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From:   Sargun Dhillon <sargun@sargun.me>
Date:   Tue, 27 Oct 2020 23:31:39 -0700
Message-ID: <CAMp4zn_Qt2MYuoLojn5ikRkr-J5yGimirjevoAvorK5wfzrBHg@mail.gmail.com>
Subject: Re: For review: seccomp_user_notif(2) manual page
To:     Jann Horn <jannh@google.com>
Cc:     "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>,
        Tycho Andersen <tycho@tycho.pizza>,
        Kees Cook <keescook@chromium.org>,
        Christian Brauner <christian@brauner.io>,
        linux-man <linux-man@vger.kernel.org>,
        lkml <linux-kernel@vger.kernel.org>,
        Aleksa Sarai <cyphar@cyphar.com>,
        Alexei Starovoitov <ast@kernel.org>,
        Will Drewry <wad@chromium.org>, bpf <bpf@vger.kernel.org>,
        Song Liu <songliubraving@fb.com>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andy Lutomirski <luto@amacapital.net>,
        Linux Containers <containers@lists.linux-foundation.org>,
        Giuseppe Scrivano <gscrivan@redhat.com>,
        Robert Sesek <rsesek@google.com>
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On Tue, Oct 27, 2020 at 3:28 AM Jann Horn <jannh@google.com> wrote:
>
> On Tue, Oct 27, 2020 at 7:14 AM Michael Kerrisk (man-pages)
> <mtk.manpages@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.