For review: seccomp_user_notif(2) manual page

[Jann Horn]

On Wed, Oct 28, 2020 at 7:32 AM Sargun Dhillon <sargun at sargun.me> wrote:
> On Tue, Oct 27, 2020 at 3:28 AM Jann Horn <jannh at google.com> wrote:
> > On Tue, Oct 27, 2020 at 7:14 AM Michael Kerrisk (man-pages)
> > <mtk.manpages at gmail.com> wrote:
> > > On 10/26/20 4:54 PM, Jann Horn wrote:
> > > > I'm a bit on the fence now on whether non-blocking mode should use
> > > > ENOTCONN or not... I guess if we returned ENOENT even when there are
> > > > no more listeners, you'd have to disambiguate through the poll()
> > > > revents, which would be kinda ugly?
> > >
> > > I must confess, I'm not quite clear on which two cases you
> > > are trying to distinguish. Can you elaborate?
> >
> > Let's say someone writes a program whose responsibilities are just to
> > handle seccomp events and to listen on some other fd for commands. And
> > this is implemented with an event loop. Then once all the target
> > processes are gone (including zombie reaping), we'll start getting
> > EPOLLERR.
> >
> > If NOTIF_RECV starts returning -ENOTCONN at this point, the event loop
> > can just call into the seccomp logic without any arguments; it can
> > just call NOTIF_RECV one more time, see the -ENOTCONN, and terminate.
> > The downside is that there's one more error code userspace has to
> > special-case.
> > This would be more consistent with what we'd be doing in the blocking case.
> >
> > If NOTIF_RECV keeps returning -ENOENT, the event loop has to also tell
> > the seccomp logic what the revents are.
> >
> > I guess it probably doesn't really matter much.
>
> So, in practice, if you're emulating a blocking syscall (such as open,
> perf_event_open, or any of a number of other syscalls), you probably
> have to do it on a separate thread in the supervisor because you want
> to continue to be able to receive new notifications if any other process
> generates a seccomp notification event that you need to handle.
>
> In addition to that, some of these syscalls are preemptible, so you need
> to poll SECCOMP_IOCTL_NOTIF_ID_VALID to make sure that the program
> under supervision hasn't left the syscall.
>
> If we're to implement a mechanism that makes the seccomp ioctl receive
> non-blocking, it would be valuable to address this problem as well (getting
> a notification when the supervisor is processing a syscall and needs to
> preempt it). In the best case, this can be a minor inconvenience, and
> in the worst case this can result in weird errors where you're keeping
> resources open that the container expects to be closed.

Does "a notification" mean signals? Or would you want to have a second
thread in userspace that poll()s for cancellation events on the
seccomp fd and then somehow takes care of interrupting the first
thread, or something like that?

Either way, I think your proposal goes beyond the scope of patching
the existing weirdness, and should be a separate patch.