* nothing to do, we use a static value in start() */
}

/**
 * This function is called for each "step" of a sequence
 *
 */
static int my_seq_show(struct seq_file *s, void *v)
{
	loff_t *spos = (loff_t *) v;
	
	seq_printf(s, "%Ld\n", *spos);
	return 0;
}

/**
 * This structure gather "function" to manage the sequence
 *
 */
static struct seq_operations my_seq_ops = {
	.start = my_seq_start,
	.next  = my_seq_next,
	.stop  = my_seq_stop,
	.show  = my_seq_show
};

/**
 * This function is called when the /proc file is open.
 *
 */
static int my_open(struct inode *inode, struct file *file)
{
	return seq_open(file, &my_seq_ops);
};

/**
 * This structure gather "function" that manage the /proc file
 *
 */
static struct file_operations my_file_ops = {
	.owner   = THIS_MODULE,
	.open    = my_open,
	.read    = seq_read,
	.llseek  = seq_lseek,
	.release = seq_release
};
	
	
/**
 * This function is called when the module is loaded
 *
 */
int init_module(void)
{
	struct proc_dir_entry *entry;

	entry = create_proc_entry(PROC_NAME, 0, NULL);
	if (entry) {
		entry->proc_fops = &my_file_ops;
	}
	
	return 0;
}

/**
 * This function is called when the module is unloaded.
 *
 */
void cleanup_module(void)
{
	remove_proc_entry(PROC_NAME, NULL);
}

If you want more information, you can read this web page:

  *

    http://lwn.net/Articles/22355/

  *

    http://www.kernelnewbies.org/documents/seq_file_howto.txt

You can also read the code of fs/seq_file.c in the linux kernel.

------------------------------------------------------------------------


  Chapter 6. Using /proc For Input


  6.1. TODO: Write a chapter about sysfs

This is just a placeholder for now. Finally I'd like to see a (yet to be
written) chapter about sysfs instead here. If you are familiar with
sysfs and would like to take part in writing this chapter, feel free to
contact us (the LKMPG maintainers) for further details.

------------------------------------------------------------------------


  Chapter 7. Talking To Device Files


  7.1. Talking to Device Files (writes and IOCTLs)

Device files are supposed to represent physical devices. Most physical
devices are used for output as well as input, so there has to be some
mechanism for device drivers in the kernel to get the output to send to
the device from processes. This is done by opening the device file for
output and writing to it, just like writing to a file. In the following
example, this is implemented by device_write.

This is not always enough. Imagine you had a serial port connected to a
modem (even if you have an internal modem, it is still implemented from
the CPU's perspective as a serial port connected to a modem, so you
don't have to tax your imagination too hard). The natural thing to do
would be to use the device file to write things to the modem (either
modem commands or data to be sent through the phone line) and read
things from the modem (either responses for commands or the data
received through the phone line). However, this leaves open the question
of what to do when you need to talk to the serial port itself, for
example to send the rate at which data is sent and received.

The answer in Unix is to use a special function called ioctl (short for
Input Output ConTroL). Every device can have its own ioctl commands,
which can be read ioctl's (to send information from a process to the
kernel), write ioctl's (to return information to a process), [10]
<#FTN.AEN914> both or neither. The ioctl function is called with three
parameters: the file descriptor of the appropriate device file, the
ioctl number, and a parameter, which is of type long so you can use a
cast to use it to pass anything. [11] <#FTN.AEN919>

The ioctl number encodes the major device number, the type of the ioctl,
the command, and the type of the parameter. This ioctl number is usually
created by a macro call (_IO, _IOR, _IOW or _IOWR --- depending on the
type) in a header file. This header file should then be included both by
the programs which will use ioctl (so they can generate the appropriate
ioctl's) and by the kernel module (so it can understand it). In the
example below, the header file is chardev.h and the program which uses
it is ioctl.c.

If you want to use ioctls in your own kernel modules, it is best to
receive an official ioctl assignment, so if you accidentally get
somebody else's ioctls, or if they get yours, you'll know something is
wrong. For more information, consult the kernel source tree at
Documentation/ioctl-number.txt.

*Example 7-1. chardev.c*

/*
 *  chardev.c - Create an input/output character device
 */

#include <linux/kernel.h>	/* We're doing kernel work */
#include <linux/module.h>	/* Specifically, a module */
#include <linux/fs.h>
#include <asm/uaccess.h>	/* for get_user and put_user */

#include "chardev.h"
#define SUCCESS 0
#define DEVICE_NAME "char_dev"
#define BUF_LEN 80

/* 
 * Is the device open right now? Used to prevent
 * concurent access into the same device 
 */
static int Device_Open = 0;

/* 
 * The message the device will give when asked 
 */
static char Message[BUF_LEN];

/* 
 * How far did the process reading the message get?
 * Useful if the message is larger than the size of the
 * buffer we get to fill in device_read. 
 */
static char *Message_Ptr;

/* 
 * This is called whenever a process attempts to open the device file 
 */
static int device_open(struct inode *inode, struct file *file)
{
#ifdef DEBUG
	printk(KERN_INFO "device_open(%p)\n", file);
#endif

	/* 
	 * We don't want to talk to two processes at the same time 
	 */
	if (Device_Open)
		return -EBUSY;

	Device_Open++;
	/*
	 * Initialize the message 
	 */
	Message_Ptr = Message;
	try_module_get(THIS_MODULE);
	return SUCCESS;
}

static int device_release(struct inode *inode, struct file *file)
{
#ifdef DEBUG
	printk(KERN_INFO "device_release(%p,%p)\n", inode, file);
#endif

	/* 
	 * We're now ready for our next caller 
	 */
	Device_Open--;

	module_put(THIS_MODULE);
	return SUCCESS;
}

/* 
 * This function is called whenever a process which has already opened the
 * device file attempts to read from it.
 */
static ssize_t device_read(struct file *file,	/* see include/linux/fs.h   */
			   char __user * buffer,	/* buffer to be
							 * filled with data */
			   size_t length,	/* length of the buffer     */
			   loff_t * offset)
{
	/* 
	 * Number of bytes actually written to the buffer 
	 */
	int bytes_read = 0;

#ifdef DEBUG
	printk(KERN_INFO "device_read(%p,%p,%d)\n", file, buffer, length);
#endif

	/* 
	 * If we're at the end of the message, return 0
	 * (which signifies end of file) 
	 */
	if (*Message_Ptr == 0)
		return 0;

	/* 
	 * Actually put the data into the buffer 
	 */
	while (length && *Message_Ptr) {

		/* 
		 * Because the buffer is in the user data segment,
		 * not the kernel data segment, assignment wouldn't
		 * work. Instead, we have to use put_user which
		 * copies data from the kernel data segment to the
		 * user data segment. 
		 */
		put_user(*(Message_Ptr++), buffer++);
		length--;
		bytes_read++;
	}

#ifdef DEBUG
	printk(KERN_INFO "Read %d bytes, %d left\n", bytes_read, length);
#endif

	/* 
	 * Read functions are supposed to return the number
	 * of bytes actually inserted into the buffer 
	 */
	return bytes_read;
}

/* 
 * This function is called when somebody tries to
 * write into our device file. 
 */
static ssize_t
device_write(struct file *file,
	     const char __user * buffer, size_t length, loff_t * offset)
{
	int i;

#ifdef DEBUG
	printk(KERN_INFO "device_write(%p,%s,%d)", file, buffer, length);
#endif

	for (i = 0; i < length && i < BUF_LEN; i++)
		get_user(Message[i], buffer + i);

	Message_Ptr = Message;

	/* 
	 * Again, return the number of input characters used 
	 */
	return i;
}

/* 
 * This function is called whenever a process tries to do an ioctl on our
 * device file. We get two extra parameters (additional to the inode and file
 * structures, which all device functions get): the number of the ioctl called
 * and the parameter given to the ioctl function.
 *
 * If the ioctl is write or read/write (meaning output is returned to the
 * calling process), the ioctl call returns the output of this function.
 *
 */
int device_ioctl(struct inode *inode,	/* see include/linux/fs.h */
		 struct file *file,	/* ditto */
		 unsigned int ioctl_num,	/* number and param for ioctl */
		 unsigned long ioctl_param)
{
	int i;
	char *temp;
	char ch;

	/* 
	 * Switch according to the ioctl called 
	 */
	switch (ioctl_num) {
	case IOCTL_SET_MSG:
		/* 
		 * Receive a pointer to a message (in user space) and set that
		 * to be the device's message.  Get the parameter given to 
		 * ioctl by the process. 
		 */
		temp = (char *)ioctl_param;

		/* 
		 * Find the length of the message 
		 */
		get_user(ch, temp);
		for (i = 0; ch && i < BUF_LEN; i++, temp++)
			get_user(ch, temp);

		device_write(file, (char *)ioctl_param, i, 0);
		break;

	case IOCTL_GET_MSG:
		/* 
		 * Give the current message to the calling process - 
		 * the parameter we got is a pointer, fill it. 
		 */
		i = device_read(file, (char *)ioctl_param, 99, 0);

		/* 
		 * Put a zero at the end of the buffer, so it will be 
		 * properly terminated 
		 */
		put_user('\0', (char *)ioctl_param + i);
		break;

	case IOCTL_GET_NTH_BYTE:
		/* 
		 * This ioctl is both input (ioctl_param) and 
		 * output (the return value of this function) 
		 */
		return Message[ioctl_param];
		break;
	}

	return SUCCESS;
}

/* Module Declarations */

/* 
 * This structure will hold the functions to be called
 * when a process does something to the device we
 * created. Since a pointer to this structure is kept in
 * the devices table, it can't be local to
 * init_module. NULL is for unimplemented functions. 
 */
struct file_operations Fops = {
	.read = device_read,
	.write = device_write,
	.ioctl = device_ioctl,
	.open = device_open,
	.release = device_release,	/* a.k.a. close */
};

/* 
 * Initialize the module - Register the character device 
 */
int init_module()
{
	int ret_val;
	/* 
	 * Register the character device (atleast try) 
	 */
	ret_val = register_chrdev(MAJOR_NUM, DEVICE_NAME, &Fops);

	/* 
	 * Negative values signify an error 
	 */
	if (ret_val < 0) {
		printk(KERN_ALERT "%s failed with %d\n",
		       "Sorry, registering the character device ", ret_val);
		return ret_val;
	}

	printk(KERN_INFO "%s The major device number is %d.\n",
	       "Registeration is a success", MAJOR_NUM);
	printk(KERN_INFO "If you want to talk to the device driver,\n");
	printk(KERN_INFO "you'll have to create a device file. \n");
	printk(KERN_INFO "We suggest you use:\n");
	printk(KERN_INFO "mknod %s c %d 0\n", DEVICE_FILE_NAME, MAJOR_NUM);
	printk(KERN_INFO "The device file name is important, because\n");
	printk(KERN_INFO "the ioctl program assumes that's the\n");
	printk(KERN_INFO "file you'll use.\n");

	return 0;
}

/* 
 * Cleanup - unregister the appropriate file from /proc 
 */
void cleanup_module()
{
	int ret;

	/* 
	 * Unregister the device 
	 */
	ret = unregister_chrdev(MAJOR_NUM, DEVICE_NAME);

	/* 
	 * If there's an error, report it 
	 */
	if (ret < 0)
		printk(KERN_ALERT "Error: unregister_chrdev: %d\n", ret);
}

*Example 7-2. chardev.h*

/*
 *  chardev.h - the header file with the ioctl definitions.
 *
 *  The declarations here have to be in a header file, because
 *  they need to be known both to the kernel module
 *  (in chardev.c) and the process calling ioctl (ioctl.c)
 */

#ifndef CHARDEV_H
#define CHARDEV_H

#include <linux/ioctl.h>

/* 
 * The major device number. We can't rely on dynamic 
 * registration any more, because ioctls need to know 
 * it. 
 */
#define MAJOR_NUM 100

/* 
 * Set the message of the device driver 
 */
#define IOCTL_SET_MSG _IOR(MAJOR_NUM, 0, char *)
/*
 * _IOR means that we're creating an ioctl command 
 * number for passing information from a user process
 * to the kernel module. 
 *
 * The first arguments, MAJOR_NUM, is the major device 
 * number we're using.
 *
 * The second argument is the number of the command 
 * (there could be several with different meanings).
 *
 * The third argument is the type we want to get from 
 * the process to the kernel.
 */

/* 
 * Get the message of the device driver 
 */
#define IOCTL_GET_MSG _IOR(MAJOR_NUM, 1, char *)
/* 
 * This IOCTL is used for output, to get the message 
 * of the device driver. However, we still need the 
 * buffer to place the message in to be input, 
 * as it is allocated by the process.
 */

/* 
 * Get the n'th byte of the message 
 */
#define IOCTL_GET_NTH_BYTE _IOWR(MAJOR_NUM, 2, int)
/* 
 * The IOCTL is used for both input and output. It 
 * receives from the user a number, n, and returns 
 * Message[n]. 
 */

/* 
 * The name of the device file 
 */
#define DEVICE_FILE_NAME "char_dev"

#endif

*Example 7-3. ioctl.c*

/*
 *  ioctl.c - the process to use ioctl's to control the kernel module
 *
 *  Until now we could have used cat for input and output.  But now
 *  we need to do ioctl's, which require writing our own process.
 */

/* 
 * device specifics, such as ioctl numbers and the
 * major device file. 
 */
#include "chardev.h"

#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>		/* open */
#include <unistd.h>		/* exit */
#include <sys/ioctl.h>		/* ioctl */

/* 
 * Functions for the ioctl calls 
 */

ioctl_set_msg(int file_desc, char *message)
{
	int ret_val;

	ret_val = ioctl(file_desc, IOCTL_SET_MSG, message);

	if (ret_val < 0) {
		printf("ioctl_set_msg failed:%d\n", ret_val);
		exit(-1);
	}
}

ioctl_get_msg(int file_desc)
{
	int ret_val;
	char message[100];

	/* 
	 * Warning - this is dangerous because we don't tell
	 * the kernel how far it's allowed to write, so it
	 * might overflow the buffer. In a real production
	 * program, we would have used two ioctls - one to tell
	 * the kernel the buffer length and another to give
	 * it the buffer to fill
	 */
	ret_val = ioctl(file_desc, IOCTL_GET_MSG, message);

	if (ret_val < 0) {
		printf("ioctl_get_msg failed:%d\n", ret_val);
		exit(-1);
	}

	printf("get_msg message:%s\n", message);
}

ioctl_get_nth_byte(int file_desc)
{
	int i;
	char c;

	printf("get_nth_byte message:");

	i = 0;
	do {
		c = ioctl(file_desc, IOCTL_GET_NTH_BYTE, i++);

		if (c < 0) {
			printf
			    ("ioctl_get_nth_byte failed at the %d'th byte:\n",
			     i);
			exit(-1);
		}

		putchar(c);
	} while (c != 0);
	putchar('\n');
}

/* 
 * Main - Call the ioctl functions 
 */
main()
{
	int file_desc, ret_val;
	char *msg = "Message passed by ioctl\n";

	file_desc = open(DEVICE_FILE_NAME, 0);
	if (file_desc < 0) {
		printf("Can't open device file: %s\n", DEVICE_FILE_NAME);
		exit(-1);
	}

	ioctl_get_nth_byte(file_desc);
	ioctl_get_msg(file_desc);
	ioctl_set_msg(file_desc, msg);

	close(file_desc);
}

------------------------------------------------------------------------


  Chapter 8. System Calls


  8.1. System Calls

So far, the only thing we've done was to use well defined kernel
mechanisms to register /proc files and device handlers. This is fine if
you want to do something the kernel programmers thought you'd want, such
as write a device driver. But what if you want to do something unusual,
to change the behavior of the system in some way? Then, you're mostly on
your own.

This is where kernel programming gets dangerous. While writing the
example below, I killed the open() system call. This meant I couldn't
open any files, I couldn't run any programs, and I couldn't *shutdown*
the computer. I had to pull the power switch. Luckily, no files died. To
ensure you won't lose any files either, please run *sync* right before
you do the *insmod* and the *rmmod*.

Forget about /proc files, forget about device files. They're just minor
details. The /real/ process to kernel communication mechanism, the one
used by all processes, is system calls. When a process requests a
service from the kernel (such as opening a file, forking to a new
process, or requesting more memory), this is the mechanism used. If you
want to change the behaviour of the kernel in interesting ways, this is
the place to do it. By the way, if you want to see which system calls a
program uses, run *strace <arguments>*.

In general, a process is not supposed to be able to access the kernel.
It can't access kernel memory and it can't call kernel functions. The
hardware of the CPU enforces this (that's the reason why it's called
`protected mode').

System calls are an exception to this general rule. What happens is that
the process fills the registers with the appropriate values and then
calls a special instruction which jumps to a previously defined location
in the kernel (of course, that location is readable by user processes,
it is not writable by them). Under Intel CPUs, this is done by means of
interrupt 0x80. The hardware knows that once you jump to this location,
you are no longer running in restricted user mode, but as the operating
system kernel --- and therefore you're allowed to do whatever you want.

The location in the kernel a process can jump to is called
/system_call/. The procedure at that location checks the system call
number, which tells the kernel what service the process requested. Then,
it looks at the table of system calls (sys_call_table) to see the
address of the kernel function to call. Then it calls the function, and
after it returns, does a few system checks and then return back to the
process (or to a different process, if the process time ran out). If you
want to read this code, it's at the source file
arch/$<$architecture$>$/kernel/entry.S, after the line ENTRY(system_call).

So, if we want to change the way a certain system call works, what we
need to do is to write our own function to implement it (usually by
adding a bit of our own code, and then calling the original function)
and then change the pointer at sys_call_table to point to our function.
Because we might be removed later and we don't want to leave the system
in an unstable state, it's important for cleanup_module to restore the
table to its original state.

The source code here is an example of such a kernel module. We want to
`spy' on a certain user, and to printk() a message whenever that user
opens a file. Towards this end, we replace the system call to open a
file with our own function, called our_sys_open. This function checks
the uid (user's id) of the current process, and if it's equal to the uid
we spy on, it calls printk() to display the name of the file to be
opened. Then, either way, it calls the original open() function with the
same parameters, to actually open the file.

The init_module function replaces the appropriate location in
sys_call_table and keeps the original pointer in a variable. The
cleanup_module function uses that variable to restore everything back to
normal. This approach is dangerous, because of the possibility of two
kernel modules changing the same system call. Imagine we have two kernel
modules, A and B. A's open system call will be A_open and B's will be
B_open. Now, when A is inserted into the kernel, the system call is
replaced with A_open, which will call the original sys_open when it's
done. Next, B is inserted into the kernel, which replaces the system
call with B_open, which will call what it thinks is the original system
call, A_open, when it's done.

Now, if B is removed first, everything will be well---it will simply
restore the system call to A_open, which calls the original. However, if
A is removed and then B is removed, the system will crash. A's removal
will restore the system call to the original, sys_open, cutting B out of
the loop. Then, when B is removed, it will restore the system call to
what /it/ thinks is the original, A_open, which is no longer in memory.
At first glance, it appears we could solve this particular problem by
checking if the system call is equal to our open function and if so not
changing it at all (so that B won't change the system call when it's
removed), but that will cause an even worse problem. When A is removed,
it sees that the system call was changed to B_open so that it is no
longer pointing to A_open, so it won't restore it to sys_open before it
is removed from memory. Unfortunately, B_open will still try to call
A_open which is no longer there, so that even without removing B the
system would crash.

Note that all the related problems make syscall stealing unfeasiable for
production use. In order to keep people from doing potential harmful
things sys_call_table is no longer exported. This means, if you want to
do something more than a mere dry run of this example, you will have to
patch your current kernel in order to have sys_call_table exported. In
the example directory you will find a README and the patch. As you can
imagine, such modifications are not to be taken lightly. Do not try this
on valueable systems (ie systems that you do not own - or cannot restore
easily). You'll need to get the complete sourcecode of this guide as a
tarball in order to get the patch and the README. Depending on your
kernel version, you might even need to hand apply the patch. Still here?
Well, so is this chapter. If Wyle E. Coyote was a kernel hacker, this
would be the first thing he'd try. ;)

*Example 8-1. syscall.c*

/*
 *  syscall.c
 *
 *  System call "stealing" sample.
 */

/* 
 * Copyright (C) 2001 by Peter Jay Salzman 
 */

/* 
 * The necessary header files 
 */

/*
 * Standard in kernel modules 
 */
#include <linux/kernel.h>	/* We're doing kernel work */
#include <linux/module.h>	/* Specifically, a module, */
#include <linux/moduleparam.h>	/* which will have params */
#include <linux/unistd.h>	/* The list of system calls */

/* 
 * For the current (process) structure, we need
 * this to know who the current user is. 
 */
#include <linux/sched.h>
#include <asm/uaccess.h>

/* 
 * The system call table (a table of functions). We
 * just define this as external, and the kernel will
 * fill it up for us when we are insmod'ed
 *
 * sys_call_table is no longer exported in 2.6.x kernels.
 * If you really want to try this DANGEROUS module you will
 * have to apply the supplied patch against your current kernel
 * and recompile it.
 */
extern void *sys_call_table[];

/* 
 * UID we want to spy on - will be filled from the
 * command line 
 */
static int uid;
module_param(uid, int, 0644);

/* 
 * A pointer to the original system call. The reason
 * we keep this, rather than call the original function
 * (sys_open), is because somebody else might have
 * replaced the system call before us. Note that this
 * is not 100% safe, because if another module
 * replaced sys_open before us, then when we're inserted
 * we'll call the function in that module - and it
 * might be removed before we are.
 *
 * Another reason for this is that we can't get sys_open.
 * It's a static variable, so it is not exported. 
 */
asmlinkage int (*original_call) (const char *, int, int);

/* 
 * The function we'll replace sys_open (the function
 * called when you call the open system call) with. To
 * find the exact prototype, with the number and type
 * of arguments, we find the original function first
 * (it's at fs/open.c).
 *
 * In theory, this means that we're tied to the
 * current version of the kernel. In practice, the
 * system calls almost never change (it would wreck havoc
 * and require programs to be recompiled, since the system
 * calls are the interface between the kernel and the
 * processes).
 */
asmlinkage int our_sys_open(const char *filename, int flags, int mode)
{
	int i = 0;
	char ch;

	/* 
	 * Check if this is the user we're spying on 
	 */
	if (uid == current->uid) {
		/* 
		 * Report the file, if relevant 
		 */
		printk("Opened file by %d: ", uid);
		do {
			get_user(ch, filename + i);
			i++;
			printk("%c", ch);
		} while (ch != 0);
		printk("\n");
	}

	/* 
	 * Call the original sys_open - otherwise, we lose
	 * the ability to open files 
	 */
	return original_call(filename, flags, mode);
}

/* 
 * Initialize the module - replace the system call 
 */
int init_module()
{
	/* 
	 * Warning - too late for it now, but maybe for
	 * next time... 
	 */
	printk(KERN_ALERT "I'm dangerous. I hope you did a ");
	printk(KERN_ALERT "sync before you insmod'ed me.\n");
	printk(KERN_ALERT "My counterpart, cleanup_module(), is even");
	printk(KERN_ALERT "more dangerous. If\n");
	printk(KERN_ALERT "you value your file system, it will ");
	printk(KERN_ALERT "be \"sync; rmmod\" \n");
	printk(KERN_ALERT "when you remove this module.\n");

	/* 
	 * Keep a pointer to the original function in
	 * original_call, and then replace the system call
	 * in the system call table with our_sys_open 
	 */
	original_call = sys_call_table[__NR_open];
	sys_call_table[__NR_open] = our_sys_open;

	/* 
	 * To get the address of the function for system
	 * call foo, go to sys_call_table[__NR_foo]. 
	 */

	printk(KERN_INFO "Spying on UID:%d\n", uid);

	return 0;
}

/* 
 * Cleanup - unregister the appropriate file from /proc 
 */
void cleanup_module()
{
	/* 
	 * Return the system call back to normal 
	 */
	if (sys_call_table[__NR_open] != our_sys_open) {
		printk(KERN_ALERT "Somebody else also played with the ");
		printk(KERN_ALERT "open system call\n");
		printk(KERN_ALERT "The system may be left in ");
		printk(KERN_ALERT "an unstable state.\n");
	}

	sys_call_table[__NR_open] = original_call;
}

------------------------------------------------------------------------


  Chapter 9. Blocking Processes


  9.1. Blocking Processes

What do you do when somebody asks you for something you can't do right
away? If you're a human being and you're bothered by a human being, the
only thing you can say is: "Not right now, I'm busy. /Go away!/". But if
you're a kernel module and you're bothered by a process, you have
another possibility. You can put the process to sleep until you can
service it. After all, processes are being put to sleep by the kernel
and woken up all the time (that's the way multiple processes appear to
run on the same time on a single CPU).

This kernel module is an example of this. The file (called /proc/sleep)
can only be opened by a single process at a time. If the file is already
open, the kernel module calls wait_event_interruptible[12]
<#FTN.AEN1089>. This function changes the status of the task (a task is
the kernel data structure which holds information about a process and
the system call it's in, if any) to /TASK_INTERRUPTIBLE/, which means
that the task will not run until it is woken up somehow, and adds it to
WaitQ, the queue of tasks waiting to access the file. Then, the function
calls the scheduler to context switch to a different process, one which
has some use for the CPU.

When a process is done with the file, it closes it, and module_close is
called. That function wakes up all the processes in the queue (there's
no mechanism to only wake up one of them). It then returns and the
process which just closed the file can continue to run. In time, the
scheduler decides that that process has had enough and gives control of
the CPU to another process. Eventually, one of the processes which was
in the queue will be given control of the CPU by the scheduler. It
starts at the point right after the call to
module_interruptible_sleep_on[13] <#FTN.AEN1097>. It can then proceed to
set a global variable to tell all the other processes that the file is
still open and go on with its life. When the other processes get a piece
of the CPU, they'll see that global variable and go back to sleep.

So we'll use *tail -f * to keep the file open in the background, while
trying to access it with another process (again in the background, so
that we need not switch to a different vt). As soon as the first
background process is killed with *kill %1 *, the second is woken up, is
able to access the file and finally terminates.

To make our life more interesting, module_close doesn't have a monopoly
on waking up the processes which wait to access the file. A signal, such
as *Ctrl*+*c* (/SIGINT/) can also wake up a process. [14] <#FTN.AEN1121>
In that case, we want to return with /-EINTR/ immediately. This is
important so users can, for example, kill the process before it receives
the file.

There is one more point to remember. Some times processes don't want to
sleep, they want either to get what they want immediately, or to be told
it cannot be done. Such processes use the /O_NONBLOCK/ flag when opening
the file. The kernel is supposed to respond by returning with the error
code /-EAGAIN/ from operations which would otherwise block, such as
opening the file in this example. The program *cat_noblock*, available
in the source directory for this chapter, can be used to open a file
with /O_NONBLOCK/.

hostname:~/lkmpg-examples/09-BlockingProcesses# insmod sleep.ko
hostname:~/lkmpg-examples/09-BlockingProcesses# cat_noblock /proc/sleep
Last input:
hostname:~/lkmpg-examples/09-BlockingProcesses# tail -f /proc/sleep &
Last input:
Last input:
Last input:
Last input:
Last input:
Last input:
Last input:
tail: /proc/sleep: file truncated
[1] 6540
hostname:~/lkmpg-examples/09-BlockingProcesses# cat_noblock /proc/sleep
Open would block
hostname:~/lkmpg-examples/09-BlockingProcesses# kill %1
[1]+  Terminated              tail -f /proc/sleep
hostname:~/lkmpg-examples/09-BlockingProcesses# cat_noblock /proc/sleep
Last input:
hostname:~/lkmpg-examples/09-BlockingProcesses#

*Example 9-1. sleep.c*

/*
 *  sleep.c - create a /proc file, and if several processes try to open it at
 *  the same time, put all but one to sleep
 */

#include <linux/kernel.h>	/* We're doing kernel work */
#include <linux/module.h>	/* Specifically, a module */
#include <linux/proc_fs.h>	/* Necessary because we use proc fs */
#include <linux/sched.h>	/* For putting processes to sleep and 
				   waking them up */
#include <asm/uaccess.h>	/* for get_user and put_user */

/* 
 * The module's file functions 
 */

/* 
 * Here we keep the last message received, to prove that we can process our
 * input
 */
#define MESSAGE_LENGTH 80
static char Message[MESSAGE_LENGTH];

static struct proc_dir_entry *Our_Proc_File;
#define PROC_ENTRY_FILENAME "sleep"

/* 
 * Since we use the file operations struct, we can't use the special proc
 * output provisions - we have to use a standard read function, which is this
 * function
 */
static ssize_t module_output(struct file *file,	/* see include/linux/fs.h   */
			     char *buf,	/* The buffer to put data to 
					   (in the user segment)    */
			     size_t len,	/* The length of the buffer */
			     loff_t * offset)
{
	static int finished = 0;
	int i;
	char message[MESSAGE_LENGTH + 30];

	/* 
	 * Return 0 to signify end of file - that we have nothing 
	 * more to say at this point.
	 */
	if (finished) {
		finished = 0;
		return 0;
	}

	/* 
	 * If you don't understand this by now, you're hopeless as a kernel
	 * programmer.
	 */
	sprintf(message, "Last input:%s\n", Message);
	for (i = 0; i < len && message[i]; i++)
		put_user(message[i], buf + i);

	finished = 1;
	return i;		/* Return the number of bytes "read" */
}

/* 
 * This function receives input from the user when the user writes to the /proc
 * file.
 */
static ssize_t module_input(struct file *file,	/* The file itself */
			    const char *buf,	/* The buffer with input */
			    size_t length,	/* The buffer's length */
			    loff_t * offset)
{				/* offset to file - ignore */
	int i;

	/* 
	 * Put the input into Message, where module_output will later be 
	 * able to use it
	 */
	for (i = 0; i < MESSAGE_LENGTH - 1 && i < length; i++)
		get_user(Message[i], buf + i);
	/* 
	 * we want a standard, zero terminated string 
	 */
	Message[i] = '\0';

	/* 
	 * We need to return the number of input characters used 
	 */
	return i;
}

/* 
 * 1 if the file is currently open by somebody 
 */
int Already_Open = 0;

/* 
 * Queue of processes who want our file 
 */
DECLARE_WAIT_QUEUE_HEAD(WaitQ);
/* 
 * Called when the /proc file is opened 
 */
static int module_open(struct inode *inode, struct file *file)
{
	/* 
	 * If the file's flags include O_NONBLOCK, it means the process doesn't
	 * want to wait for the file.  In this case, if the file is already 
	 * open, we should fail with -EAGAIN, meaning "you'll have to try 
	 * again", instead of blocking a process which would rather stay awake.
	 */
	if ((file->f_flags & O_NONBLOCK) && Already_Open)
		return -EAGAIN;

	/* 
	 * This is the correct place for try_module_get(THIS_MODULE) because 
	 * if a process is in the loop, which is within the kernel module,
	 * the kernel module must not be removed.
	 */
	try_module_get(THIS_MODULE);

	/* 
	 * If the file is already open, wait until it isn't 
	 */

	while (Already_Open) {
		int i, is_sig = 0;

		/* 
		 * This function puts the current process, including any system
		 * calls, such as us, to sleep.  Execution will be resumed right
		 * after the function call, either because somebody called 
		 * wake_up(&WaitQ) (only module_close does that, when the file 
		 * is closed) or when a signal, such as Ctrl-C, is sent 
		 * to the process
		 */
		wait_event_interruptible(WaitQ, !Already_Open);

		/* 
		 * If we woke up because we got a signal we're not blocking, 
		 * return -EINTR (fail the system call).  This allows processes
		 * to be killed or stopped.
		 */

/*
 * Emmanuel Papirakis:
 *
 * This is a little update to work with 2.2.*.  Signals now are contained in
 * two words (64 bits) and are stored in a structure that contains an array of
 * two unsigned longs.  We now have to make 2 checks in our if.
 *
 * Ori Pomerantz:
 *
 * Nobody promised me they'll never use more than 64 bits, or that this book
 * won't be used for a version of Linux with a word size of 16 bits.  This code
 * would work in any case.
 */
		for (i = 0; i < _NSIG_WORDS && !is_sig; i++)
			is_sig =
			    current->pending.signal.sig[i] & ~current->
			    blocked.sig[i];

		if (is_sig) {
			/* 
			 * It's important to put module_put(THIS_MODULE) here,
			 * because for processes where the open is interrupted
			 * there will never be a corresponding close. If we 
			 * don't decrement the usage count here, we will be 
			 * left with a positive usage count which we'll have no
			 * way to bring down to zero, giving us an immortal 
			 * module, which can only be killed by rebooting 
			 * the machine.
			 */
			module_put(THIS_MODULE);
			return -EINTR;
		}
	}

	/* 
	 * If we got here, Already_Open must be zero 
	 */

	/* 
	 * Open the file 
	 */
	Already_Open = 1;
	return 0;		/* Allow the access */
}

/* 
 * Called when the /proc file is closed 
 */
int module_close(struct inode *inode, struct file *file)
{
	/* 
	 * Set Already_Open to zero, so one of the processes in the WaitQ will
	 * be able to set Already_Open back to one and to open the file. All 
	 * the other processes will be called when Already_Open is back to one,
	 * so they'll go back to sleep.
	 */
	Already_Open = 0;

	/* 
	 * Wake up all the processes in WaitQ, so if anybody is waiting for the
	 * file, they can have it.
	 */
	wake_up(&WaitQ);

	module_put(THIS_MODULE);

	return 0;		/* success */
}

/*
 * This function decides whether to allow an operation (return zero) or not
 * allow it (return a non-zero which indicates why it is not allowed).
 *
 * The operation can be one of the following values:
 * 0 - Execute (run the "file" - meaningless in our case)
 * 2 - Write (input to the kernel module)
 * 4 - Read (output from the kernel module)
 *
 * This is the real function that checks file permissions. The permissions
 * returned by ls -l are for reference only, and can be overridden here.
 */
static int module_permission(struct inode *inode, int op, struct nameidata *nd)
{
	/* 
	 * We allow everybody to read from our module, but only root (uid 0)
	 * may write to it
	 */
	if (op == 4 || (op == 2 && current->euid == 0))
		return 0;

	/* 
	 * If it's anything else, access is denied 
	 */
	return -EACCES;
}

/* 
 * Structures to register as the /proc file, with pointers to all the relevant
 * functions.
 */

/* 
 * File operations for our proc file. This is where we place pointers to all
 * the functions called when somebody tries to do something to our file. NULL
 * means we don't want to deal with something.
 */
static struct file_operations File_Ops_4_Our_Proc_File = {
	.read = module_output,	/* "read" from the file */
	.write = module_input,	/* "write" to the file */
	.open = module_open,	/* called when the /proc file is opened */
	.release = module_close,	/* called when it's closed */
};

/* 
 * Inode operations for our proc file.  We need it so we'll have somewhere to
 * specify the file operations structure we want to use, and the function we
 * use for permissions. It's also possible to specify functions to be called
 * for anything else which could be done to an inode (although we don't bother,
 * we just put NULL).
 */

static struct inode_operations Inode_Ops_4_Our_Proc_File = {
	.permission = module_permission,	/* check for permissions */
};

/* 
 * Module initialization and cleanup 
 */

/* 
 * Initialize the module - register the proc file 
 */

int init_module()
{

	Our_Proc_File = create_proc_entry(PROC_ENTRY_FILENAME, 0644, NULL);
	
	if (Our_Proc_File == NULL) {
		remove_proc_entry(PROC_ENTRY_FILENAME, &proc_root);
		printk(KERN_ALERT "Error: Could not initialize /proc/test\n");
		return -ENOMEM;
	}
	
	Our_Proc_File->owner = THIS_MODULE;
	Our_Proc_File->proc_iops = &Inode_Ops_4_Our_Proc_File;
	Our_Proc_File->proc_fops = &File_Ops_4_Our_Proc_File;
	Our_Proc_File->mode = S_IFREG | S_IRUGO | S_IWUSR;
	Our_Proc_File->uid = 0;
	Our_Proc_File->gid = 0;
	Our_Proc_File->size = 80;
	
	printk(KERN_INFO "/proc/test created\n");
	
	return 0;
}

/* 
 * Cleanup - unregister our file from /proc.  This could get dangerous if
 * there are still processes waiting in WaitQ, because they are inside our
 * open function, which will get unloaded. I'll explain how to avoid removal
 * of a kernel module in such a case in chapter 10.
 */
void cleanup_module()
{
	remove_proc_entry(PROC_ENTRY_FILENAME, &proc_root);
	
	printk(KERN_INFO "/proc/test removed\n");
}

*Example 9-2. cat_noblock.c*

/* cat_noblock.c - open a file and display its contents, but exit rather than
 * wait for input */


/* Copyright (C) 1998 by Ori Pomerantz */



#include <stdio.h>    /* standard I/O */
#include <fcntl.h>    /* for open */
#include <unistd.h>   /* for read */ 
#include <stdlib.h>   /* for exit */
#include <errno.h>    /* for errno */

#define MAX_BYTES 1024*4


main(int argc, char *argv[])
{
  int    fd;  /* The file descriptor for the file to read */
  size_t bytes; /* The number of bytes read */
  char   buffer[MAX_BYTES]; /* The buffer for the bytes */  


  /* Usage */
  if (argc != 2) {
    printf("Usage: %s <filename>\n", argv[0]);
    puts("Reads the content of a file, but doesn't wait for input");
    exit(-1);
  }

  /* Open the file for reading in non blocking mode */ 
  fd = open(argv[1], O_RDONLY | O_NONBLOCK);

  /* If open failed */
  if (fd == -1) {
    if (errno = EAGAIN)
      puts("Open would block");
    else
      puts("Open failed");
    exit(-1);
  }

  /* Read the file and output its contents */
  do {
    int i;

    /* Read characters from the file */
    bytes = read(fd, buffer, MAX_BYTES);

    /* If there's an error, report it and die */
    if (bytes == -1) {
      if (errno = EAGAIN)
	puts("Normally I'd block, but you told me not to");
      else
	puts("Another read error");
      exit(-1);
    }

    /* Print the characters */
    if (bytes > 0) {
      for(i=0; i<bytes; i++)
	putchar(buffer[i]);
    }

    /* While there are no errors and the file isn't over */
  } while (bytes > 0);
}

------------------------------------------------------------------------


  Chapter 10. Replacing Printks


  10.1. Replacing printk

In Section 1.2.1.2 <#USINGX>, I said that X and kernel module
programming don't mix. That's true for developing kernel modules, but in
actual use, you want to be able to send messages to whichever tty[15]
<#FTN.AEN1169> the command to load the module came from.

The way this is done is by using current, a pointer to the currently
running task, to get the current task's tty structure. Then, we look
inside that tty structure to find a pointer to a string write function,
which we use to write a string to the tty.

*Example 10-1. print_string.c*

/* 
 *  print_string.c - Send output to the tty we're running on, regardless if it's
 *  through X11, telnet, etc.  We do this by printing the string to the tty
 *  associated with the current task.
 */
#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/init.h>
#include <linux/sched.h>	/* For current */
#include <linux/tty.h>		/* For the tty declarations */
#include <linux/version.h>	/* For LINUX_VERSION_CODE */

MODULE_LICENSE("GPL");
MODULE_AUTHOR("Peter Jay Salzman");

static void print_string(char *str)
{
	struct tty_struct *my_tty;

	/* 
	 * tty struct went into signal struct in 2.6.6 
	 */
#if ( LINUX_VERSION_CODE <= KERNEL_VERSION(2,6,5) )
	/* 
	 * The tty for the current task 
	 */
	my_tty = current->tty;
#else
	/* 
	 * The tty for the current task, for 2.6.6+ kernels 
	 */
	my_tty = current->signal->tty;
#endif

	/* 
	 * If my_tty is NULL, the current task has no tty you can print to 
	 * (ie, if it's a daemon).  If so, there's nothing we can do.
	 */
	if (my_tty != NULL) {

		/* 
		 * my_tty->driver is a struct which holds the tty's functions,
		 * one of which (write) is used to write strings to the tty. 
		 * It can be used to take a string either from the user's or 
		 * kernel's memory segment.
		 *
		 * The function's 1st parameter is the tty to write to,
		 * because the same function would normally be used for all 
		 * tty's of a certain type.  The 2nd parameter controls 
		 * whether the function receives a string from kernel
		 * memory (false, 0) or from user memory (true, non zero). 
		 * BTW: this param has been removed in Kernels > 2.6.9
		 * The (2nd) 3rd parameter is a pointer to a string.
		 * The (3rd) 4th parameter is the length of the string.
		 *
		 * As you will see below, sometimes it's necessary to use
		 * preprocessor stuff to create code that works for different
		 * kernel versions. The (naive) approach we've taken here 
		 * does not scale well. The right way to deal with this 
		 * is described in section 2 of 
		 * linux/Documentation/SubmittingPatches
		 */
		((my_tty->driver)->write) (my_tty,	/* The tty itself */
#if ( LINUX_VERSION_CODE <= KERNEL_VERSION(2,6,9) )		
					   0,	/* Don't take the string 
						   from user space        */
#endif
					   str,	/* String                 */
					   strlen(str));	/* Length */

		/* 
		 * ttys were originally hardware devices, which (usually) 
		 * strictly followed the ASCII standard.  In ASCII, to move to
		 * a new line you need two characters, a carriage return and a
		 * line feed.  On Unix, the ASCII line feed is used for both 
		 * purposes - so we can't just use \n, because it wouldn't have
		 * a carriage return and the next line will start at the
		 * column right after the line feed.
		 *
		 * This is why text files are different between Unix and 
		 * MS Windows.  In CP/M and derivatives, like MS-DOS and 
		 * MS Windows, the ASCII standard was strictly adhered to,
		 * and therefore a newline requirs both a LF and a CR.
		 */

#if ( LINUX_VERSION_CODE <= KERNEL_VERSION(2,6,9) )		
		((my_tty->driver)->write) (my_tty, 0, "\015\012", 2);
#else
		((my_tty->driver)->write) (my_tty, "\015\012", 2);
#endif
	}
}

static int __init print_string_init(void)
{
	print_string("The module has been inserted.  Hello world!");
	return 0;
}

static void __exit print_string_exit(void)
{
	print_string("The module has been removed.  Farewell world!");
}

module_init(print_string_init);
module_exit(print_string_exit);

------------------------------------------------------------------------


  10.2. Flashing keyboard LEDs

In certain conditions, you may desire a simpler and more direct way to
communicate to the external world. Flashing keyboard LEDs can be such a
solution: It is an immediate way to attract attention or to display a
status condition. Keyboard LEDs are present on every hardware, they are
always visible, they do not need any setup, and their use is rather
simple and non-intrusive, compared to writing to a tty or a file.

The following source code illustrates a minimal kernel module which,
when loaded, starts blinking the keyboard LEDs until it is unloaded.

*Example 10-2. kbleds.c*

/* 
 *  kbleds.c - Blink keyboard leds until the module is unloaded.
 */

#include <linux/module.h>
#include <linux/config.h>
#include <linux/init.h>
#include <linux/tty.h>		/* For fg_console, MAX_NR_CONSOLES */
#include <linux/kd.h>		/* For KDSETLED */
#include <linux/vt.h>
#include <linux/console_struct.h>	/* For vc_cons */

MODULE_DESCRIPTION("Example module illustrating the use of Keyboard LEDs.");
MODULE_AUTHOR("Daniele Paolo Scarpazza");
MODULE_LICENSE("GPL");

struct timer_list my_timer;
struct tty_driver *my_driver;
char kbledstatus = 0;

#define BLINK_DELAY   HZ/5
#define ALL_LEDS_ON   0x07
#define RESTORE_LEDS  0xFF

/*
 * Function my_timer_func blinks the keyboard LEDs periodically by invoking
 * command KDSETLED of ioctl() on the keyboard driver. To learn more on virtual 
 * terminal ioctl operations, please see file:
 *     /usr/src/linux/drivers/char/vt_ioctl.c, function vt_ioctl().
 *
 * The argument to KDSETLED is alternatively set to 7 (thus causing the led 
 * mode to be set to LED_SHOW_IOCTL, and all the leds are lit) and to 0xFF
 * (any value above 7 switches back the led mode to LED_SHOW_FLAGS, thus
 * the LEDs reflect the actual keyboard status).  To learn more on this, 
 * please see file:
 *     /usr/src/linux/drivers/char/keyboard.c, function setledstate().
 * 
 */

static void my_timer_func(unsigned long ptr)
{
	int *pstatus = (int *)ptr;

	if (*pstatus == ALL_LEDS_ON)
		*pstatus = RESTORE_LEDS;
	else
		*pstatus = ALL_LEDS_ON;

	(my_driver->ioctl) (vc_cons[fg_console].d->vc_tty, NULL, KDSETLED,
			    *pstatus);

	my_timer.expires = jiffies + BLINK_DELAY;
	add_timer(&my_timer);
}

static int __init kbleds_init(void)
{
	int i;

	printk(KERN_INFO "kbleds: loading\n");
	printk(KERN_INFO "kbleds: fgconsole is %x\n", fg_console);
	for (i = 0; i < MAX_NR_CONSOLES; i++) {
		if (!vc_cons[i].d)
			break;
		printk(KERN_INFO "poet_atkm: console[%i/%i] #%i, tty %lx\n", i,
		       MAX_NR_CONSOLES, vc_cons[i].d->vc_num,
		       (unsigned long)vc_cons[i].d->vc_tty);
	}
	printk(KERN_INFO "kbleds: finished scanning consoles\n");

	my_driver = vc_cons[fg_console].d->vc_tty->driver;
	printk(KERN_INFO "kbleds: tty driver magic %x\n", my_driver->magic);

	/*
	 * Set up the LED blink timer the first time
	 */
	init_timer(&my_timer);
	my_timer.function = my_timer_func;
	my_timer.data = (unsigned long)&kbledstatus;
	my_timer.expires = jiffies + BLINK_DELAY;
	add_timer(&my_timer);

	return 0;
}

static void __exit kbleds_cleanup(void)
{
	printk(KERN_INFO "kbleds: unloading...\n");
	del_timer(&my_timer);
	(my_driver->ioctl) (vc_cons[fg_console].d->vc_tty, NULL, KDSETLED,
			    RESTORE_LEDS);
}

module_init(kbleds_init);
module_exit(kbleds_cleanup);

If none of the examples in this chapter fit your debugging needs there
might yet be some other tricks to try. Ever wondered what
CONFIG_LL_DEBUG in *make menuconfig * is good for? If you activate that
you get low level access to the serial port. While this might not sound
very powerful by itself, you can patch kernel/printk.c or any other
essential syscall to use printascii, thus makeing it possible to trace
virtually everything what your code does over a serial line. If you find
yourself porting the kernel to some new and former unsupported
architecture this is usually amongst the first things that should be
implemented. Logging over a netconsole might also be worth a try.

While you have seen lots of stuff that can be used to aid debugging
here, there are some things to be aware of. Debugging is almost always
intrusive. Adding debug code can change the situation enough to make the
bug seem to dissappear. Thus you should try to keep debug code to a
minimum and make sure it does not show up in production code.

------------------------------------------------------------------------


  Chapter 11. Scheduling Tasks


  11.1. Scheduling Tasks

Very often, we have "housekeeping" tasks which have to be done at a
certain time, or every so often. If the task is to be done by a process,
we do it by putting it in the crontab file. If the task is to be done by
a kernel module, we have two possibilities. The first is to put a
process in the crontab file which will wake up the module by a system
call when necessary, for example by opening a file. This is terribly
inefficient, however -- we run a new process off of crontab, read a new
executable to memory, and all this just to wake up a kernel module which
is in memory anyway.

Instead of doing that, we can create a function that will be called once
for every timer interrupt. The way we do this is we create a task, held
in a workqueue_struct structure, which will hold a pointer to the
function. Then, we use queue_delayed_work to put that task on a task
list called my_workqueue, which is the list of tasks to be executed on
the next timer interrupt. Because we want the function to keep on being
executed, we need to put it back on my_workqueue whenever it is called,
for the next timer interrupt.

There's one more point we need to remember here. When a module is
removed by *rmmod*, first its reference count is checked. If it is zero,
module_cleanup is called. Then, the module is removed from memory with
all its functions. Things need to be shut down properly, or bad things
will happen. See the code below how this can be done in a safe way.

*Example 11-1. sched.c*

/*
 *  sched.c - scheduale a function to be called on every timer interrupt.
 *
 *  Copyright (C) 2001 by Peter Jay Salzman
 */

/* 
 * The necessary header files 
 */

/* 
 * Standard in kernel modules 
 */
#include <linux/kernel.h>	/* We're doing kernel work */
#include <linux/module.h>	/* Specifically, a module */
#include <linux/proc_fs.h>	/* Necessary because we use the proc fs */
#include <linux/workqueue.h>	/* We scheduale tasks here */
#include <linux/sched.h>	/* We need to put ourselves to sleep 
				   and wake up later */
#include <linux/init.h>		/* For __init and __exit */
#include <linux/interrupt.h>	/* For irqreturn_t */

struct proc_dir_entry *Our_Proc_File;
#define PROC_ENTRY_FILENAME "sched"
#define MY_WORK_QUEUE_NAME "WQsched.c"

/* 
 * The number of times the timer interrupt has been called so far 
 */
static int TimerIntrpt = 0;

static void intrpt_routine(void *);

static int die = 0;		/* set this to 1 for shutdown */

/* 
 * The work queue structure for this task, from workqueue.h 
 */
static struct workqueue_struct *my_workqueue;

static struct work_struct Task;
static DECLARE_WORK(Task, intrpt_routine, NULL);

/* 
 * This function will be called on every timer interrupt. Notice the void*
 * pointer - task functions can be used for more than one purpose, each time
 * getting a different parameter.
 */
static void intrpt_routine(void *irrelevant)
{
	/* 
	 * Increment the counter 
	 */
	TimerIntrpt++;

	/* 
	 * If cleanup wants us to die
	 */
	if (die == 0)
		queue_delayed_work(my_workqueue, &Task, 100);
}

/* 
 * Put data into the proc fs file. 
 */
ssize_t
procfile_read(char *buffer,
	      char **buffer_location,
	      off_t offset, int buffer_length, int *eof, void *data)
{
	int len;		/* The number of bytes actually used */

	/* 
	 * It's static so it will still be in memory 
	 * when we leave this function
	 */
	static char my_buffer[80];

	/* 
	 * We give all of our information in one go, so if anybody asks us
	 * if we have more information the answer should always be no.
	 */
	if (offset > 0)
		return 0;

	/* 
	 * Fill the buffer and get its length 
	 */
	len = sprintf(my_buffer, "Timer called %d times so far\n", TimerIntrpt);

	/* 
	 * Tell the function which called us where the buffer is 
	 */
	*buffer_location = my_buffer;

	/* 
	 * Return the length 
	 */
	return len;
}

/* 
 * Initialize the module - register the proc file 
 */
int __init init_module()
{
	/*
	 * Create our /proc file
	 */
	Our_Proc_File = create_proc_entry(PROC_ENTRY_FILENAME, 0644, NULL);
	
	if (Our_Proc_File == NULL) {
		remove_proc_entry(PROC_ENTRY_FILENAME, &proc_root);
		printk(KERN_ALERT "Error: Could not initialize /proc/%s\n",
		       PROC_ENTRY_FILENAME);
		return -ENOMEM;
	}

	Our_Proc_File->read_proc = procfile_read;
	Our_Proc_File->owner = THIS_MODULE;
	Our_Proc_File->mode = S_IFREG | S_IRUGO;
	Our_Proc_File->uid = 0;
	Our_Proc_File->gid = 0;
	Our_Proc_File->size = 80;

	/* 
	 * Put the task in the work_timer task queue, so it will be executed at
	 * next timer interrupt
	 */
	my_workqueue = create_workqueue(MY_WORK_QUEUE_NAME);
	queue_delayed_work(my_workqueue, &Task, 100);
	
	
	printk(KERN_INFO "/proc/%s created\n", PROC_ENTRY_FILENAME);
	
	return 0;
}

/* 
 * Cleanup
 */
void __exit cleanup_module()
{
	/* 
	 * Unregister our /proc file 
	 */
	remove_proc_entry(PROC_ENTRY_FILENAME, &proc_root);
	printk(KERN_INFO "/proc/%s removed\n", PROC_ENTRY_FILENAME);

	die = 1;		/* keep intrp_routine from queueing itself */
	cancel_delayed_work(&Task);	/* no "new ones" */
	flush_workqueue(my_workqueue);	/* wait till all "old ones" finished */
	destroy_workqueue(my_workqueue);

	/* 
	 * Sleep until intrpt_routine is called one last time. This is 
	 * necessary, because otherwise we'll deallocate the memory holding 
	 * intrpt_routine and Task while work_timer still references them.
	 * Notice that here we don't allow signals to interrupt us.
	 *
	 * Since WaitQ is now not NULL, this automatically tells the interrupt
	 * routine it's time to die.
	 */

}

/* 
 * some work_queue related functions
 * are just available to GPL licensed Modules 
 */
MODULE_LICENSE("GPL");

------------------------------------------------------------------------


  Chapter 12. Interrupt Handlers


  12.1. Interrupt Handlers

------------------------------------------------------------------------


    12.1.1. Interrupt Handlers

Except for the last chapter, everything we did in the kernel so far
we've done as a response to a process asking for it, either by dealing
with a special file, sending an ioctl(), or issuing a system call. But
the job of the kernel isn't just to respond to process requests. Another
job, which is every bit as important, is to speak to the hardware
connected to the machine.

There are two types of interaction between the CPU and the rest of the
computer's hardware. The first type is when the CPU gives orders to the
hardware, the other is when the hardware needs to tell the CPU
something. The second, called interrupts, is much harder to implement
because it has to be dealt with when convenient for the hardware, not
the CPU. Hardware devices typically have a very small amount of RAM, and
if you don't read their information when available, it is lost.

Under Linux, hardware interrupts are called IRQ's (/I/nterrupt/R/e
/q/uests)[16] <#FTN.AEN1272>. There are two types of IRQ's, short and
long. A short IRQ is one which is expected to take a /very/ short period
of time, during which the rest of the machine will be blocked and no
other interrupts will be handled. A long IRQ is one which can take
longer, and during which other interrupts may occur (but not interrupts
from the same device). If at all possible, it's better to declare an
interrupt handler to be long.

When the CPU receives an interrupt, it stops whatever it's doing (unless
it's processing a more important interrupt, in which case it will deal
with this one only when the more important one is done), saves certain
parameters on the stack and calls the interrupt handler. This means that
certain things are not allowed in the interrupt handler itself, because
the system is in an unknown state. The solution to this problem is for
the interrupt handler to do what needs to be done immediately, usually
read something from the hardware or send something to the hardware, and
then schedule the handling of the new information at a later time (this
is called the "bottom half") and return. The kernel is then guaranteed
to call the bottom half as soon as possible -- and when it does,
everything allowed in kernel modules will be allowed.

The way to implement this is to call request_irq() to get your interrupt
handler called when the relevant IRQ is received. [17]
<#FTN.AEN1289>This function receives the IRQ number, the name of the
function, flags, a name for /proc/interrupts and a parameter to pass to
the interrupt handler. Usually there is a certain number of IRQs
available. How many IRQs there are is hardware-dependent. The flags can
include /SA_SHIRQ/ to indicate you're willing to share the IRQ with
other interrupt handlers (usually because a number of hardware devices
sit on the same IRQ) and /SA_INTERRUPT / to indicate this is a fast
interrupt. This function will only succeed if there isn't already a
handler on this IRQ, or if you're both willing to share.

Then, from within the interrupt handler, we communicate with the
hardware and then use queue_work() mark_bh(BH_IMMEDIATE) to schedule the
bottom half.

------------------------------------------------------------------------


    12.1.2. Keyboards on the Intel Architecture

The rest of this chapter is completely Intel specific. If you're not
running on an Intel platform, it will not work. Don't even try to
compile the code here.

I had a problem with writing the sample code for this chapter. On one
hand, for an example to be useful it has to run on everybody's computer
with meaningful results. On the other hand, the kernel already includes
device drivers for all of the common devices, and those device drivers
won't coexist with what I'm going to write. The solution I've found was
to write something for the keyboard interrupt, and disable the regular
keyboard interrupt handler first. Since it is defined as a static symbol
in the kernel source files (specifically, drivers/char/keyboard.c),
there is no way to restore it. Before *insmod*'ing this code, do on
another terminal *sleep 120; reboot* if you value your file system.

This code binds itself to IRQ 1, which is the IRQ of the keyboard
controlled under Intel architectures. Then, when it receives a keyboard
interrupt, it reads the keyboard's status (that's the purpose of the
*inb(0x64)*) and the scan code, which is the value returned by the
keyboard. Then, as soon as the kernel thinks it's feasible, it runs
got_char which gives the code of the key used (the first seven bits of
the scan code) and whether it has been pressed (if the 8th bit is zero)
or released (if it's one).

*Example 12-1. intrpt.c*

/*
 *  intrpt.c - An interrupt handler.
 *
 *  Copyright (C) 2001 by Peter Jay Salzman
 */

/* 
 * The necessary header files 
 */

/* 
 * Standard in kernel modules 
 */
#include <linux/kernel.h>	/* We're doing kernel work */
#include <linux/module.h>	/* Specifically, a module */
#include <linux/sched.h>
#include <linux/workqueue.h>
#include <linux/interrupt.h>	/* We want an interrupt */
#include <asm/io.h>

#define MY_WORK_QUEUE_NAME "WQsched.c"

static struct workqueue_struct *my_workqueue;

/* 
 * This will get called by the kernel as soon as it's safe
 * to do everything normally allowed by kernel modules.
 */
static void got_char(void *scancode)
{
	printk(KERN_INFO "Scan Code %x %s.\n",
	       (int)*((char *)scancode) & 0x7F,
	       *((char *)scancode) & 0x80 ? "Released" : "Pressed");
}

/* 
 * This function services keyboard interrupts. It reads the relevant
 * information from the keyboard and then puts the non time critical
 * part into the work queue. This will be run when the kernel considers it safe.
 */
irqreturn_t irq_handler(int irq, void *dev_id, struct pt_regs *regs)
{
	/* 
	 * This variables are static because they need to be
	 * accessible (through pointers) to the bottom half routine.
	 */
	static int initialised = 0;
	static unsigned char scancode;
	static struct work_struct task;
	unsigned char status;

	/* 
	 * Read keyboard status
	 */
	status = inb(0x64);
	scancode = inb(0x60);

	if (initialised == 0) {
		INIT_WORK(&task, got_char, &scancode);
		initialised = 1;
	} else {
		PREPARE_WORK(&task, got_char, &scancode);
	}

	queue_work(my_workqueue, &task);

	return IRQ_HANDLED;
}

/* 
 * Initialize the module - register the IRQ handler 
 */
int init_module()
{
	my_workqueue = create_workqueue(MY_WORK_QUEUE_NAME);

	/* 
	 * Since the keyboard handler won't co-exist with another handler,
	 * such as us, we have to disable it (free its IRQ) before we do
	 * anything.  Since we don't know where it is, there's no way to
	 * reinstate it later - so the computer will have to be rebooted
	 * when we're done.
	 */
	free_irq(1, NULL);

	/* 
	 * Request IRQ 1, the keyboard IRQ, to go to our irq_handler.
	 * SA_SHIRQ means we're willing to have othe handlers on this IRQ.
	 * SA_INTERRUPT can be used to make the handler into a fast interrupt.
	 */
	return request_irq(1,	/* The number of the keyboard IRQ on PCs */
			   irq_handler,	/* our handler */
			   SA_SHIRQ, "test_keyboard_irq_handler",
			   (void *)(irq_handler));
}

/* 
 * Cleanup 
 */
void cleanup_module()
{
	/* 
	 * This is only here for completeness. It's totally irrelevant, since
	 * we don't have a way to restore the normal keyboard interrupt so the
	 * computer is completely useless and has to be rebooted.
	 */
	free_irq(1, NULL);
}

/* 
 * some work_queue related functions are just available to GPL licensed Modules
 */
MODULE_LICENSE("GPL");
                

------------------------------------------------------------------------


  Chapter 13. Symmetric Multi Processing


  13.1. Symmetrical Multi-Processing

One of the easiest and cheapest ways to improve hardware performance is
to put more than one CPU on the board. This can be done either making
the different CPU's take on different jobs (asymmetrical
multi-processing) or by making them all run in parallel, doing the same
job (symmetrical multi-processing, a.k.a. SMP). Doing asymmetrical
multi-processing effectively requires specialized knowledge about the
tasks the computer should do, which is unavailable in a general purpose
operating system such as Linux. On the other hand, symmetrical
multi-processing is relatively easy to implement.

By relatively easy, I mean exactly that: not that it's /really/ easy. In
a symmetrical multi-processing environment, the CPU's share the same
memory, and as a result code running in one CPU can affect the memory
used by another. You can no longer be certain that a variable you've set
to a certain value in the previous line still has that value; the other
CPU might have played with it while you weren't looking. Obviously, it's
impossible to program like this.

In the case of process programming this normally isn't an issue, because
a process will normally only run on one CPU at a time[18]
<#FTN.AEN1344>. The kernel, on the other hand, could be called by
different processes running on different CPU's.

In version 2.0.x, this isn't a problem because the entire kernel is in
one big spinlock. This means that if one CPU is in the kernel and
another CPU wants to get in, for example because of a system call, it
has to wait until the first CPU is done. This makes Linux SMP safe[19]
<#FTN.AEN1347>, but inefficient.

In version 2.2.x, several CPU's can be in the kernel at the same time.
This is something module writers need to be aware of.

------------------------------------------------------------------------


  Chapter 14. Common Pitfalls


  14.1. Common Pitfalls

Before I send you on your way to go out into the world and write kernel
modules, there are a few things I need to warn you about. If I fail to
warn you and something bad happens, please report the problem to me for
a full refund of the amount I was paid for your copy of the book.

Using standard libraries

    You can't do that. In a kernel module you can only use kernel
    functions, which are the functions you can see in /proc/kallsyms.

Disabling interrupts

    You might need to do this for a short time and that is OK, but if
    you don't enable them afterwards, your system will be stuck and
    you'll have to power it off.

Sticking your head inside a large carnivore

    I probably don't have to warn you about this, but I figured I will
    anyway, just in case.

------------------------------------------------------------------------


  Appendix A. Changes: 2.0 To 2.2


  A.1. Changes between 2.4 and 2.6

------------------------------------------------------------------------


    A.1.1. Changes between 2.4 and 2.6

I don't know the entire kernel well enough to document all of the
changes. Some hints for porting can be found by comparing this version
of the LKMPG with it's counterpart for kernel 2.4. Apart from that,
anybody who needs to port drivers from 2.4 to 2.6 kernels might want to
visit http://lwn.net/Articles/driver-porting/
<http://lwn.net/Articles/driver-porting/>. If you still can't find an
example that exactly meets your needs there, find a driver that's
similar to your driver and present in both kernel versions. File
comparison tools like *xxdiff * or *meld * can be a great help then.
Also check if your driver is covered by docs in linux/Documentation/ .
Before starting with porting and in case you're stuck it's a good idea
to find an appropiate mailinglist and ask people there for pointers.

------------------------------------------------------------------------


  Appendix B. Where To Go From Here


  B.1. Where From Here?

I could easily have squeezed a few more chapters into this book. I could
have added a chapter about creating new file systems, or about adding
new protocol stacks (as if there's a need for that -- you'd have to dig
underground to find a protocol stack not supported by Linux). I could
have added explanations of the kernel mechanisms we haven't touched
upon, such as bootstrapping or the disk interface.

However, I chose not to. My purpose in writing this book was to provide
initiation into the mysteries of kernel module programming and to teach
the common techniques for that purpose. For people seriously interested
in kernel programming, I recommend Juan-Mariano de Goyeneche's list of
kernel resources
<http://jungla.dit.upm.es/~jmseyas/linux/kernel/hackers-docs.html>.
Also, as Linus said, the best way to learn the kernel is to read the
source code yourself.

If you're interested in more examples of short kernel modules, I
recommend Phrack magazine. Even if you're not interested in security,
and as a programmer you should be, the kernel modules there are good
examples of what you can do inside the kernel, and they're short enough
not to require too much effort to understand.

I hope I have helped you in your quest to become a better programmer, or
at least to have fun through technology. And, if you do write useful
kernel modules, I hope you publish them under the GPL, so I can use them
too.

If you'd like to contribute to this guide, please contact one the
maintainers for details. As you've already seen, there's a placeholder
chapter now, waiting to be filled with examples for sysfs.

------------------------------------------------------------------------


  Index


    Symbols

/etc/conf.modules, How Do Modules Get Into The Kernel? <x44.htm>
/etc/modules.conf, How Do Modules Get Into The Kernel? <x44.htm>
/proc filesystem, The /proc File System <c708.htm#AEN710>
/proc/interrupts, Interrupt Handlers <c1254.htm#AEN1263>
/proc/kallsyms, Functions available to modules <c425.htm#AEN441>, Name
Space <c425.htm#AEN483>, Common Pitfalls <c1350.htm#AEN1352>
/proc/meminfo, The /proc File System <c708.htm#AEN710>
/proc/modules, How Do Modules Get Into The Kernel? <x44.htm>, The /proc
File System <c708.htm#AEN710>
2.6 changes, Changes between 2.4 and 2.6 <a1387.htm#AEN1389>
_IO, Talking to Device Files (writes and IOCTLs) <c890.htm#AEN892>
_IOR, Talking to Device Files (writes and IOCTLs) <c890.htm#AEN892>
_IOW, Talking to Device Files (writes and IOCTLs) <c890.htm#AEN892>
_IOWR, Talking to Device Files (writes and IOCTLs) <c890.htm#AEN892>
__exit, Hello World (part 3): The __init and __exit Macros <x245.htm>
__init, Hello World (part 3): The __init and __exit Macros <x245.htm>
__initdata, Hello World (part 3): The __init and __exit Macros <x245.htm>
__initfunction(), Hello World (part 3): The __init and __exit Macros
<x245.htm>

------------------------------------------------------------------------


    B

blocking processes, Blocking Processes <c1050.htm#AEN1052>
blocking, how to avoid, Blocking Processes <c1050.htm#AEN1052>
bottom half, Interrupt Handlers <c1254.htm#AEN1263>
busy, Blocking Processes <c1050.htm#AEN1052>

------------------------------------------------------------------------


    C

carnivore

    large, Common Pitfalls <c1350.htm#AEN1352>

cleanup_module(), Hello, World (part 1): The Simplest Module
<c119.htm#AEN121>
code space, Code space <c425.htm#AEN500>
coffee, Major and Minor Numbers <c425.htm#AEN530>
copy_from_user, Read and Write a /proc File <x769.htm>
copy_to_user, Read and Write a /proc File <x769.htm>
CPU

    multiple, Symmetrical Multi-Processing <c1324.htm#AEN1326>

crontab, Scheduling Tasks <c1209.htm#AEN1211>
ctrl-c, Blocking Processes <c1050.htm#AEN1052>
current task, Replacing printk <c1159.htm#AEN1161>

------------------------------------------------------------------------


    D

DEFAULT_MESSAGE_LOGLEVEL, Introducing printk() <c119.htm#INTRODUCINGPRINTK>
defining ioctls, Talking to Device Files (writes and IOCTLs)
<c890.htm#AEN892>
device file

    character, Character Device Drivers <c567.htm#AEN569>

device files

    input to, Talking to Device Files (writes and IOCTLs) <c890.htm#AEN892>

    write to, Talking to Device Files (writes and IOCTLs) <c890.htm#AEN892>

------------------------------------------------------------------------


    E

EAGAIN, Blocking Processes <c1050.htm#AEN1052>
EINTR, Blocking Processes <c1050.htm#AEN1052>
ENTRY(system call), System Calls <c976.htm#AEN978>
entry.S, System Calls <c976.htm#AEN978>

------------------------------------------------------------------------


    F

file, The file structure <c567.htm#AEN599>
filesystem

    /proc, The /proc File System <c708.htm#AEN710>

    registration, Manage /proc file with standard filesystem <x810.htm>

filesystem registration, Manage /proc file with standard filesystem
<x810.htm>
file_operations, The file_operations Structure <c567.htm#AEN574>
file_operations structure, Manage /proc file with standard filesystem
<x810.htm>

------------------------------------------------------------------------


    G

get_user, Read and Write a /proc File <x769.htm>

------------------------------------------------------------------------


    H

handlers

    interrupt, Interrupt Handlers <c1254.htm#AEN1256>

housekeeping, Scheduling Tasks <c1209.htm#AEN1211>
Hurd, Code space <c425.htm#AEN500>

------------------------------------------------------------------------


    I

inb, Keyboards on the Intel Architecture <c1254.htm#KEYBOARD>
init_module(), Hello, World (part 1): The Simplest Module <c119.htm#AEN121>
inode, The file structure <c567.htm#AEN599>, The /proc File System
<c708.htm#AEN710>
inode_operations structure, Manage /proc file with standard filesystem
<x810.htm>
insmod, Compiling Kernel Modules <x181.htm>, System Calls <c976.htm#AEN978>
Intel architecture

    keyboard, Keyboards on the Intel Architecture <c1254.htm#KEYBOARD>

interrupt 0x80, System Calls <c976.htm#AEN978>
interrupt handlers, Interrupt Handlers <c1254.htm#AEN1256>
interruptible_sleep_on, Blocking Processes <c1050.htm#AEN1052>
interrupts

    disabling, Common Pitfalls <c1350.htm#AEN1352>

ioctl, Talking to Device Files (writes and IOCTLs) <c890.htm#AEN892>

    defining, Talking to Device Files (writes and IOCTLs) <c890.htm#AEN892>

    official assignment, Talking to Device Files (writes and IOCTLs)
    <c890.htm#AEN892>

------------------------------------------------------------------------


    K

kernel

    versions, Changes between 2.4 and 2.6 <a1387.htm#AEN1389>

kernel versions, Writing Modules for Multiple Kernel Versions
<c567.htm#AEN691>
kerneld, How Do Modules Get Into The Kernel? <x44.htm>
KERNEL_VERSION, Writing Modules for Multiple Kernel Versions
<c567.htm#AEN691>
keyboard, Keyboards on the Intel Architecture <c1254.htm#KEYBOARD>
keyboard LEDs

    flashing, Flashing keyboard LEDs <x1194.htm>

kmod, How Do Modules Get Into The Kernel? <x44.htm>

------------------------------------------------------------------------


    L

libraries

    standard, Common Pitfalls <c1350.htm#AEN1352>

library function, Functions available to modules <c425.htm#AEN441>
LINUX_VERSION_CODE, Writing Modules for Multiple Kernel Versions
<c567.htm#AEN691>

------------------------------------------------------------------------


    M

major number, Major and Minor Numbers <c425.htm#AEN530>

    dynamic allocation, Registering A Device <c567.htm#AEN621>

memory segments, Read and Write a /proc File <x769.htm>
microkernel, Code space <c425.htm#AEN500>
minor number, Major and Minor Numbers <c425.htm#AEN530>
mknod, Major and Minor Numbers <c425.htm#AEN530>
modem, Talking to Device Files (writes and IOCTLs) <c890.htm#AEN892>
MODULE_AUTHOR(), Hello World (part 4): Licensing and Module
Documentation <x279.htm>
module_cleanup, Scheduling Tasks <c1209.htm#AEN1211>
MODULE_DESCRIPTION(), Hello World (part 4): Licensing and Module
Documentation <x279.htm>
module_exit, Hello World (part 2) <x217.htm>
module_init, Hello World (part 2) <x217.htm>
module_interruptible_sleep_on, Blocking Processes <c1050.htm#AEN1052>
MODULE_LICENSE(), Hello World (part 4): Licensing and Module
Documentation <x279.htm>
module_permissions, Manage /proc file with standard filesystem <x810.htm>
module_sleep_on, Blocking Processes <c1050.htm#AEN1052>
MODULE_SUPPORTED_DEVICE(), Hello World (part 4): Licensing and Module
Documentation <x279.htm>
module_wake_up, Blocking Processes <c1050.htm#AEN1052>
MOD_DEC_USE_COUNT, Unregistering A Device <c567.htm#AEN651>
MOD_INC_USE_COUNT, Unregistering A Device <c567.htm#AEN651>
MOD_IN_USE, Unregistering A Device <c567.htm#AEN651>
monolithic kernel, Code space <c425.htm#AEN500>
multi-processing, Symmetrical Multi-Processing <c1324.htm#AEN1326>
multi-tasking, Blocking Processes <c1050.htm#AEN1052>
multitasking, Blocking Processes <c1050.htm#AEN1052>

------------------------------------------------------------------------


    N

namespace pollution, Name Space <c425.htm#AEN483>
Neutrino, Code space <c425.htm#AEN500>
non-blocking, Blocking Processes <c1050.htm#AEN1052>

------------------------------------------------------------------------


    O

official ioctl assignment, Talking to Device Files (writes and IOCTLs)
<c890.htm#AEN892>
O_NONBLOCK, Blocking Processes <c1050.htm#AEN1052>

------------------------------------------------------------------------


    P

permission, Manage /proc file with standard filesystem <x810.htm>
pointer

    current, Manage /proc file with standard filesystem <x810.htm>

printk

    replacing, Replacing printk <c1159.htm#AEN1161>

printk(), Introducing printk() <c119.htm#INTRODUCINGPRINTK>
proc file

    kallsyms, Common Pitfalls <c1350.htm#AEN1352>

processes

    blocking, Blocking Processes <c1050.htm#AEN1052>

    killing, Blocking Processes <c1050.htm#AEN1052>

    waking up, Blocking Processes <c1050.htm#AEN1052>

processing

    multi, Symmetrical Multi-Processing <c1324.htm#AEN1326>

proc_register, The /proc File System <c708.htm#AEN710>
proc_register_dynamic, The /proc File System <c708.htm#AEN710>
putting processes to sleep, Blocking Processes <c1050.htm#AEN1052>
put_user, Read and Write a /proc File <x769.htm>

------------------------------------------------------------------------


    Q

queue_delayed_work, Scheduling Tasks <c1209.htm#AEN1211>
queue_work, Interrupt Handlers <c1254.htm#AEN1263>

------------------------------------------------------------------------


    R

read

    in the kernel, Read and Write a /proc File <x769.htm>

reference count, Scheduling Tasks <c1209.htm#AEN1211>
refund policy, Common Pitfalls <c1350.htm#AEN1352>
register_chrdev, Registering A Device <c567.htm#AEN621>
request_irq(), Interrupt Handlers <c1254.htm#AEN1263>
rmmod, System Calls <c976.htm#AEN978>, Scheduling Tasks <c1209.htm#AEN1211>

    preventing, Unregistering A Device <c567.htm#AEN651>

------------------------------------------------------------------------


    S

SA_INTERRUPT, Interrupt Handlers <c1254.htm#AEN1263>
SA_SHIRQ, Interrupt Handlers <c1254.htm#AEN1263>
scheduler, Blocking Processes <c1050.htm#AEN1052>
scheduling tasks, Scheduling Tasks <c1209.htm#AEN1211>
segment

    memory, Read and Write a /proc File <x769.htm>

seq_file, Manage /proc file with seq_file <x861.htm>
serial port, Talking to Device Files (writes and IOCTLs) <c890.htm#AEN892>
shutdown, System Calls <c976.htm#AEN978>
SIGINT, Blocking Processes <c1050.htm#AEN1052>
signal, Blocking Processes <c1050.htm#AEN1052>
sleep

    putting processes to, Blocking Processes <c1050.htm#AEN1052>

sleep_on, Blocking Processes <c1050.htm#AEN1052>
SMP, Symmetrical Multi-Processing <c1324.htm#AEN1326>
source file

    chardev.c, Talking to Device Files (writes and IOCTLs)
    <c890.htm#AEN892>

    chardev.h, Talking to Device Files (writes and IOCTLs)
    <c890.htm#AEN892>

    hello-1.c, Hello, World (part 1): The Simplest Module <c119.htm#AEN121>

    hello-2.c, Hello World (part 2) <x217.htm>

    hello-3.c, Hello World (part 3): The __init and __exit Macros
    <x245.htm>

    hello-4.c, Hello World (part 4): Licensing and Module Documentation
    <x279.htm>

    hello-5.c, Passing Command Line Arguments to a Module <x323.htm>

    intrpt.c, Keyboards on the Intel Architecture <c1254.htm#KEYBOARD>

    ioctl.c, Talking to Device Files (writes and IOCTLs) <c890.htm#AEN892>

    print_string.c, Replacing printk <c1159.htm#AEN1161>

    sched.c, Scheduling Tasks <c1209.htm#AEN1211>

    sleep.c, Blocking Processes <c1050.htm#AEN1052>

    start.c, Modules Spanning Multiple Files <x351.htm>

    stop.c, Modules Spanning Multiple Files <x351.htm>

    syscall.c, System Calls <c976.htm#AEN978>

source files

    multiple, Modules Spanning Multiple Files <x351.htm>, Building
    modules for a precompiled kernel <x380.htm>

standard libraries, Common Pitfalls <c1350.htm#AEN1352>
strace, Functions available to modules <c425.htm#AEN441>, System Calls
<c976.htm#AEN978>
struct

    tty, Replacing printk <c1159.htm#AEN1161>

struct file_operations, Manage /proc file with standard filesystem
<x810.htm>
struct inode_operations, Manage /proc file with standard filesystem
<x810.htm>
symbol table, Name Space <c425.htm#AEN483>
symmetrical multi-processing, Symmetrical Multi-Processing
<c1324.htm#AEN1326>
sync, System Calls <c976.htm#AEN978>
system call, Functions available to modules <c425.htm#AEN441>, System
Calls <c976.htm#AEN978>

    open, System Calls <c976.htm#AEN978>

system calls, System Calls <c976.htm#AEN978>
sys_call_table, System Calls <c976.htm#AEN978>
sys_open, System Calls <c976.htm#AEN978>

------------------------------------------------------------------------


    T

task, Scheduling Tasks <c1209.htm#AEN1211>

    current, Replacing printk <c1159.htm#AEN1161>

tasks

    scheduling, Scheduling Tasks <c1209.htm#AEN1211>

TASK_INTERRUPTIBLE, Blocking Processes <c1050.htm#AEN1052>
try_module_get, System Calls <c976.htm#AEN978>
tty_structure, Replacing printk <c1159.htm#AEN1161>

------------------------------------------------------------------------


    W

waking up processes, Blocking Processes <c1050.htm#AEN1052>
workqueue_struct, Scheduling Tasks <c1209.htm#AEN1211>
write

    in the kernel, Read and Write a /proc File <x769.htm>


      Notes

[1] <#AEN62>	

In earlier versions of linux, this was known as kerneld.

[2] <#AEN74>	

If such a file exists. Note that the acual behavoir might be
distribution-dependent. If you're interested in the details,read the man
pages that came with module-init-tools, and see for yourself what's
really going on. You could use something like *strace modprobe dummy *
to find out how dummy.ko gets loaded. FYI: The dummy.ko I'm talking
about here is part of the mainline kernel and can be found in the
networking section. It needs to be compiled as a module (and installed,
of course) for this to work.

[3] <#AEN87>	

If you are modifying the kernel, to avoid overwriting your existing
modules you may want to use the EXTRAVERSION variable in the kernel
Makefile to create a seperate directory.

[4] <#AEN467>	

It's an invaluable tool for figuring out things like what files a
program is trying to access. Ever have a program bail silently because
it couldn't find a file? It's a PITA!

[5] <#AEN514>	

I'm a physicist, not a computer scientist, Jim!

[6] <#AEN520>	

This isn't quite the same thing as `building all your modules into the
kernel', although the idea is the same.

[7] <#AEN630>	

This is by convention. When writing a driver, it's OK to put the device
file in your current directory. Just make sure you place it in /dev for
a production driver

[8] <#AEN736>	

In version 2.0, in version 2.2 this is done automatically if we set the
inode to zero.

[9] <#AEN814>	

The difference between the two is that file operations deal with the
file itself, and inode operations deal with ways of referencing the
file, such as creating links to it.

[10] <#AEN914>	

Notice that here the roles of read and write are reversed /again/, so in
ioctl's read is to send information to the kernel and write is to
receive information from the kernel.

[11] <#AEN919>	

This isn't exact. You won't be able to pass a structure, for example,
through an ioctl --- but you will be able to pass a pointer to the
structure.

[12] <#AEN1089>	

The easiest way to keep a file open is to open it with *tail -f*.

[13] <#AEN1097>	

This means that the process is still in kernel mode -- as far as the
process is concerned, it issued the open system call and the system call
hasn't returned yet. The process doesn't know somebody else used the CPU
for most of the time between the moment it issued the call and the
moment it returned.

[14] <#AEN1121>	

This is because we used module_interruptible_sleep_on. We could have
used module_sleep_on instead, but that would have resulted is extremely
angry users whose *Ctrl*+*c*s are ignored.

[15] <#AEN1169>	

/T/ele/ty/pe, originally a combination keyboard-printer used to
communicate with a Unix system, and today an abstraction for the text
stream used for a Unix program, whether it's a physical terminal, an
xterm on an X display, a network connection used with telnet, etc.

[16] <#AEN1272>	

This is standard nomencalture on the Intel architecture where Linux
originated.

[17] <#AEN1289>	

In practice IRQ handling can be a bit more complex. Hardware is often
designed in a way that chains two interrupt controllers, so that all the
IRQs from interrupt controller B are cascaded to a certain IRQ from
interrupt controller A. Of course that requires that the kernel finds
out which IRQ it really was afterwards and that adds overhead. Other
architectures offer some special, very low overhead, so called "fast
IRQ" or FIQs. To take advantage of them requires handlers to be written
in assembler, so they do not really fit into the kernel. They can be
made to work similar to the others, but after that procedure, they're no
longer any faster than "common" IRQs. SMP enabled kernels running on
systems with more than one processor need to solve another truckload of
problems. It's not enough to know if a certain IRQs has happend, it's
also important for what CPU(s) it was for. People still interested in
more details, might want to do a web search for "APIC" now ;)

[18] <#AEN1344>	

The exception is threaded processes, which can run on several CPU's at once.

[19] <#AEN1347>	

Meaning it is safe to use it with SMP