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

[James Almer <jamrial@gmail.com>]

On 4/30/2024 9:38 PM, Michael Niedermayer wrote:
> 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

Should this happen if the internal decoder failed to generate a frame? 
Can it be avoided?

> 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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