Re: [FFmpeg-devel] [PATCH v3 1/4] avformat/movenc: fix assert failure in get_cluster_duration()

["\"zhilizhao(赵志立)\"" <quinkblack@foxmail.com>]

> On Mar 1, 2022, at 9:32 PM, zhilizhao(赵志立) <quinkblack@foxmail.com> wrote:
> 
> 
>> On Feb 27, 2022, at 2:49 PM, Gyan Doshi <ffmpeg@gyani.pro> wrote:
>> 
>> 
>> 
>> On 2022-02-27 12:04 pm, "zhilizhao(赵志立)" wrote:
>>> Ping.
>>> 
>>>> On Dec 31, 2021, at 7:36 PM, Zhao Zhili <quinkblack@foxmail.com> wrote:
>>>> 
>>>> When editlist is disabled, the workaournd method of shift dts to
>>>> zero and increase the first sample duration doesn't work if the
>>>> timestamp is larger than mp4 spec restriction (e.g., sample_delta
>>>> in stts entry). Further more, it triggers get_cluster_duration()
>>>> assert failure. This patch will drop large offsets between
>>>> multiple tracks.
>>>> ---
>>>> libavformat/movenc.c | 13 ++++++++++++-
>>>> 1 file changed, 12 insertions(+), 1 deletion(-)
>>>> 
>>>> diff --git a/libavformat/movenc.c b/libavformat/movenc.c
>>>> index 0f912dd012..f5bb785b01 100644
>>>> --- a/libavformat/movenc.c
>>>> +++ b/libavformat/movenc.c
>>>> @@ -5917,7 +5917,18 @@ int ff_mov_write_packet(AVFormatContext *s, AVPacket *pkt)
>>>>         * to signal the difference in starting time without an edit list.
>>>>         * Thus move the timestamp for this first sample to 0, increasing
>>>>         * its duration instead. */
>>>> -        trk->cluster[trk->entry].dts = trk->start_dts = 0;
>>>> +        if (pkt->dts + pkt->duration <= INT32_MAX) {
>>>> +            trk->cluster[trk->entry].dts = trk->start_dts = 0;
>>>> +        } else {
>>>> +            /* Impossible to write a sample duration >= UINT32_MAX.
>> 
>> As per 14496-12 (2005), sample_delta is stored as
>> 
>> unsigned int(32) sample_delta;
>> 
>> The only restriction is that no delta shall be zero except the last sample, which means UINT32_MAX  is allowed.
>> INT32_MAX had been set as max allowed delta in mov.c since inception. See my recent changes in mov.c correcting that.
>> 
>> Will this patch break remuxing of files with large deltas?
>> Let's not clamp if the spec does not call for it.
> 
> Since get_cluster_duration() checks dts against INT_MAX, remove
> the clamp at here can still trigger abort() after.
> 
> ```
>    av_assert0(next_dts <= INT_MAX);
> 
> ```
> 
> The prototype of get_cluster_duration() which returns int needs
> to be fixed, and a bunch of places.
> 
> And then we can get regression issues: movenc.c output files with
> delta larger than INT32_MAX can be dangerous, which is not the
> case of mov.c demuxer.
> 
> It’s a clever trick here, by using a large sample duration to
> handle start time difference between multiple tracks. Since it’s
> not a standard method like editlist, I think we can turn off the
> strategy safely (FFmpeg abort before the patch, so it’s already
> turned off in a hard way).
> 
> What do you think?

Ping. Another idea is return error instead of abort().

> 
>> 
>> Regards,
>> Gyan
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