Re: [FFmpeg-devel] [PATCH] avformat/movenc: Check that cts fits in 32bit

[Michael Niedermayer <michael@niedermayer.cc>]

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On Sun, Mar 31, 2024 at 01:30:26PM -0300, James Almer wrote:
> On 3/31/2024 8:40 AM, Michael Niedermayer wrote:
> > Fixes: Assertion av_rescale_rnd(start_dts, mov->movie_timescale, track->timescale, AV_ROUND_DOWN) <= 0 failed at libavformat/movenc.c:3694
> > Fixes: poc2
> > 
> > Found-by: Wang Dawei and Zhou Geng, from Zhongguancun Laboratory
> > Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
> > ---
> >   libavformat/movenc.c | 6 ++++++
> >   1 file changed, 6 insertions(+)
> > 
> > diff --git a/libavformat/movenc.c b/libavformat/movenc.c
> > index ae94d8d5959..5617a2620c5 100644
> > --- a/libavformat/movenc.c
> > +++ b/libavformat/movenc.c
> > @@ -6194,6 +6194,12 @@ int ff_mov_write_packet(AVFormatContext *s, AVPacket *pkt)
> >       if (ret < 0)
> >           return ret;
> > +    if (pkt->pts != AV_NOPTS_VALUE &&
> > +        (uint64_t)pkt->dts - pkt->pts != (int32_t)((uint64_t)pkt->dts - pkt->pts)) {
> > +        av_log(s, AV_LOG_WARNING, "pts/dts pair unsupported\n");
> > +        return AVERROR_PATCHWELCOME;
> > +    }
> 
> Any such check should happen in check_pkt(), called directly above. And
> afaict there's no reason to not support 64bit cts. Even in
> mov_write_edts_tag() we check for it and write a version 1 of the box that
> supports 64bit values.
> 
> Maybe the problem is that MOVIentry.cts is an int, when it should be an
> int64_t like start_cts? Can you test the following?

changing cts to 64bit does avoid the assert with the test sample but

If you chaneg cts to 64bit consider cts is assigned to MOVCtts duration (32bit)
in mov_write_ctts_tag() and also compared to it.
its also written with avio_wb32() later

theres also
avio_wb32(pb, track->cluster[i].cts);
in mov_write_trun_tag()

so the suggestion would avoid the assert but the code is not correct, now one can
maybe add support for this by switching to 64bit variants. But this needs to be
backported too.
and "64bit cts support" seems not ideal both because of complexity and beacause
its not a bugfix

I would prefer to keep a simple (easy backportable) bugfix for the releases.

thx

[...]

-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

What does censorship reveal? It reveals fear. -- Julian Assange

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