Re: [PATCH 0/5] Add pidfd support to the fanotify API

[Jan Kara <jack@suse.cz>]

On Sat 22-05-21 09:32:36, Matthew Bobrowski wrote:
> On Fri, May 21, 2021 at 12:40:56PM +0200, Jan Kara wrote:
> > On Fri 21-05-21 20:15:35, Matthew Bobrowski wrote:
> > > On Thu, May 20, 2021 at 03:55:27PM +0200, Jan Kara wrote:
> > > There's one thing that I'd like to mention, and it's something in
> > > regards to the overall approach we've taken that I'm not particularly
> > > happy about and I'd like to hear all your thoughts. Basically, with
> > > this approach the pidfd creation is done only once an event has been
> > > queued and the notification worker wakes up and picks up the event
> > > from the queue processes it. There's a subtle latency introduced when
> > > taking such an approach which at times leads to pidfd creation
> > > failures. As in, by the time pidfd_create() is called the struct pid
> > > has already been reaped, which then results in FAN_NOPIDFD being
> > > returned in the pidfd info record.
> > > 
> > > Having said that, I'm wondering what the thoughts are on doing pidfd
> > > creation earlier on i.e. in the event allocation stages? This way, the
> > > struct pid is pinned earlier on and rather than FAN_NOPIDFD being
> > > returned in the pidfd info record because the struct pid has been
> > > already reaped, userspace application will atleast receive a valid
> > > pidfd which can be used to check whether the process still exists or
> > > not. I think it'll just set the expectation better from an API
> > > perspective.
> > 
> > Yes, there's this race. OTOH if FAN_NOPIDFD is returned, the listener can
> > be sure the original process doesn't exist anymore. So is it useful to
> > still receive pidfd of the dead process?
> 
> Well, you're absolutely right. However, FWIW I was approaching this
> from two different angles:
> 
> 1) I wanted to keep the pattern in which the listener checks for the
>    existence/recycling of the process consistent. As in, the listener
>    would receive the pidfd, then send the pidfd a signal via
>    pidfd_send_signal() and check for -ESRCH which clearly indicates
>    that the target process has terminated.
> 
> 2) I didn't want to mask failed pidfd creation because of early
>    process termination and other possible failures behind a single
>    FAN_NOPIDFD. IOW, if we take the -ESRCH approach above, the
>    listener can take clear corrective branches as what's to be done
>    next if a race is to have been detected, whereas simply returning
>    FAN_NOPIDFD at this stage can mean multiple things.
> 
> Now that I've written the above and keeping in mind that we'd like to
> refrain from doing anything in the event allocation stages, perhaps we
> could introduce a different error code for detecting early process
> termination while attempting to construct the info record. WDYT?

Sure, I wouldn't like to overengineer it but having one special fd value for
"process doesn't exist anymore" and another for general "creating pidfd
failed" looks OK to me.

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR