Re: [Qemu-devel] [PATCH for-1.7] seccomp: setting "-sandbox on" by default

[Will Drewry <wad@chromium.org>]

On Thu, Dec 5, 2013 at 7:15 AM, Stefan Hajnoczi <stefanha@gmail.com> wrote:
> On Wed, Dec 04, 2013 at 11:21:12AM -0200, Eduardo Otubo wrote:
>> On 12/04/2013 07:39 AM, Stefan Hajnoczi wrote:
>> >On Fri, Nov 22, 2013 at 11:00:24AM -0500, Paul Moore wrote:
>> >>>Developers will only be happy with seccomp if it's easy and rewarding to
>> >>>support/debug.
>> >>
>> >>Agreed.
>> >>
>> >>As a developer, how do you feel about the audit/syslog based approach I
>> >>mentioned earlier?
>> >
>> >I used the commands you posted (I think that's what you mean).  They
>> >produce useful output.
>> >
>> >The problem is that without an error message on stderr or from the
>> >shell, no one will think "QEMU process dead and hung == check seccomp"
>> >immediately.  It's frustrating to deal with a "silent" failure.
>>
>> The process dies with a SIGKILL, and sig handling in Qemu is hard to
>> implement due to dozen of external linked libraries that has their
>> own signal masks and conflicts with seccomp. I've already tried this
>> approach in the past (you can find in the list by searching for
>> debug mode)
>
> I now realize we may be talking past each other.  Dying with
> SIGKILL/SIGSYS is perfectly reasonable and I would be happy with that
> :-).
>
> But I think there's a bug in seccomp: a multi-threaded process can be
> left in a zombie state.  In my case the primary thread was killed by
> seccomp but another thread was deadlocked on a futex.
>
> The result is the process isn't quite dead yet.  The shell will not reap
> it and we're stuck with a zombie.
>
> I can reproduce it reliably when I run "qemu-system-x86_64 -sandbox on"
> on Fedora 20 (qemu-system-x86-1.6.1-2).
>
> Should seccomp use do_group_exit() for SIGKILL?

Is the problem that the SECCOMP_RET_KILL didn't take down the thread
group (which would be a departure from how seccomp(mode=1) worked) and
causes the deadlock somehow, or is it that the other thread is
deadlocked?

Regardless, adding a SECCOMP_RET_TGKILL probably isn't a bad idea :)

cheers!
will