Re: [FFmpeg-devel] [PATCH] avcodec/hevc_ps: fix the problem of memcmp losing effectiveness

[James Almer <jamrial@gmail.com>]

On 3/28/2024 10:15 AM, tong1.wu-at-intel.com@ffmpeg.org wrote:
> From: Tong Wu <tong1.wu@intel.com>
> 
> HEVCHdrParams* receives a pointer which points to a dynamically
> allocated memory block. It causes the memcmp always returning 1.
> Add a function to do the comparision. A condition is also added to
> avoid malloc(0).
> 
> Signed-off-by: Tong Wu <tong1.wu@intel.com>
> ---
>   libavcodec/hevc_ps.c | 20 ++++++++++++++++----
>   libavcodec/hevc_ps.h |  4 +++-
>   2 files changed, 19 insertions(+), 5 deletions(-)
> 
> diff --git a/libavcodec/hevc_ps.c b/libavcodec/hevc_ps.c
> index cbef3ef4cd..8b3a27a17c 100644
> --- a/libavcodec/hevc_ps.c
> +++ b/libavcodec/hevc_ps.c
> @@ -449,6 +449,16 @@ static void uninit_vps(FFRefStructOpaque opaque, void *obj)
>       av_freep(&vps->hdr);
>   }
>   
> +static int compare_vps(const HEVCVPS *vps1, const HEVCVPS *vps2)
> +{
> +    if ((!vps1->hdr && !vps2->hdr) ||
> +        (vps1->hdr && vps2->hdr && !memcmp(vps1->hdr, vps2->hdr, sizeof(*vps1->hdr)))) {

I think this should be vps1->vps_num_hrd_parameters * 
sizeof(*vps1->hdr), and done after the memcmp below to ensure 
vps_num_hrd_parameters is the same value in both structs.

> +        return !memcmp(vps1, vps2, offsetof(HEVCVPS, hdr));
> +    }
> +
> +    return 0;
> +}
> +
>   int ff_hevc_decode_nal_vps(GetBitContext *gb, AVCodecContext *avctx,
>                              HEVCParamSets *ps)
>   {
> @@ -545,9 +555,11 @@ int ff_hevc_decode_nal_vps(GetBitContext *gb, AVCodecContext *avctx,
>               goto err;
>           }
>   
> -        vps->hdr = av_calloc(vps->vps_num_hrd_parameters, sizeof(*vps->hdr));
> -        if (!vps->hdr)
> -            goto err;
> +        if (vps->vps_num_hrd_parameters) {
> +            vps->hdr = av_calloc(vps->vps_num_hrd_parameters, sizeof(*vps->hdr));
> +            if (!vps->hdr)
> +                goto err;
> +        }
>   
>           for (i = 0; i < vps->vps_num_hrd_parameters; i++) {
>               int common_inf_present = 1;
> @@ -569,7 +581,7 @@ int ff_hevc_decode_nal_vps(GetBitContext *gb, AVCodecContext *avctx,
>       }
>   
>       if (ps->vps_list[vps_id] &&
> -        !memcmp(ps->vps_list[vps_id], vps, sizeof(*vps))) {
> +        compare_vps(ps->vps_list[vps_id], vps)) {
>           ff_refstruct_unref(&vps);
>       } else {
>           remove_vps(ps, vps_id);
> diff --git a/libavcodec/hevc_ps.h b/libavcodec/hevc_ps.h
> index cc75aeb8d3..0d8eaf2b3e 100644
> --- a/libavcodec/hevc_ps.h
> +++ b/libavcodec/hevc_ps.h
> @@ -153,7 +153,6 @@ typedef struct PTL {
>   
>   typedef struct HEVCVPS {
>       unsigned int vps_id;
> -    HEVCHdrParams *hdr;
>   
>       uint8_t vps_temporal_id_nesting_flag;
>       int vps_max_layers;
> @@ -175,6 +174,9 @@ typedef struct HEVCVPS {
>   
>       uint8_t data[4096];
>       int data_size;
> +    /* Put this at the end of the structure to make it easier to calculate the
> +     * size before this pointer, which is used for memcmp */
> +    HEVCHdrParams *hdr;
>   } HEVCVPS;
>   
>   typedef struct ScalingList {
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