fs/pipe: Fix lockdep false-positive in watchqueue pipe_write()
When you try to splice between a normal pipe and a notification pipe,
get_pipe_info(..., true) fails, so splice() falls back to treating the
notification pipe like a normal pipe - so we end up in
iter_file_splice_write(), which first locks the input pipe, then calls
vfs_iter_write(), which locks the output pipe.
Lockdep complains about that, because we're taking a pipe lock while
already holding another pipe lock.
I think this probably (?) can't actually lead to deadlocks, since you'd
need another way to nest locking a normal pipe into locking a
watch_queue pipe, but the lockdep annotations don't make that clear.
Bail out earlier in pipe_write() for notification pipes, before taking
the pipe lock.
Reported-and-tested-by: <syzbot+011e4ea1da6692cf881c@syzkaller.appspotmail.com>
Closes: https://syzkaller.appspot.com/bug?extid=
011e4ea1da6692cf881c
Fixes:
c73be61cede5 ("pipe: Add general notification queue support")
Signed-off-by: Jann Horn <jannh@google.com>
Link: https://lore.kernel.org/r/20231124150822.2121798-1-jannh@google.com
Signed-off-by: Christian Brauner <brauner@kernel.org>