[PATCH 0/4] configfs: add a basic selftest

[Breno Leitao <leitao@debian.org>]

configfs has had no selftest since day one, which makes maintaining it
harder than it needs to be.

The good news is that it is not hard to test: nearly all of it is
reachable from userspace through mkdir/rmdir/read/write and symlink(2).
This series leverages the sample configfs module to do exactly that.

The sample implements no ->allow_link(), so patch 2 adds a subsystem that
accepts symlinks and patches 3 and 4 build the symlink coverage on it.

Patch 4 races symlink(2) against rmdir(2) of the target and reproduces the
use-after-free syzbot reported, so it fails until these patch land:

  1507d5b57e40 ("configfs: pin the symlink target's dirent instead of chasing ->ci_dentry")
  2251d0ed97c2 ("configfs: unhash the dentry before dropping the item in rmdir")

Both are in configfs-next.  The first trips the s_count WARN_ON()
in configfs_get(), the second the KASAN use-after-free in
config_item_get().

Signed-off-by: Breno Leitao <leitao@debian.org>
---
Breno Leitao (4):
      selftests/configfs: add tests for the userspace interface
      samples/configfs: add a subsystem that accepts symlinks
      selftests/configfs: cover symlink and unlink
      selftests/configfs: race symlink against rmdir of the target

 MAINTAINERS                                        |   1 +
 samples/configfs/configfs_sample.c                 | 121 ++++++
 tools/testing/selftests/Makefile                   |   1 +
 .../selftests/filesystems/configfs/.gitignore      |   2 +
 .../selftests/filesystems/configfs/Makefile        |   8 +
 .../testing/selftests/filesystems/configfs/config  |   5 +
 .../selftests/filesystems/configfs/configfs_test.c | 481 +++++++++++++++++++++
 7 files changed, 619 insertions(+)
---
base-commit: a8406e6c0b793ce0788019683837c40855b55995
change-id: 20260826-config_selftest-9d84a5b1ec26

Best regards,
--  
Breno Leitao <leitao@debian.org>