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

[Matthew Bobrowski <repnop@google.com>]

On Tue, Jun 01, 2021 at 01:46:28PM +0200, Christian Brauner wrote:
> On Tue, Jun 01, 2021 at 09:03:26PM +1000, Matthew Bobrowski wrote:
> > On Wed, May 26, 2021 at 08:05:29PM +0200, Christian Brauner wrote:
> > > On Wed, May 26, 2021 at 09:20:55AM +1000, Matthew Bobrowski wrote:
> > > > 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:
> > > 
> > > Ok, I see. You already do have a reference to a struct pid and in that
> > > case we should just always return a pidfd to the caller. For
> > > pidfd_open() for example we only report an error when
> > > find_get_pid(<pidnr>) doesn't find a struct pid to refer to. But in your
> > > case here you already have a struct pid so I think we should just keep
> > > this simple and always return a pidfd to the caller and in fact do
> > > burden them with figuring out that the process is gone via
> > > pidfd_send_signal() instead of complicating our lives here.
> > 
> > Ah, actually Christian... Before, I go ahead and send through the updated
> > series. Given what you've mentioned above I'm working with the assumption
> > that you're OK with dropping the pid_has_task() check from pidfd_create()
> > [0]. Is that right?
> > 
> > If so, I don't know how I feel about this given that pidfd_create() is now
> > to be exposed to the rest of the kernel and the pidfd API, as it stands,
> > doesn't support the creation of pidfds for non-thread-group leaders. I
> > suppose what I don't want is other kernel subsystems, if any, thinking it's
> > OK to call pidfd_create() with an arbitrary struct pid and setting the
> > expectation that a fully functional pidfd will be returned.
> > 
> > The way I see it, I think we've got two options here:
> > 
> > 1) Leave the pid_has_task() check within pidfd_create() and perform another
> >    explicit pid_has_task() check from the fanotify code before calling
> >    pidfd_create(). If it returns false, we set something like FAN_NOPIDFD
> >    indicating to userspace that there's no such process when the event was
> >    created.
> > 
> > 2) Scrap using pidfd_create() all together and implement a fanotify
> >    specific pidfd creation wrapper which would allow for more
> >    control. Something along the lines of what you've done in kernel/fork.c
> >    [1]. Not the biggest fan of this idea just yet given the possibility of
> >    it leading to an API drift over time.
> > 
> > WDYT?
> 
> Hm, why would you have to drop the pid_has_task() check again?

Because of the race that I brielfy decscribed here [0]. The race exists
because we perform the pidfd creation during the notification queue
processing and not in the event allocation stages (for reasons that Jan has
already covered here [1]). So, tl;dr there is the case where the fanotify
calls pidfd_create() and the check for pid_has_task() fails because the
struct pid that we're hanging onto within an event no longer contains a
task of type PIDTYPE_TGID...

[0] https://www.spinics.net/lists/linux-api/msg48630.html
[1] https://www.spinics.net/lists/linux-api/msg48632.html

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