[FFmpeg-devel] [PATCH] avcodec/ac3: Explicitly return to discard large amounts of nonsense bytes

Michael Niedermayer michael at niedermayer.cc
Fri Feb 8 02:19:30 EET 2019


On Thu, Feb 07, 2019 at 01:28:35AM +0100, Hendrik Leppkes wrote:
> On Thu, Jan 31, 2019 at 1:25 AM Michael Niedermayer
> <michael at niedermayer.cc> wrote:
> >
> > Changes 19sec to 10ms (12559) runtime, 17sec to 177ms (12570)
> > Fixes: Timeout
> > Fixes: 12559/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_AC3_fuzzer-5666516266123264
> > Fixes: 12561/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_AC3_FIXED_fuzzer-5682923041193984
> > Fixes: 12570/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_EAC3_fuzzer-5194734308425728
> >
> > Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
> > Signed-off-by: Michael Niedermayer <michael at niedermayer.cc>
> > ---
> >  libavcodec/ac3dec.c | 2 ++
> >  1 file changed, 2 insertions(+)
> >
> > diff --git a/libavcodec/ac3dec.c b/libavcodec/ac3dec.c
> > index f844a463ee..eaa327a3ee 100644
> > --- a/libavcodec/ac3dec.c
> > +++ b/libavcodec/ac3dec.c
> > @@ -1490,6 +1490,8 @@ static int ac3_decode_frame(AVCodecContext * avctx, void *data,
> >      }
> >      if (i >= buf_size)
> >          return AVERROR_INVALIDDATA;
> > +    if (i > 10)
> > +        return i;
> >      buf += i;
> >      buf_size -= i;
> >
> 
> How exactly does this speed up the decoder, anyway? Isn't the garbage
> immediately skipped right after this return?

no, the return is limited to the (super) frame size so it re scans the garbage
over and over again

thx

[...]

-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Those who are too smart to engage in politics are punished by being
governed by those who are dumber. -- Plato 
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