[FFmpeg-devel] [PATCH] avcodec/videodsp_template: Fix overflow of addition

[Reimar Döffinger]

On 18.06.2019, at 16:25, Michael Niedermayer <michael at niedermayer.cc> wrote:

> Fixes: addition of unsigned offset to 0x7f56fc26a9b6 overflowed to 0x7f56fc26a8be*
> Fixes: clusterfuzz-testcase-minimized-mediasource_MP4_AVC1_pipeline_integration_fuzzer-4917949056679936
> 
> Reported-by: Matt Wolenetz <wolenetz at google.com>
> Reviewed-by: Matt Wolenetz <wolenetz at google.com>
> Signed-off-by: Michael Niedermayer <michael at niedermayer.cc>
> ---
> libavcodec/videodsp_template.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/libavcodec/videodsp_template.c b/libavcodec/videodsp_template.c
> index 94c1b7188d..eae2f1d51b 100644
> --- a/libavcodec/videodsp_template.c
> +++ b/libavcodec/videodsp_template.c
> @@ -44,7 +44,7 @@ void FUNC(ff_emulated_edge_mc)(uint8_t *buf, const uint8_t *src,
>         src_y = 1 - block_h;
>     }
>     if (src_x >= w) {
> -        src  += (w - 1 - src_x) * sizeof(pixel);
> +        src  -= (1 + src_x - w) * sizeof(pixel);

This is really non-obvious and someone might be tempted to "simplify", especially since the old way matched the code a few lines below.
I'd suggest adding a comment.

>         src_x = w - 1;
>     } else if (src_x <= -block_w) {
>         src  += (1 - block_w - src_x) * sizeof(pixel);