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Date:   Tue, 18 Jan 2022 20:30:53 +0300
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Subject: Re: [PATCH 1/8] signal: Make SIGKILL during coredumps an explicit
 special case
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To:     "Eric W. Biederman" <ebiederm@xmission.com>
Cc:     linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org,
        linux-api@vger.kernel.org,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Alexey Gladkov <legion@kernel.org>,
        Kyle Huey <me@kylehuey.com>, Oleg Nesterov <oleg@redhat.com>,
        Kees Cook <keescook@chromium.org>,
        Al Viro <viro@ZenIV.linux.org.uk>
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From:   Dmitry Osipenko <digetx@gmail.com>
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11.01.2022 20:20, Eric W. Biederman пишет:
> Dmitry Osipenko <digetx@gmail.com> writes:
> 
>> 08.01.2022 21:13, Eric W. Biederman пишет:
>>> Dmitry Osipenko <digetx@gmail.com> writes:
>>>
>>>> 05.01.2022 22:58, Eric W. Biederman пишет:
>>>>>
>>>>> I have not yet been able to figure out how to run gst-pluggin-scanner in
>>>>> a way that triggers this yet.  In truth I can't figure out how to
>>>>> run gst-pluggin-scanner in a useful way.
>>>>>
>>>>> I am going to set up some unit tests and see if I can reproduce your
>>>>> hang another way, but if you could give me some more information on what
>>>>> you are doing to trigger this I would appreciate it.
>>>>
>>>> Thanks, Eric. The distro is Arch Linux, but it's a development
>>>> environment where I'm running latest GStreamer from git master. I'll try
>>>> to figure out the reproduction steps and get back to you.
>>>
>>> Thank you.
>>>
>>> Until I can figure out why this is causing problems I have dropped the
>>> following two patches from my queue:
>>>  signal: Make SIGKILL during coredumps an explicit special case
>>>  signal: Drop signals received after a fatal signal has been processed
>>>
>>> I have replaced them with the following two patches that just do what
>>> is needed for the rest of the code in the series:
>>>  signal: Have prepare_signal detect coredumps using
>>>  signal: Make coredump handling explicit in complete_signal
>>>
>>> Perversely my failure to change the SIGKILL handling when coredumps are
>>> happening proves to me that I need to change the SIGKILL handling when
>>> coredumps are happening to make the code more maintainable.
>>
>> Eric, thank you again. I started to look at the reproduction steps and
>> haven't completed it yet. Turned out the problem affects only older
>> NVIDIA Tegra2 Cortex-A9 CPU that lacks support of ARM NEON instructions
>> set, hence the problem isn't visible on x86 and other CPUs out of the
>> box. I'll need to check whether the problem could be simulated on all
>> arches or maybe it's specific to VFP exception handling of ARM32.
> 
> It sounds like the gstreamer plugins only fail on certain hardware on
> arm32, and things don't hang in coredumps unless the plugins fail.
> That does make things tricky to minimize.
> 
> I have just verified that the known problematic code is not
> in linux-next for Jan 11 2022.
> 
> If folks as they have time can double check linux-next and verify all is
> well I would appreciate it.  I don't expect that there are problems but
> sometimes one problem hides another.

Hello Eric,

I reproduced the trouble on x86_64.

Here are the reproduction steps, using ArchLinux and linux-next-20211224:

```
sudo pacman -S base-devel git mesa glu meson wget
git clone https://github.com/grate-driver/gstreamer.git
cd gstreamer
git checkout sigill
meson --prefix=/usr -Dgst-plugins-base:playback=enabled -Dgst-devtools:validate=disabled build
cd build
sudo ninja install
wget https://www.peach.themazzone.com/big_buck_bunny_720p_h264.mov
rm -r ~/.cache/gstreamer-1.0
gst-play-1.0 ./big_buck_bunny_720p_h264.mov
```

The SIGILL, thrown by [1], causes the hang. There is no hang using v5.16.1 kernel. 

[1] https://github.com/grate-driver/gstreamer/commit/006f9a2ee6dcf7b31c9b5413815d6054d82a3b2f