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

[Nuo Mi <nuomi2021@gmail.com>]

On Tue, Jan 30, 2024 at 8:56 PM Frank Plowman <post@frankplowman.com> wrote:

> 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?
>
Even the specification editor is uncertain about what will be included in
the future.
Perhaps logic later than the point needs a total rewrite.
It's not an error in terms of invalid data; we simply do not know how to
handle it and are asking others to provide the patch.

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