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unprivileged btrfs subvol list #893

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unprivileged btrfs subvol list #893

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Commits on Sep 23, 2024

  1. libbtrfsutil: don't check for UID 0 in subvolume_info() 

    btrfs_util_subvolume_info() explicitly checks whether geteuid() == 0 to
decide whether to use the unprivileged BTRFS_IOC_GET_SUBVOL_INFO ioctl
or the privileged BTRFS_IOC_TREE_SEARCH ioctl. This breaks in user
namespaces:

  $ unshare -r python3 -c 'import btrfsutil; print(btrfsutil.subvolume_info("/"))'
  Traceback (most recent call last):
    File "<string>", line 1, in <module>
  btrfsutil.BtrfsUtilError: [BtrfsUtilError 12 Errno 1] Could not search B-tree: Operation not permitted: '/'

The unprivileged ioctl has been supported since Linux 4.18. Let's try
the unprivileged ioctl first, then fall back to the privileged version
only if it isn't supported.

Signed-off-by: Omar Sandoval <[email protected]>
    @osandov @maharmstone
    osandov authored and maharmstone committed Sep 23, 2024
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  2. libbtrfsutil: don't check for UID 0 in subvolume iterator 

    The subvolume iterator API explicitly checks whether geteuid() == 0 to
decide whether to use the unprivileged BTRFS_IOC_GET_SUBVOL_ROOTREF and
BTRFS_IOC_INO_LOOKUP_USER ioctls or the privileged BTRFS_IOC_TREE_SEARCH
ioctl. This breaks in user namespaces:

  $ unshare -r python3 -c 'import btrfsutil; print(list(btrfsutil.SubvolumeIterator("/home")))'
  Traceback (most recent call last):
    File "<string>", line 1, in <module>
  btrfsutil.BtrfsUtilError: [BtrfsUtilError 12 Errno 1] Could not search B-tree: Operation not permitted

Instead of the explicit check, let's try the privileged mode first, and
if it fails with a permission error, fall back to the unprivileged mode
(which has been supported since Linux 4.18). Note that we have to try
the privileged mode first, since even for privileged users, the
unprivileged mode may omit some subvolumes that are hidden by filesystem
mounts.

Signed-off-by: Omar Sandoval <[email protected]>
    @osandov @maharmstone
    osandov authored and maharmstone committed Sep 23, 2024
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  3. btrfs-progs: subvol list: remove unused raw layout code 

    It hasn't been used since commit 9005b60 ("btrfs-progs: use
libbtrfsutil for subvol show").

Signed-off-by: Omar Sandoval <[email protected]>
    @osandov @maharmstone
    osandov authored and maharmstone committed Sep 23, 2024
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  4. btrfs-progs: subvol list: remove unused filters 

    BTRFS_LIST_FILTER_ROOTID hasn't been used since commit 9e73a41
("btrfs-progs: use libbtrfsutil for get-default"), and
BTRFS_LIST_FILTER_BY_PARENT hasn't been used since commit 9005b60
("btrfs-progs: use libbtrfsutil for subvol show").

Signed-off-by: Omar Sandoval <[email protected]>
    @osandov @maharmstone
    osandov authored and maharmstone committed Sep 23, 2024
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  5. btrfs-progs: subvol list: document and test actual behavior of paths 

    The way btrfs subvol list prints paths and what the -o and -a flags do
are all nonsense.

Apparently, very early versions of Btrfs had a concept of a "top level"
of subvolumes rather than the root filesystem tree that we have today;
see commit 4ff9e2a ("Add btrfs-list for listing subvolumes"). The
original subvol list code tracked the ID of that top level subvolume.
Eventually, 5 became the only possibility for the top level, and -o, -a,
and path printing were based on that. Commit 4f5ebb3 ("Btrfs-progs:
fix to make list specified directory's subvolumes work") broke this and
changed the top level to be the same as the parent subvolume ID, which
gave us the illogical behavior we have today.

It has been this way for a decade, so we're probably stuck with it. But
let's at least document precisely what these all do in preparation for
adding sensible options. Let's also add tests in preparation for the
upcoming changes.

Signed-off-by: Omar Sandoval <[email protected]>
    @osandov @maharmstone
    osandov authored and maharmstone committed Sep 23, 2024
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  6. btrfs-progs: subvol list: use libbtrfsutil 

    btrfs subvol list has its own subvolume walking implementation that we
can replace with a libbtrfsutil subvolume iterator. Most of the changed
lines are removing the old implementation and mechanically updating the
comparators, filters, and printers to use libbtrfsutil's subvolume info.
The interesting parts are:

1. We can replace the red-black tree of subvolumes with an array that we
   qsort.
2. Listing deleted subvolumes needs a different codepath, but we don't
   need a filter for it anymore.
3. We need some hacks to maintain the weird path behavior documented in
   the previous commit.

In addition to removing a bunch of redundant code, this also prepares us
for allowing subvol list by unprivileged users in some cases.

Signed-off-by: Omar Sandoval <[email protected]>
    @osandov @maharmstone
    osandov authored and maharmstone committed Sep 23, 2024
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  7. btrfs-progs: subvol list: add sane -O and -A options 

    Now that we've documented the current nonsensical behavior, add a couple
of options that actually make sense: -O lists all subvolumes below a
path (which is what people think -o does), and -A lists all subvolumes
with no path munging (which is what people think the default or -a do).
-O can even be used by unprivileged users.

-O and -A also renames the "top level" in the default output to what it
actually is now: the "parent".

Signed-off-by: Omar Sandoval <[email protected]>
    @osandov @maharmstone
    osandov authored and maharmstone committed Sep 23, 2024
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