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. You're not being thick at all Amir, and perhaps this is my fault for not articulating the problem at hand correctly. > I still don't understand what's racy about this. Won't the event reader > get a valid pidfd? I guess this depends, right? As the logic/implementation currently stands in this specific patch series, pidfd_create() will _NOT_ return a valid pidfd unless the struct pid still holds reference to a task of type PIDTYPE_TGID. This is thanks to the extra pid_hash_task() check that I thought was appropriate to incorporate into pidfd_create() seeing as though: - With the pidfd_create() declaration now being added to linux/pid.h, we effectively are giving the implicit OK for it to be called from other kernel subsystems, and hence why the caller should be subject to the same restrictions/verifications imposed by the API specification i.e. "Currently, the process identified by @pid must be a thread-group leader...". Not enforcing the pid_has_task() check in pidfd_create() effectively says that the pidfd creation can be done for any struct pid types i.e. PIDTYPE_PID, PIDTYPE_PGID, etc. This leads to assumptions being made by the callers, which effectively then could lead to undefined/unexpected behavior. There definitely can be cases whereby the underlying task(s) associated with a struct pid have been freed as a result of process being killed early. As in, the process is killed before the pid_has_task() check is performed from within pidfd_create() when called from fanotify. This is precisely the race that I'm referring to here, and in such cases as the code currently stands, the event listener will _NOT_ receive a valid pidfd. > Can't the event reader verify that the pidfd points to a dead process? This was the idea, as in, the burden of checking whether a process has been killed before the event listener receives the event should be on the event listener. However, we're trying to come up with a good way to effectively communicate that the above race I've attempted to articulate has actually occurred. As in, do we: a) Drop the pid_has_task() check in pidfd_create() so that a pidfd can be returned for all passed struct pids? That way, even if the above race is experienced the caller will still receive a pidfd and the event listener can do whatever it needs to with it. b) Before calling into pidfd_create(), perform an explicit pid_has_task() check for PIDTYPE_TGID and if that returns false, then set FAN_NOPIDFD and save ourselves from calling into pidfd_create() all together. The event listener in this case doesn't have to perform the signal check to determine whether the process has already been killed. c) Scrap calling into pidfd_create() all together and write a simple fanotify wrapper that contains the pidfd creation logic we need. > 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. Well sort of, as it depends on the approach that we decide to go ahead with to report such early process termination cases. If we return FAN_NOPIDFD, which signifies that the process died before a pidfd could be created, then there's no point for the listener to step into checking the pidfd because the pidfd already == FAN_NOPIDFD. If we simply return a pidfd regardless of the early termination of the process, then sure the event listener will need to check each pidfd that is supplied. > > 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 Maybe I'm going down a rabbit hole and overthinking this whole thing, IDK... :( /M
