Re: [FFmpeg-devel] [PATCH] avcodec/vvcdec: check pred flag to fix undefined beavhiours

[Andreas Rheinhardt <andreas.rheinhardt@outlook.com>]

Nuo Mi:
> libavcodec/vvc/vvc_inter.c:823:18: runtime error: signed integer overflow: 1426128896 + 1426128896 cannot be represented in type 'int'
> SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior libavcodec/vvc/vvc_inter.c:823:18
> ---
>  libavcodec/vvc/vvc_inter.c | 15 +++++++++------
>  1 file changed, 9 insertions(+), 6 deletions(-)
> 
> diff --git a/libavcodec/vvc/vvc_inter.c b/libavcodec/vvc/vvc_inter.c
> index d5be32aa14..48c566b097 100644
> --- a/libavcodec/vvc/vvc_inter.c
> +++ b/libavcodec/vvc/vvc_inter.c
> @@ -816,13 +816,16 @@ static void derive_affine_mvc(MvField *mvc, const VVCFrameContext *fc, const MvF
>      const int hs = fc->ps.sps->hshift[1];
>      const int vs = fc->ps.sps->vshift[1];
>      const MvField* mv2 = ff_vvc_get_mvf(fc, x0 + hs * sbw, y0 + vs * sbh);
> +
>      *mvc = *mv;
> -    mvc->mv[0].x += mv2->mv[0].x;
> -    mvc->mv[0].y += mv2->mv[0].y;
> -    mvc->mv[1].x += mv2->mv[1].x;
> -    mvc->mv[1].y += mv2->mv[1].y;
> -    ff_vvc_round_mv(mvc->mv + 0, 0, 1);
> -    ff_vvc_round_mv(mvc->mv + 1, 0, 1);
> +    for (int i = L0; i <= L1; i++) {
> +        PredFlag mask = 1 << i;
> +        if (mv2->pred_flag & mask) {
> +            mvc->mv[i].x += mv2->mv[i].x;
> +            mvc->mv[i].y += mv2->mv[i].y;
> +            ff_vvc_round_mv(mvc->mv + i, 0, 1);
> +        }
> +    }
>  }
>  
>  static void pred_affine_blk(VVCLocalContext *lc)

This adds branches; the UB alone could also be fixed/suppressed with
casting to unsigned. Would the latter cause any problems or are the
values set here unused lateron?

- Andreas

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