[FFmpeg-devel] [PATCH] mpeg12dec: don't assert on unknown chroma format

[Hendrik Leppkes]

On Wed, Sep 30, 2015 at 4:35 PM, Paul B Mahol <onemda at gmail.com> wrote:
> On 9/30/15, Hendrik Leppkes <h.leppkes at gmail.com> wrote:
>> On Wed, Sep 30, 2015 at 2:51 PM, Michael Niedermayer <michaelni at gmx.at>
>> wrote:
>>> On Wed, Sep 30, 2015 at 02:39:21PM +0200, Hendrik Leppkes wrote:
>>>> On Wed, Sep 30, 2015 at 2:33 PM, Michael Niedermayer <michaelni at gmx.at>
>>>> wrote:
>>>> > On Wed, Sep 30, 2015 at 02:29:43PM +0200, Hendrik Leppkes wrote:
>>>> >> On Wed, Sep 30, 2015 at 2:10 PM, Michael Niedermayer
>>>> >> <michaelni at gmx.at> wrote:
>>>> >> > On Wed, Sep 30, 2015 at 12:42:59PM +0200, Hendrik Leppkes wrote:
>>>> >> >> The chroma format can be still unset in postinit when a badly cut
>>>> >> >> stream
>>>> >> >> starts with a slice instead of a sequence header. This is a common
>>>> >> >> occurance when feeding avcodec from a Live TV stream.
>>>> >> >> ---
>>>> >> >>  libavcodec/mpeg12dec.c | 1 -
>>>> >> >>  1 file changed, 1 deletion(-)
>>>> >> >>
>>>> >> >> diff --git a/libavcodec/mpeg12dec.c b/libavcodec/mpeg12dec.c
>>>> >> >> index 5d916d1..b3c2c45 100644
>>>> >> >> --- a/libavcodec/mpeg12dec.c
>>>> >> >> +++ b/libavcodec/mpeg12dec.c
>>>> >> >> @@ -1389,7 +1389,6 @@ static int
>>>> >> >> mpeg_decode_postinit(AVCodecContext *avctx)
>>>> >> >>              case 1: avctx->chroma_sample_location =
>>>> >> >> AVCHROMA_LOC_LEFT; break;
>>>> >> >>              case 2:
>>>> >> >>              case 3: avctx->chroma_sample_location =
>>>> >> >> AVCHROMA_LOC_TOPLEFT; break;
>>>> >> >> -            default: av_assert0(0);
>>>> >> >>              }
>>>> >> >>          } // MPEG-2
>>>> >> >
>>>> >> > This assert double checks that the context init which uses
>>>> >> > width/height/chroma format is done after the chroma format and w/h
>>>> >> > has
>>>> >> > been read from the headers
>>>> >> > if this is reached without the headers being read then the code is
>>>> >> > buggy and removing the assert will not fix that bug
>>>> >> >
>>>> >> > also if there is no sequence header how is width/height set ?
>>>> >> > theres a check for width/height before mpeg_decode_postinit is
>>>> >> > called
>>>> >> >
>>>> >>
>>>> >> The dimensions are set by the caller in the avctx before opening the
>>>> >> codec, which apparently inits the mpeg context as well.
>>>> >>
>>>> >> Feel free to fix it differently, error out or something, but I can
>>>> >> trip an assert with input data, which should never happen.
>>>> >
>>>> > how can this be reproduced ?
>>>> >
>>>>
>>>> I can only trigger it with API usage, not through the CLI tools, so I
>>>> cannot produce a test case.
>>>
>>> is this with a unmodified up to date ffmpeg ?
>>> i fixed this assert failing a while ago
>>> (b54e03c9dc2a05324c08b503bfe7535c49c0f281)
>>>
>>>
>>
>> Its up to date, but slightly modified - but nothing pertinent to the
>> mpeg2 decoder as far as I know.
>> I'll just add this to the list of modifications then. I can clearly
>> trigger this assert in a release build (which is hilarious on its own,
>> asserts are a debugging tool), which crashes the application, so thats
>> no good.
>
> WTF, can you just give sample/link something?

Like I said earlier in the thread, it happens with API usage in a Live
TV scenario. I cannot produce a sample of that.

- Hendrik