Re: [FFmpeg-devel] [PATCH v3 2/2] lavc/vvc: Ensure subpictures don't overlap

[Frank Plowman <post@frankplowman.com>]

On 22/02/2025 07:51, Nuo Mi wrote:
> From: Frank Plowman <post@frankplowman.com>
> 
> This is essentially a re-implementation of
> https://patchwork.ffmpeg.org/project/ffmpeg/patch/20241005223955.54158-1-post@frankplowman.com/
> 
> That patch was not applied last time.  Instead we opted to identify
> issues which could be caused by invalid subpicture layouts and remedy
> those issues where they manifest, either through error detection or code
> hardening.  This was primarily implemented in the set
> https://patchwork.ffmpeg.org/project/ffmpeg/list/?series=13381.
> 
> This has worked to some degree, however issues with subpicture layouts
> continue to crop up from the fuzzer and I've fixed a number of bugs
> related to subpicture layouts since then.  I think it's best to return
> to the initial plan and simply check if the subpicture layout is valid
> initially.
> 
> This implementation is also lighter than the first time -- by doing a
> bit more logic in pps_subpic_less_than_one_tile_slice, we are able to
> store a tile_in_subpic map rather than a ctu_in_subpic map.  This
> reduces the size of the map to the point it becomes possible to allocate
> it on the stack.  Similar to 8bd66a8c9587af61c7b46558be3c4ee317c1af5a,
> the layout is also validated in the slice map construction code, rather
> than in the CBS, which avoids duplicating some logic.
> 
> Signed-off-by: Frank Plowman <post@frankplowman.com>
> ---
>  libavcodec/vvc/ps.c | 55 ++++++++++++++++++++++++++++++++++++---------
>  1 file changed, 45 insertions(+), 10 deletions(-)
> 
> diff --git a/libavcodec/vvc/ps.c b/libavcodec/vvc/ps.c
> index d8794277d2..9b8af5524b 100644
> --- a/libavcodec/vvc/ps.c
> +++ b/libavcodec/vvc/ps.c
> @@ -450,25 +450,50 @@ static void subpic_tiles(int *tile_x, int *tile_y, int *tile_x_end, int *tile_y_
>          (*tile_y_end)++;
>  }
>  
> -static void pps_subpic_less_than_one_tile_slice(VVCPPS *pps, const VVCSPS *sps, const int i, const int tx, const int ty, int *off)
> +static bool mark_tile_as_used(bool *tile_in_subpic, const int tx, const int ty, const int tile_columns)
>  {
> +    const size_t tile_idx = ty * tile_columns + tx;
> +    if (tile_in_subpic[tile_idx]) {
> +        /* the tile is covered by other subpictures */
> +        return false;
> +    }
> +    tile_in_subpic[tile_idx] = true;
> +    return true;
> +}
> +
> +static int pps_subpic_less_than_one_tile_slice(VVCPPS *pps, const VVCSPS *sps, const int i, const int tx, const int ty, int *off, bool *tile_in_subpic)
> +{
> +    const int subpic_bottom = sps->r->sps_subpic_ctu_top_left_y[i] + sps->r->sps_subpic_height_minus1[i];
> +    const int tile_bottom = pps->row_bd[ty] + pps->r->row_height_val[ty] - 1;
> +    const bool is_final_subpic_in_tile = subpic_bottom == tile_bottom;
> +
> +    if (is_final_subpic_in_tile && !mark_tile_as_used(tile_in_subpic, tx, ty, pps->r->num_tile_columns))
> +        return AVERROR_INVALIDDATA;
> +
>      pps->num_ctus_in_slice[i] = pps_add_ctus(pps, off,
>          sps->r->sps_subpic_ctu_top_left_x[i], sps->r->sps_subpic_ctu_top_left_y[i],
>          sps->r->sps_subpic_width_minus1[i] + 1, sps->r->sps_subpic_height_minus1[i] + 1);
> +
> +    return 0;
>  }
>  
> -static void pps_subpic_one_or_more_tiles_slice(VVCPPS *pps, const int tile_x, const int tile_y, const int x_end, const int y_end, const int i, int *off)
> +static int pps_subpic_one_or_more_tiles_slice(VVCPPS *pps, const int tile_x, const int tile_y, const int x_end, const int y_end,
> +    const int i, int *off, bool *tile_in_subpic)
>  {
>      for (int ty = tile_y; ty < y_end; ty++) {
>          for (int tx = tile_x; tx < x_end; tx++) {
> +            if (!mark_tile_as_used(tile_in_subpic, tx, ty, pps->r->num_tile_columns))
> +                return AVERROR_INVALIDDATA;
> +
>              pps->num_ctus_in_slice[i] += pps_add_ctus(pps, off,
>                  pps->col_bd[tx], pps->row_bd[ty],
>                  pps->r->col_width_val[tx], pps->r->row_height_val[ty]);
>          }
>      }
> +    return 0;
>  }
>  
> -static void pps_subpic_slice(VVCPPS *pps, const VVCSPS *sps, const int i, int *off)
> +static int pps_subpic_slice(VVCPPS *pps, const VVCSPS *sps, const int i, int *off, bool *tile_in_subpic)
>  {
>      int tx, ty, x_end, y_end;
>  
> @@ -477,19 +502,30 @@ static void pps_subpic_slice(VVCPPS *pps, const VVCSPS *sps, const int i, int *o
>  
>      subpic_tiles(&tx, &ty, &x_end, &y_end, sps, pps, i);
>      if (ty + 1 == y_end && sps->r->sps_subpic_height_minus1[i] + 1 < pps->r->row_height_val[ty])
> -        pps_subpic_less_than_one_tile_slice(pps, sps, i, tx, ty, off);
> +        return pps_subpic_less_than_one_tile_slice(pps, sps, i, tx, ty, off, tile_in_subpic);
>      else
> -        pps_subpic_one_or_more_tiles_slice(pps, tx, ty, x_end, y_end, i, off);
> +        return pps_subpic_one_or_more_tiles_slice(pps, tx, ty, x_end, y_end, i, off, tile_in_subpic);
>  }
>  
> -static void pps_single_slice_per_subpic(VVCPPS *pps, const VVCSPS *sps, int *off)
> +static int pps_single_slice_per_subpic(VVCPPS *pps, const VVCSPS *sps, int *off)
>  {
>      if (!sps->r->sps_subpic_info_present_flag) {
>          pps_single_slice_picture(pps, off);
>      } else {
> -        for (int i = 0; i < pps->r->pps_num_slices_in_pic_minus1 + 1; i++)
> -            pps_subpic_slice(pps, sps, i, off);
> +        bool tile_in_subpic[VVC_MAX_TILES_PER_AU] = {0};
> +        for (int i = 0; i < pps->r->pps_num_slices_in_pic_minus1 + 1; i++) {
> +            const int ret = pps_subpic_slice(pps, sps, i, off, tile_in_subpic);
> +            if (ret < 0)
> +                return ret;
> +        }
> +
> +        // We only use tile_in_subpic to check that the subpictures don't overlap
> +        // here; we don't use tile_in_subpic to check that the subpictures cover
> +        // every tile.  It is possible to avoid doing this work here because the
> +        // covering property of subpictures is already guaranteed by the mechanisms
> +        // which check every CTU belongs to a slice.
>      }
> +    return 0;
>  }
>  
>  static int pps_one_tile_slices(VVCPPS *pps, const int tile_idx, int i, int *off)
> @@ -540,8 +576,7 @@ static int pps_rect_slice(VVCPPS *pps, const VVCSPS *sps)
>      int tile_idx = 0, off = 0;
>  
>      if (r->pps_single_slice_per_subpic_flag) {
> -        pps_single_slice_per_subpic(pps, sps, &off);
> -        return 0;
> +        return pps_single_slice_per_subpic(pps, sps, &off);
>      }
>  
>      for (int i = 0; i < r->pps_num_slices_in_pic_minus1 + 1; i++) {

Thanks, LGTM!

-- 
Frank

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