Userspace generally expects APIs that return -EMSGSIZE to allow for them
to adjust their buffer size and retry the operation. However, the
fscontext log would previously clear the message even in the -EMSGSIZE
case.

Given that it is very cheap for us to check whether the buffer is too
small before we remove the message from the ring buffer, let's just do
that instead. While we're at it, refactor some fscontext_read() into a
separate helper to make the ring buffer logic a bit easier to read.

Fixes: 007ec26cdc9f ("vfs: Implement logging through fs_context")
Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
---
Changes in v2:
- Refactor message fetching to fetch_message_locked() which returns
  ERR_PTR() in error cases. [Al Viro]
- v1: <https://lore.kernel.org/r/20250806-fscontext-log-cleanups-v1-0-880597d42a5a@cyphar.com>

---
Aleksa Sarai (2):
      fscontext: do not consume log entries when returning -EMSGSIZE
      selftests/filesystems: add basic fscontext log tests

 fs/fsopen.c                                    |  54 +++++-----
 tools/testing/selftests/filesystems/.gitignore |   1 +
 tools/testing/selftests/filesystems/Makefile   |   2 +-
 tools/testing/selftests/filesystems/fclog.c    | 135 +++++++++++++++++++++++++
 4 files changed, 167 insertions(+), 25 deletions(-)
---
base-commit: 66639db858112bf6b0f76677f7517643d586e575
change-id: 20250806-fscontext-log-cleanups-50f0143674ae

Best regards,
-- 
Aleksa Sarai <cyphar@cyphar.com>