[mythtv] Seg Fault in mediacodec

[Aman Gupta]

On Tue, Jul 3, 2018 at 11:35 AM David Engel <david at istwok.net> wrote:

> On Tue, Jul 03, 2018 at 10:37:04AM -0400, Peter Bennett wrote:
> >
> >
> > On 07/02/2018 09:38 PM, David Engel wrote:
> > > On Mon, Jul 02, 2018 at 05:14:52PM -0400, Aman Gupta wrote:
> > > > On Mon, Jul 2, 2018 at 3:58 PM Peter Bennett <pb.mythtv at gmail.com>
> wrote:
> > > > > Hi Aman
> > > > >
> > > > > Thank you for all your help so far.
> > > > >
> > > > > I upgraded my Shield to Android 8.0. Now I get a seg fault every
> time I
> > > > > play 1080 interlaced recordings. Progressive recordings are fine,
> and
> > > > > 480 interlaced are fine. Also 4K recordings are fine. With 1080
> > > > > interlaced it gets a seg fault in avcodec_receive_frame. This
> happens at
> > > > > about the fifth frame received in the video. It starts off fine,
> with
> > > > > avcodec_receive_frame returning EAGAIN until some number of
> packets have
> > > > > been sent, then it receives a few frames, then there is a seg
> fault in
> > > > > memcpy. There are no null addresses being passed to memcpy, but
> > > > > something must be invalid. This was working fine with Android 7. I
> am
> > > > > using the default buffer allocation: AVCodecContext->get_buffer2 =
> > > > > get_avf_buffer. avcodec_receive_frame should allocate the buffers
> using
> > > > > that routine so they should be valid.
> > > > >
> > > > > [backtrace snipped]
> > > > >
> > > > You can try the mpv-android apk from GitHub to compare. I'm pretty
> sure it
> > > > works fine on Oreo.
> > > mpv-android almost always crashes for me after a split second on 1080i
> > > content.  When it doesn't crash, it just exits playback, also after a
> > > split second.  720p content works fine, just like with MythTV.
> > >
> > > David
> > Thank you for doing that test.
> >
> > This indicates the problem is somewhere in ffmpeg rather than mythtv
> code. I
> > don't know what the trigger is, interlaced video or mpeg2 video, but it
> does
> > not affect 480 interlaced mpeg2 video.
>
> I have a few channels using 1080i and h.264 encoding.  It happens with
> recordings from them as well.


I'm traveling and don't have access to my devices to try this myself, but
it sounds like a regression in the copy based decoder. Would be useful to
know if the same issue occurs on an Oreo MiBox to narrow down the
regression to Oreo itself vs Nvidia.

You can also try setting the ffmpeg loglevel to trace which will show much
more detailed info about each output buffer being returned from the
decoder. A segv in memcpy is pretty low level so it's unclear if this is
something we would be able to fix in ffmpeg ourselves.

Aman


>
> Davi
>
> > The issue of the illegal state exception while fast forwarding is still
> > there so I should remove the code that discards frames, as that did not
> fix
> > it consistently.
> >
> > I will wait a while to see if FFmpeg can give us a fix, before looking
> into
> > it further.
> >
> > Peter
>
> --
> David Engel
> david at istwok.net
>
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