[FFmpeg-devel] [PATCH] avcodec/cbs_h264_syntax_template: fix off by 1 error with slice_group_change_cycle

[James Almer]

On 3/22/2020 8:00 PM, Michael Niedermayer wrote:
> Fixes: assertion failure
> Fixes: 20390/clusterfuzz-testcase-minimized-ffmpeg_BSF_H264_REDUNDANT_PPS_fuzzer-5683400772157440
> 
> 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/cbs_h264_syntax_template.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/libavcodec/cbs_h264_syntax_template.c b/libavcodec/cbs_h264_syntax_template.c
> index 878d348b94..b7e796e9a8 100644
> --- a/libavcodec/cbs_h264_syntax_template.c
> +++ b/libavcodec/cbs_h264_syntax_template.c
> @@ -1365,7 +1365,7 @@ static int FUNC(slice_header)(CodedBitstreamContext *ctx, RWContext *rw,
>          pic_size = (sps->pic_width_in_mbs_minus1 + 1) *
>                     (sps->pic_height_in_map_units_minus1 + 1);
>          max = (pic_size + pps->slice_group_change_rate_minus1) /
> -              (pps->slice_group_change_rate_minus1 + 1);
> +              (pps->slice_group_change_rate_minus1 + 1) + 1;

The spec says

"The value of slice_group_change_cycle is represented in the bitstream
by the following number of bits Ceil( Log2( PicSizeInMapUnits ÷
SliceGroupChangeRate + 1 ) )

The value of slice_group_change_cycle shall be in the range of 0 to
Ceil( PicSizeInMapUnits ÷ SliceGroupChangeRate ), inclusive."

Are you trying to change the amount of bits read, the max allowed value,
or both?

>          bits = av_log2(2 * max - 1);

This should use av_ceil_log2() IMO, so it's clear what it's trying to do.

>  
>          u(bits, slice_group_change_cycle, 0, max);
>