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

[Hendrik Leppkes]

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?

- Hendrik