Re: [FFmpeg-devel] [PATCH 2/3] avcodec/cbs_h266_syntax_template: sanity check num_multi_layer_olss

[James Almer <jamrial@gmail.com>]

On 1/27/2024 8:56 PM, Michael Niedermayer wrote:
> On Sat, Jan 27, 2024 at 09:25:16AM -0300, James Almer wrote:
>> On 1/26/2024 6:46 PM, Michael Niedermayer wrote:
>>> It is not possible to encode a index into an empty list. Thus
>>> this must be invalid at this point or before.
>>> Its likely a broader earlier check can be used here, someone knowing
>>> VVC should look at that. Its not immedeatly obvious from the spec
>>> by looking for numlayerolss
>>
>> Can you check if the following fixes it?
>>
>>> diff --git a/libavcodec/cbs_h266_syntax_template.c b/libavcodec/cbs_h266_syntax_template.c
>>> index 549d021211..40572dadb5 100644
>>> --- a/libavcodec/cbs_h266_syntax_template.c
>>> +++ b/libavcodec/cbs_h266_syntax_template.c
>>> @@ -793,6 +793,7 @@ static int FUNC(vps) (CodedBitstreamContext *ctx, RWContext *rw,
>>>       {
>>>           //calc NumMultiLayerOlss
>>>           int m;
>>> +        int num_layers_in_ols = 0;
>>>           uint8_t dependency_flag[VVC_MAX_LAYERS][VVC_MAX_LAYERS];
>>>           uint16_t num_output_layers_in_ols[VVC_MAX_TOTAL_NUM_OLSS];
>>>           uint8_t num_sub_layers_in_layer_in_ols[VVC_MAX_TOTAL_NUM_OLSS][VVC_MAX_TOTAL_NUM_OLSS];
>>> @@ -895,7 +896,6 @@ static int FUNC(vps) (CodedBitstreamContext *ctx, RWContext *rw,
>>>                   return AVERROR_INVALIDDATA;
>>>           }
>>>           for (i = 1; i < total_num_olss; i++) {
>>> -            int num_layers_in_ols = 0;
>>>               if (current->vps_each_layer_is_an_ols_flag) {
>>>                   num_layers_in_ols = 1;
>>>               } else if (current->vps_ols_mode_idc == 0 ||
>>
>> num_layers_in_ols is not meant to be reset on every loop.
> 
> replacing my patch by yours does not change
> num_multi_layer_olss from being 0
> and if its 0 then "num_multi_layer_olss - 1" causes problems as a limit
> 
> more precissely this:
> i can also send you the file if you want?

No, this should be looked at by someone more familiar with VVC.
And my patch should be applied either way. The current code is wrong.
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