On Wed, Jun 02, 2021 at 10:18:36AM +0300, Amir Goldstein wrote:

On Wed, Jun 2, 2021 at 9:30 AM Matthew Bobrowski [off-list ref] wrote:

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On Tue, Jun 01, 2021 at 01:46:28PM +0200, Christian Brauner wrote:

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On Tue, Jun 01, 2021 at 09:03:26PM +1000, Matthew Bobrowski wrote:

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On Wed, May 26, 2021 at 08:05:29PM +0200, Christian Brauner wrote:

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On Wed, May 26, 2021 at 09:20:55AM +1000, Matthew Bobrowski wrote:

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On Tue, May 25, 2021 at 12:31:33PM +0200, Christian Brauner wrote:

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On Mon, May 24, 2021 at 10:47:46AM +0200, Jan Kara wrote:

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On Sat 22-05-21 09:32:36, Matthew Bobrowski wrote:

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On Fri, May 21, 2021 at 12:40:56PM +0200, Jan Kara wrote:

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On Fri 21-05-21 20:15:35, Matthew Bobrowski wrote:

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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

Sorry for being thich. I still don't understand what's racy about this.
Won't the event reader get a valid pidfd?
Can't the event reader verify that the pidfd points to a dead process?
I don't mind returning FAN_NOPIDFD for convenience, but user
will have to check the pidfd that it got anyway, because process
can die at any time between reading the event and acting on the
pidfd.

(Replying to this part of the thread so we don't have to many parallel
replies.)

Hm, so quoting from link [0] Matthew posted so we all have some context
here:
"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."

I don't think that's a race and even if I don't think that it's a
meaningful one. So when the event is queued the process is still alive
but when the notification is actually delivered the process is dead.

And your point, Matthew, seems to be that the caller should always get a
pidfd back even if the process has already exited _and_ been reaped,
i.e. is dead because it was alive when the event was generated.

I think that's no how it needs to work and I have a hard time seeing
this as a good argument. What's problematic about just returning
FAN_NOPIDFD in that case? After all the process is gone. All the caller
can do with such a pidfd is to send it a signal and immediately realize
that the process is gone, i.e. -ESRCH anyway.

quoted

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

I warmly recommend that you use lore.kernel.org for archive links.

Thanks,
Amir.