Messages in this thread | | | From | Rodrigo Campos <> | Date | Mon, 2 May 2022 16:15:07 +0200 | Subject | Re: [PATCH v3 1/2] seccomp: Add wait_killable semantic to seccomp user notifier |
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On Fri, Apr 29, 2022 at 4:32 AM Sargun Dhillon <sargun@sargun.me> wrote: > diff --git a/Documentation/userspace-api/seccomp_filter.rst b/Documentation/userspace-api/seccomp_filter.rst > index 539e9d4a4860..204cf5ba511a 100644 > --- a/Documentation/userspace-api/seccomp_filter.rst > +++ b/Documentation/userspace-api/seccomp_filter.rst > @@ -271,6 +271,14 @@ notifying process it will be replaced. The supervisor can also add an FD, and > respond atomically by using the ``SECCOMP_ADDFD_FLAG_SEND`` flag and the return > value will be the injected file descriptor number. > > +The notifying process can be preempted, resulting in the notification being > +aborted. This can be problematic when trying to take actions on behalf of the > +notifying process that are long-running and typically retryable (mounting a > +filesytem). Alternatively, the at filter installation time, the > +``SECCOMP_FILTER_FLAG_WAIT_KILLABLE_RECV`` flag can be set. This flag makes it > +such that when a user notification is received by the supervisor, the notifying > +process will ignore non-fatal signals until the response is sent.
Maybe:
This flags ignores non-fatal signals that arrive after the supervisor received the notification
I mean, I want to make it clear that if a signal arrives before the notification was received by the supervisor, then it will be interrupted anyways.
> diff --git a/kernel/seccomp.c b/kernel/seccomp.c > index db10e73d06e0..9291b0843cb2 100644 > --- a/kernel/seccomp.c > +++ b/kernel/seccomp.c > @@ -1485,6 +1512,9 @@ static long seccomp_notify_recv(struct seccomp_filter *filter, > mutex_lock(&filter->notify_lock); > knotif = find_notification(filter, unotif.id); > if (knotif) { > + /* Reset the process to make sure it's not stuck */ > + if (should_sleep_killable(filter, knotif)) > + complete(&knotif->ready); > knotif->state = SECCOMP_NOTIFY_INIT; > up(&filter->notif->request);
(I couldn't git-am this locally, so maybe I'm injecting this at the wrong parts mentally when looking at the other code for more context. Sorry if that is the case :))
Why do we need to complete() only in this error path? As far as I can see this is on the error path where the copy to userspace failed and we want to reset this notification.
I think that is wrong, we want to wake up the other side not just on the error path, but on the non-error path (in fact, do we want to do this on the error path? It seems like a no-op, but don't see any reason to do it).
We _need_ to call complete() in the non error path here so the other side wakes up and switches to a killable wait. As we are not doing this (for the non error path), this will basically not achieve a wait_killable() at all.
I think this was probably an oversight adapting the patch from last year. Is it possble? Because it seems that in the previous version we sent last year[1] (if you can link them next time it will be way simpler :)) you had a new ioctl() and the call to complete() was handled there, in seccomp_notify_set_wait_killable(). Now, as this is part of the filter (and as I said last year, I think this way looks better) that call to complete() was completely forgotten.
Is it possible that this is not really working as intended, then? Am I missing something?
Best, Rodrigo
[1]: https://lore.kernel.org/all/20210430204939.5152-3-sargun@sargun.me/
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