[FFmpeg-devel] [PATCH] avformat/mov: Fix decoding fragmented MP4 with multiple sample entries and empty stsc

[Dimitry Andric]

On 10 Apr 2025, at 11:03, Dimitry Andric <dimitry at unified-streaming.com> wrote:
> 
> On 3 Apr 2025, at 22:02, Dimitry Andric <dimitry at unified-streaming.com> wrote:
>> 
>> When decoding fragmented MP4 files that have an empty stsc box, and
>> instead contain sample description indexes in their tfhd boxes, the mov
>> demuxer does not notify the decoder whenever the current sample
>> description index changes. If the SPS or PPS changed sufficiently, this
>> can lead to unexpected decoding errors.
>> 
>> To fix this, in mov_finalize_packet(), when stsc_data is not available,
>> use get_frag_stream_info_from_pkt() to get at the current fragment
>> stream info, and retrieve the current sample description index from
>> there. Then use that index in a similar manner as the stsc case.
>> 
>> Signed-off-by: Dimitry Andric <dimitry at unified-streaming.com>
>> ---
>> libavformat/mov.c | 50 ++++++++++++++++++++++++++++-------------------
>> 1 file changed, 30 insertions(+), 20 deletions(-)
>> 
>> diff --git a/libavformat/mov.c b/libavformat/mov.c
>> index 452690090c..ead89192f4 100644
>> --- a/libavformat/mov.c
>> +++ b/libavformat/mov.c
>> @@ -10756,25 +10756,29 @@ static int mov_switch_root(AVFormatContext *s, int64_t target, int index)
>>    return 1;
>> }
>> 
>> -static int mov_change_extradata(AVStream *st, AVPacket *pkt)
>> +static int mov_change_extradata(AVStream *st, AVPacket *pkt, int stsd_id)
>> {
>>    MOVStreamContext *sc = st->priv_data;
>>    uint8_t *side, *extradata;
>>    int extradata_size;
>> 
>> -    /* Save the current index. */
>> -    sc->last_stsd_index = sc->stsc_data[sc->stsc_index].id - 1;
>> +    if (stsd_id > 0 &&
>> +        stsd_id - 1 < sc->stsd_count &&
>> +        stsd_id - 1 != sc->last_stsd_index) {
>> +        /* Save the current index. */
>> +        sc->last_stsd_index = stsd_id - 1;
>> 
>> -    /* Notify the decoder that extradata changed. */
>> -    extradata_size = sc->extradata_size[sc->last_stsd_index];
>> -    extradata = sc->extradata[sc->last_stsd_index];
>> -    if (st->discard != AVDISCARD_ALL && extradata_size > 0 && extradata) {
>> -        side = av_packet_new_side_data(pkt,
>> -                                       AV_PKT_DATA_NEW_EXTRADATA,
>> -                                       extradata_size);
>> -        if (!side)
>> -            return AVERROR(ENOMEM);
>> -        memcpy(side, extradata, extradata_size);
>> +        /* Notify the decoder that extradata changed. */
>> +        extradata_size = sc->extradata_size[sc->last_stsd_index];
>> +        extradata = sc->extradata[sc->last_stsd_index];
>> +        if (st->discard != AVDISCARD_ALL && extradata_size > 0 && extradata) {
>> +            side = av_packet_new_side_data(pkt,
>> +                                           AV_PKT_DATA_NEW_EXTRADATA,
>> +                                           extradata_size);
>> +            if (!side)
>> +                return AVERROR(ENOMEM);
>> +            memcpy(side, extradata, extradata_size);
>> +        }
>>    }
>> 
>>    return 0;
>> @@ -10893,13 +10897,10 @@ static int mov_finalize_packet(AVFormatContext *s, AVStream *st, AVIndexEntry *s
>> 
>>    /* Multiple stsd handling. */
>>    if (sc->stsc_data) {
>> -        if (sc->stsc_data[sc->stsc_index].id > 0 &&
>> -            sc->stsc_data[sc->stsc_index].id - 1 < sc->stsd_count &&
>> -            sc->stsc_data[sc->stsc_index].id - 1 != sc->last_stsd_index) {
>> -            int ret = mov_change_extradata(st, pkt);
>> -            if (ret < 0)
>> -                return ret;
>> -        }
>> +        int stsd_id = sc->stsc_data[sc->stsc_index].id;
>> +        int ret = mov_change_extradata(st, pkt, stsd_id);
>> +        if (ret < 0)
>> +            return ret;
>> 
>>        /* Update the stsc index for the next sample */
>>        sc->stsc_sample++;
>> @@ -10908,6 +10909,15 @@ static int mov_finalize_packet(AVFormatContext *s, AVStream *st, AVIndexEntry *s
>>            sc->stsc_index++;
>>            sc->stsc_sample = 0;
>>        }
>> +    } else {
>> +        MOVContext *mov = s->priv_data;
>> +        MOVFragmentStreamInfo *frag_stream_info = get_frag_stream_info_from_pkt(&mov->frag_index, pkt, sc->id);
>> +        if (frag_stream_info) {
>> +            int stsd_id = frag_stream_info->stsd_id;
>> +            int ret = mov_change_extradata(st, pkt, stsd_id);
>> +            if (ret < 0)
>> +                return ret;
>> +        }
>>    }
>> 
>>    return 0;
>> -- 
>> 2.43.0
>> 
> 
> Any comments on this patch?

Ping :)

-Dimitry