Re: [FFmpeg-devel] [PATCH] lavc/vvc: Add check to num_multi_layer_olss

[Frank Plowman <post@frankplowman.com>]

On 30/01/2024 12:31, Nuo Mi wrote:

> On Tue, Jan 30, 2024 at 5:41 PM<post@frankplowman.com>  wrote:
>> From: Frank Plowman<post@frankplowman.com>
>>
>> Check that vps_each_layer_is_an_ols_flag, which indicates that "at
>> least one OLS specified by the VPS contains more than one layer," is
>> set if num_multi_layer_olss is non-zero.
>>
>> Fixes:
>> 65160/clusterfuzz-testcase-minimized-ffmpeg_BSF_VVC_METADATA_fuzzer-4665241535119360
>>
>> Found-by: continuous fuzzing process
>> https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
>> Signed-off-by
>> <https://github.com/google/oss-fuzz/tree/master/projects/ffmpegSigned-off-by>:
>> Frank Plowman<post@frankplowman.com>
>> ---
>>   libavcodec/cbs_h266_syntax_template.c | 2 ++
>>   1 file changed, 2 insertions(+)
>>
>> diff --git a/libavcodec/cbs_h266_syntax_template.c
>> b/libavcodec/cbs_h266_syntax_template.c
>> index 2f3478e5e1..37dc3acba0 100644
>> --- a/libavcodec/cbs_h266_syntax_template.c
>> +++ b/libavcodec/cbs_h266_syntax_template.c
>> @@ -911,6 +911,8 @@ static int FUNC(vps) (CodedBitstreamContext *ctx,
>> RWContext *rw,
>>                   num_multi_layer_olss++;
>>               }
>>           }
>> +        if (!current->vps_each_layer_is_an_ols_flag &&
>> num_multi_layer_olss == 0)
>> +            return AVERROR_INVALIDDATA;
>>       }
> The specification does not provide information on how to obtain
> TotalNumOlss (total_num_olss) when ols_mode_idc is set to 3.
> Therefore, the earlier line "u(8, vps_num_ptls_minus1, 0, total_num_olss -
> 1)" is undefined.
> We'd better return a patch welcome error instead of printing a warning
> before vps_num_ptls_minus1 line

This is the same behaviour James suggested in an earlier patch. The spec 
says "decoders conforming to this version of this Specification shall 
ignore the OLSs with vps_ols_mode_idc equal to 3." I don't think this 
should be an error as the spec is unambiguous here. Perhaps we can 
instead skip the remainder of the VPS if vps_ols_mode_idc is 3? Or is 
there some better way to ignore these OLSs?
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