Re: [RFC][PATCH] fanotify: register event pid for pidfd reporting

[Christian Brauner]

On Mon, Jun 01, 2026 at 05:24:36PM +0200, Jan Kara wrote:
> On Sat 30-05-26 18:51:17, AnonymeMeow wrote:
> > pidfd_prepare() may still fail if userspace reads the event after the
> > task has been reaped and the pid has not been registered with pidfs.
> > Register the event pid with pidfs when creating the fanotify event if
> > pidfd reporting was requested, so pidfd_prepare() can later create a
> > pidfd for the reaped task.
> > 
> > Signed-off-by: AnonymeMeow <anonymemeow@xxxxxxxxx>
> 
> So the two patches in this series so far look good to me. This patch looks
> OK to me as well and since the overhead of allocation etc. is only once per

It's a kmem cache so should not matter at all...

> registered task, I think it's fine but let's wait a bit whether Christian
> agrees this is what needs to be done.

pidfs_register_pid() can only fail with ENOMEM or if the task is already
dead. The latter cannot happen because even for a non-empty thread group
where the thread-group leader already exited but subthreads are still
alive the thread-group leader cannot be reaped. So the only error here
is ENOMEM which means we should defo fail this, imho.

And yes, I think this is worth doing. Because you get a persistent
handle on the task that generated that event + all the exit information
even if that thask has been reaped in the meantime. I'm pretty sure if
you're using fanotify that's a rather huge win for apps that need it.