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

[Hendrik Leppkes]

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.