[FFmpeg-devel] [PATCH 2/2] avcodec/vc1_block: Fix invalid shifts in vc1_decode_i_blocks()

[Paul B Mahol]

On 6/22/19, Michael Niedermayer <michael at niedermayer.cc> wrote:
> Fixes: left shift of negative value -9
> Fixes:
> 15299/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_MSS2_fuzzer-5660922678345728
>
> 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/vc1_block.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/libavcodec/vc1_block.c b/libavcodec/vc1_block.c
> index 7e41791832..6137252580 100644
> --- a/libavcodec/vc1_block.c
> +++ b/libavcodec/vc1_block.c
> @@ -2600,13 +2600,13 @@ static void vc1_decode_i_blocks(VC1Context *v)
>                  if (v->rangeredfrm)
>                      for (k = 0; k < 6; k++)
>                          for (j = 0; j < 64; j++)
> -                            v->block[v->cur_blk_idx][block_map[k]][j] <<=
> 1;
> +                            v->block[v->cur_blk_idx][block_map[k]][j] *= 2;
>                  vc1_put_blocks_clamped(v, 1);
>              } else {
>                  if (v->rangeredfrm)
>                      for (k = 0; k < 6; k++)
>                          for (j = 0; j < 64; j++)
> -                            v->block[v->cur_blk_idx][block_map[k]][j] =
> (v->block[v->cur_blk_idx][block_map[k]][j] - 64) << 1;
> +                            v->block[v->cur_blk_idx][block_map[k]][j] =
> (v->block[v->cur_blk_idx][block_map[k]][j] - 64) * 2;
>                  vc1_put_blocks_clamped(v, 0);
>              }
>
> --
> 2.22.0
>
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This is much slower.