Don't require mount point for a non-directory to be a regular file

Previously, mounting a socket over the top of an existing socket
would fail, because create_file() opens it with creat():

    $ test -e /run/systemd/resolve/io.systemd.Resolve && echo exists
    exists
    $ bwrap \
      --bind / / \
      --bind /run/systemd/resolve/io.systemd.Resolve \
             /run/systemd/resolve/io.systemd.Resolve \
      /bin/true
    bwrap: Can't create file at /run/systemd/resolve/io.systemd.Resolve: No such device or address

Tolerate the file existing as any type that we will be able to mount
a non-directory onto.

Signed-off-by: Simon McVittie <[email protected]>

tests/test-run.sh

@@ -80,7 +80,7 @@ if [ -z "${BWRAP_MUST_WORK-}" ] && ! $RUN true; then
     skip Seems like bwrap is not working at all. Maybe setuid is not working
 fi
 
-echo "1..50"
+echo "1..51"
 
 # Test help
 ${BWRAP} --help > help.txt
@@ -398,4 +398,11 @@ command stat -c '%a' new-file-mountpoint > new-file-permissions
 assert_file_has_content new-file-permissions 444
 echo "ok - Files and directories created as mount points have expected permissions"
 
+if [ -S /dev/log ]; then
+    $RUN --bind / / --bind "$(realpath /dev/log)" "$(realpath /dev/log)" true
+    echo "ok - Can bind-mount a socket (/dev/log) onto a socket"
+else
+    echo "ok # SKIP - /dev/log is not a socket, cannot test bubblewrap#409"
+fi
+
 echo "ok - End of test"

utils.c

@@ -448,9 +448,14 @@ ensure_file (const char *path,
 
   /* We check this ahead of time, otherwise
      the create file will fail in the read-only
-     case with EROFS instead of EEXIST */
+     case with EROFS instead of EEXIST.
+
+     We're trying to set up a mount point for a non-directory, so any
+     non-directory, non-symlink is acceptable - it doesn't necessarily
+     have to be a regular file. */
   if (stat (path, &buf) ==  0 &&
-      S_ISREG (buf.st_mode))
+      !S_ISDIR (buf.st_mode) &&
+      !S_ISLNK (buf.st_mode))
     return 0;
 
   if (create_file (path, mode, NULL) != 0 &&  errno != EEXIST)