Re: [FFmpeg-devel] [PATCH 4/6] avcodec/vp3: Fix undefined pointer arithmetic

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

Andreas Rheinhardt:
> When decoding a keyframe, last_frame and golden_frame are
> not used at all and (at least when starting decoding)
> are not set at all. But due to code sharing pointer arithmetic
> on the NULL data-pointers of these frames has nevertheless
> been performed. This is undefined behaviour and causes e.g.
> "runtime error: applying non-zero offset 173440 to null pointer"
> from UBSan in the vp31, vp4, theora-coeff-level64 and theora-offset
> FATE-tests.
> 
> Fix this by reusing the current frame for unavailable frames.
> 
> Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
> ---
>  libavcodec/vp3.c | 12 ++++++++++--
>  1 file changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/libavcodec/vp3.c b/libavcodec/vp3.c
> index 33c120a58e..5ce1ecfce7 100644
> --- a/libavcodec/vp3.c
> +++ b/libavcodec/vp3.c
> @@ -2056,6 +2056,14 @@ static void render_slice(Vp3DecodeContext *s, int slice)
>  {
>      int16_t *block = s->block;
>      int motion_x = 0xdeadbeef, motion_y = 0xdeadbeef;
> +    /* When decoding keyframes, the earlier frames may not be available,
> +     * so to avoid using undefined pointer arithmetic on them we just
> +     * use the current frame instead. Nothing is ever read from these
> +     * frames in case of a keyframe. */
> +    const AVFrame *last_frame   = s->last_frame.f->data[0]   ?
> +                                      s->last_frame.f   : s->current_frame.f;
> +    const AVFrame *golden_frame = s->golden_frame.f->data[0] ?
> +                                      s->golden_frame.f : s->current_frame.f;
>      int motion_halfpel_index;
>      int first_pixel;
>  
> @@ -2065,9 +2073,9 @@ static void render_slice(Vp3DecodeContext *s, int slice)
>      for (int plane = 0; plane < 3; plane++) {
>          uint8_t *output_plane = s->current_frame.f->data[plane] +
>                                  s->data_offset[plane];
> -        const uint8_t *last_plane = s->last_frame.f->data[plane] +
> +        const uint8_t *last_plane = last_frame->data[plane] +
>                                s->data_offset[plane];
> -        const uint8_t *golden_plane = s->golden_frame.f->data[plane] +
> +        const uint8_t *golden_plane = golden_frame->data[plane] +
>                                  s->data_offset[plane];
>          ptrdiff_t stride = s->current_frame.f->linesize[plane];
>          int plane_width  = s->width  >> (plane && s->chroma_x_shift);

Will apply the remaining patches of this patchset tomorrow unless there
are objections.

- Andreas

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