fanotify: add watchdog for permission events

fanotify: add watchdog for permission events

This is to make it easier to debug issues with AV software, which time and
again deadlocks with no indication of where the issue comes from, and the
kernel being blamed for the deadlock.  Then we need to analyze dumps to
prove that the kernel is not in fact at fault.

The deadlock comes from recursion: handling the event triggers another
permission event, in some roundabout way, obviously, otherwise it would
have been found in testing.

With this patch a warning is printed when permission event is received by
userspace but not answered for more than the timeout specified in
/proc/sys/fs/fanotify/watchdog_timeout.  The watchdog can be turned off by
setting the timeout to zero (which is the default).

The timeout is very coarse (T <= t < 2T) but I guess it's good enough for
the purpose.

Overhead should be minimal.

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Link: https://patch.msgid.link/20250909143053.112171-1-mszeredi@redhat.com
Signed-off-by: Jan Kara <jack@suse.cz>