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

[Matthew Bobrowski <repnop@google.com>]

On Tue, May 25, 2021 at 12:31:33PM +0200, Christian Brauner wrote:
> On Mon, May 24, 2021 at 10:47:46AM +0200, Jan Kara wrote:
> > 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.
> 
> FAN_EPIDFD -> "creation failed"
> FAN_NOPIDFD -> "no such process"

Yes, I was thinking something along the lines of this...

With the approach that I've proposed in this series, the pidfd
creation failure trips up in pidfd_create() at the following
condition:

	if (!pid || !pid_has_task(pid, PIDTYPE_TGID))
	   	 return -EINVAL;

Specifically, the following check:
	!pid_has_task(pid, PIDTYPE_TGID)

In order to properly report either FAN_NOPIDFD/FAN_EPIDFD to
userspace, AFAIK I'll have to do one of either two things to better
distinguish between why the pidfd creation had failed:

1) Implement an additional check in pidfd_create() that effectively
   checks whether provided pid still holds reference to a struct pid
   that isn't in the process of being cleaned up. If it is being
   cleaned up, then return something like -ESRCH instead of -EINVAL so
   that the caller, in this case fanotify, can check and set
   FAN_NOPIDFD if -ESRCH is returned from pidfd_create(). I definitely
   don't feel as though returning -ESRCH from the !pid_has_task(pid,
   PIDTYPE_TGID) would be appropriate. In saying that, I'm not aware
   of a helper by which would allow us to perform such an in-flight
   check? Perhaps something needs to be introduced here, IDK...

2) Refrain from performing any further changes to pidfd_create()
   i.e. as proposed in option 1), and manually perform the pidfd
   creation from some kind of new fanotify helper, as suggested by you
   here [0]. However, I'm not convinved that I like this approach as
   we may end up slowly drifting away from pidfd creation semantics
   over time.

[0] https://www.spinics.net/lists/linux-fsdevel/msg195556.html 

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