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. (I think if would be interesting to see perf numbers and how high you need to bump the number of open fds sysctl limit to make this useable on systems with a lot of events. I'd be interested in that just in case you have something there.) > > 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 > > /M
