Re: [FFmpeg-devel] [PATCH 3/3] avutil/opt: Preserve nb_channels in opt_free

[Michael Niedermayer <michael@niedermayer.cc>]

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On Tue, Apr 30, 2024 at 06:27:23PM -0300, James Almer wrote:
> On 4/29/2024 9:48 PM, Michael Niedermayer wrote:
> > Fixes: division by 0
> > Fixes: decoder modifying demuxer channels on failure
> > Fixes: -sseof -5 -i zgclab/ffmpeg_crash/poc3
> > 
> > Found-by: Wang Dawei and Zhou Geng, from Zhongguancun Laboratory
> > Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
> > ---
> >   libavutil/opt.c | 6 ++++--
> >   1 file changed, 4 insertions(+), 2 deletions(-)
> > 
> > diff --git a/libavutil/opt.c b/libavutil/opt.c
> > index ecbf7efe5fb..24c08e4bc06 100644
> > --- a/libavutil/opt.c
> > +++ b/libavutil/opt.c
> > @@ -132,9 +132,11 @@ static void opt_free_elem(const AVOption *o, void *ptr)
> >           av_dict_free((AVDictionary **)ptr);
> >           break;
> > -    case AV_OPT_TYPE_CHLAYOUT:
> > +    case AV_OPT_TYPE_CHLAYOUT: {
> > +        int nb_channels = ((AVChannelLayout *)ptr)->nb_channels;
> >           av_channel_layout_uninit((AVChannelLayout *)ptr);
> > -        break;
> > +        ((AVChannelLayout *)ptr)->nb_channels = nb_channels;
> > +        break;}
> >       default:
> >           break;
> 
> A little bit of context would be helpful here. What's using nb_channels
> after av_opt_free was called and where?

demuxer sets nb_channels
find stream info copies codec params to context
find stream info tries opening decoder
decoder, refuses, and opt_free_elem() is called on cleanup
context now has 0 channels
context gets copied into params of demuxer
demuxer goes like i have set the channels to a non zero value let me devide by them
and oops

there is more than one position in this chain of events this can be fixed

thx

[...]
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Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Observe your enemies, for they first find out your faults. -- Antisthenes

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