[FFmpeg-devel] [PATCH v2] avcodec/hevc_ps: fix the problem of memcmp losing effectiveness

[Wu, Tong1]

>From: ffmpeg-devel <ffmpeg-devel-bounces at ffmpeg.org> On Behalf Of James
>Almer
>Sent: Friday, March 29, 2024 11:18 PM
>To: ffmpeg-devel at ffmpeg.org
>Subject: Re: [FFmpeg-devel] [PATCH v2] avcodec/hevc_ps: fix the problem of
>memcmp losing effectiveness
>
>On 3/29/2024 12:13 PM, tong1.wu-at-intel.com at ffmpeg.org wrote:
>> From: Tong Wu <tong1.wu at 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 at intel.com>
>> ---
>>   libavcodec/hevc_ps.c | 22 ++++++++++++++++++----
>>   libavcodec/hevc_ps.h |  4 +++-
>>   2 files changed, 21 insertions(+), 5 deletions(-)
>>
>> diff --git a/libavcodec/hevc_ps.c b/libavcodec/hevc_ps.c
>> index cbef3ef4cd..d3c589ec24 100644
>> --- a/libavcodec/hevc_ps.c
>> +++ b/libavcodec/hevc_ps.c
>> @@ -449,6 +449,18 @@ 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 && !memcmp(vps1, vps2, offsetof(HEVCVPS,
>hdr)))
>> +        return 1;
>> +
>> +    if (vps1->hdr && vps2->hdr && !memcmp(vps1, vps2, offsetof(HEVCVPS,
>hdr)) &&
>> +        !memcmp(vps1->hdr, vps2->hdr, vps1->vps_num_hrd_parameters *
>sizeof(*vps1->hdr)))
>> +        return 1;
>> +
>> +    return 0;
>> +}
>
>Something like this should be simpler
>
>> +static int compare_vps(const HEVCVPS *vps1, const HEVCVPS *vps2)
>> +{
>> +    if (!memcmp(vps1, vps2, offsetof(HEVCVPS, hdr)))
>> +        return !vps1->vps_num_hrd_parameters ||
>> +               !memcmp(vps1->hdr, vps2->hdr, vps1->vps_num_hrd_parameters *
>sizeof(*vps1->hdr));
>> +
>> +    return 0;
>> +}
>
>If vps1 is equal to vps2 up to hdr, then vps_num_hrd_parameters will be
>the same in both and you can safely just go ahead with the memcmp for hdr.

That makes sense. Thank you.

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