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

Michael Niedermayer michaelni at gmx.at
Wed Sep 30 22:09:58 CEST 2015


On Wed, Sep 30, 2015 at 04:32:24PM +0200, Hendrik Leppkes 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.

having the context inconsistent and fields at invalid values is also
not good.
Ill try to fix it

[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

No snowflake in an avalanche ever feels responsible. -- Voltaire
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