Re: [FFmpeg-devel] [PATCH] lavc/vvc: Only read split_cu_flag if a split is allowed

From: Frank Plowman <post@frankplowman.com>
To: ffmpeg-devel@ffmpeg.org
Subject: Re: [FFmpeg-devel] [PATCH] lavc/vvc: Only read split_cu_flag if a split is allowed
Date: Wed, 3 Apr 2024 10:58:58 +0100
Message-ID: <4a2478c3-ca80-4757-a28f-03f4e5e35b12@frankplowman.com> (raw)
In-Reply-To: <20240402214843.474910-1-post@frankplowman.com>

On 02/04/2024 22:48, Frank Plowman wrote:
> Add a check to ensure some split is possible before reading the
> split_cu_flag.  This is present in the spec, in VVCv3 section 7.3.11.4.
> Its omission could lead to infinite loops and ultimately crashing due to
> stack overflow.
> ---
>   libavcodec/vvc/vvc_ctu.c | 7 ++++++-
>   1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/libavcodec/vvc/vvc_ctu.c b/libavcodec/vvc/vvc_ctu.c
> index 8ba12c8d9f..32d8bc8f5c 100644
> --- a/libavcodec/vvc/vvc_ctu.c
> +++ b/libavcodec/vvc/vvc_ctu.c
> @@ -2095,6 +2095,7 @@ static int hls_coding_tree(VVCLocalContext *lc,
>       const int ch_type               = tree_type_curr == DUAL_TREE_CHROMA;
>       int ret;
>       VVCAllowedSplit allowed;
> +    int split_cu_flag;
>   
>       if (pps->r->pps_cu_qp_delta_enabled_flag && qg_on_y && cb_sub_div <= sh->cu_qp_delta_subdiv) {
>           lc->parse.is_cu_qp_delta_coded = 0;
> @@ -2109,7 +2110,11 @@ static int hls_coding_tree(VVCLocalContext *lc,
>   
>       can_split(lc, x0, y0, cb_width, cb_height, mtt_depth, depth_offset, part_idx,
>           last_split_mode, tree_type_curr, mode_type_curr, &allowed);
> -    if (ff_vvc_split_cu_flag(lc, x0, y0, cb_width, cb_height, ch_type, &allowed)) {
> +    if (allowed.btv || allowed.bth || allowed.ttv || allowed.tth || allowed.qt)
> +        split_cu_flag = ff_vvc_split_cu_flag(lc, x0, y0, cb_width, cb_height, ch_type, &allowed);
> +    else
> +        split_cu_flag = 0;
> +    if (split_cu_flag) {
>           VVCSplitMode split      = ff_vvc_split_mode(lc, x0, y0, cb_width, cb_height, cqt_depth, mtt_depth, ch_type, &allowed);
>           VVCModeType mode_type   = mode_type_decode(lc, x0, y0, cb_width, cb_height, split, ch_type, mode_type_curr);
>   

Retracting this patch as I missed that this logic is in fact 
implemented, just elsewhere.  There is still a bug here, but it seems 
the condition to trigger it is more complex that I thought.  Should have 
an alternative patch soon.
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