On Wed, Jun 02, 2021 at 10:48:54AM +0200, Christian Brauner wrote: > On Wed, Jun 02, 2021 at 10:18:36AM +0300, Amir Goldstein wrote: > > On Wed, Jun 2, 2021 at 9:30 AM Matthew Bobrowski <repnop@xxxxxxxxxx> wrote: > > > > > > 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 > > > > 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. To get things straight, there's no argument here. There's a discussion about what the best approach is for communicating to the event listener that a process has been killed prior to a pidfd being created by/from fanotify. I have no issues with communicating FAN_NOPIDFD to the event listener in such cases. I just want to make sure everyone else is OK with it. > > > 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 /M
