[FFmpeg-devel] [PATCH v2] avformat/mov: add option max_stts_delta

[Gyan Doshi]

On 2021-11-24 01:16 am, Michael Niedermayer wrote:
> On Tue, Nov 23, 2021 at 06:41:06PM +0530, Gyan Doshi wrote:
>> Very high stts sample deltas may occasionally be intended but usually
>> they are written in error or used to store a negative value for dts correction
>> when treated as signed 32-bit integers.
>>
>> This option lets the user set an upper limit, beyond which the delta
>> is clamped to 1. Negative values of under 1 second are used to adjust
>> dts.
>>
>> Unit is the track time scale. Default is INT_MAX which maintains current handling.
>> ---
>>   doc/demuxers.texi  |  6 ++++++
>>   libavformat/isom.h |  1 +
>>   libavformat/mov.c  | 14 +++++++++-----
>>   3 files changed, 16 insertions(+), 5 deletions(-)
>>
>> diff --git a/doc/demuxers.texi b/doc/demuxers.texi
>> index cab8a7072c..15078b9b1b 100644
>> --- a/doc/demuxers.texi
>> +++ b/doc/demuxers.texi
>> @@ -713,6 +713,12 @@ specify.
>>   
>>   @item decryption_key
>>   16-byte key, in hex, to decrypt files encrypted using ISO Common Encryption (CENC/AES-128 CTR; ISO/IEC 23001-7).
>> +
>> + at item max_stts_delta
>> +The sample offsets stored in a track's stts box are 32-bit unsigned integers. However, very large values usually indicate
>> +a value written by error or a storage of a small negative value as a way to correct accumulated DTS delay.
>> +Range is 0 to UINT_MAX. Default is INT_MAX.
>> +
>>   @end table
>>   
>>   @subsection Audible AAX
>> diff --git a/libavformat/isom.h b/libavformat/isom.h
>> index ef8f19b18c..625dea8421 100644
>> --- a/libavformat/isom.h
>> +++ b/libavformat/isom.h
>> @@ -305,6 +305,7 @@ typedef struct MOVContext {
>>       int32_t movie_display_matrix[3][3]; ///< display matrix from mvhd
>>       int have_read_mfra_size;
>>       uint32_t mfra_size;
>> +    uint32_t max_stts_delta;
>>   } MOVContext;
>>   
>>   int ff_mp4_read_descr_len(AVIOContext *pb);
>> diff --git a/libavformat/mov.c b/libavformat/mov.c
>> index 451cb78bbf..bbda07ac42 100644
>> --- a/libavformat/mov.c
>> +++ b/libavformat/mov.c
>> @@ -3965,14 +3965,17 @@ static void mov_build_index(MOVContext *mov, AVStream *st)
>>                   current_offset += sample_size;
>>                   stream_size += sample_size;
>>   
>> -                /* A negative sample duration is invalid based on the spec,
>> -                 * but some samples need it to correct the DTS. */
>> -                if (sc->stts_data[stts_index].duration < 0) {
>> +                /* STTS sample offsets are uint32 but some files store it as int32
>> +                 * with negative values used to correct DTS delays.
>> +                   There may be abnormally large values as well. */
>> +                if (sc->stts_data[stts_index].duration > mov->max_stts_delta) {
>> +                    // assume high delta is a negative correction if less than 1 second
>> +                    int32_t delta_magnitude = *((int32_t *)&sc->stts_data[stts_index].duration);
>>                       av_log(mov->fc, AV_LOG_WARNING,
>> -                           "Invalid SampleDelta %d in STTS, at %d st:%d\n",
>> +                           "Correcting too large SampleDelta %u in STTS, at %d st:%d.\n",
>>                              sc->stts_data[stts_index].duration, stts_index,
>>                              st->index);
>> -                    dts_correction += sc->stts_data[stts_index].duration - 1;
>> +                    dts_correction += (delta_magnitude < 0 && FFABS(delta_magnitude) < sc->time_scale ? delta_magnitude - 1 : 0);
>>                       sc->stts_data[stts_index].duration = 1;
>>                   }
>>                   current_dts += sc->stts_data[stts_index].duration;
>> @@ -8566,6 +8569,7 @@ static const AVOption mov_options[] = {
>>       { "decryption_key", "The media decryption key (hex)", OFFSET(decryption_key), AV_OPT_TYPE_BINARY, .flags = AV_OPT_FLAG_DECODING_PARAM },
>>       { "enable_drefs", "Enable external track support.", OFFSET(enable_drefs), AV_OPT_TYPE_BOOL,
>>           {.i64 = 0}, 0, 1, FLAGS },
>> +    { "max_stts_delta", "treat offsets above this value as invalid", OFFSET(max_stts_delta), AV_OPT_TYPE_INT, {.i64 = INT_MAX}, 0, UINT_MAX, .flags = AV_OPT_FLAG_DECODING_PARAM },
>>   
>>       { NULL },
>>   };
> The stts is used in other places, the value parsed as unsigned will cause
> problems there too
I see stts duration used in 3 places in mov.c

mov_read_stts(), which my earlier patch changed.

mov_build_index(), which this patch changes,

mov_read_trak() where frame rate is populated for CFR streams and is 
called very shortly after mov_build_index().
I don't think that can break for non-malformed files as the only 
duration entry in the stream is not likely to be > INT_MAX
In any case, after this patch, with default option value, it will see 
the same values as earlier.

> the cast is also fragile as it will break when someone tries to change it
> to int64

In what circumstances would that happen?

The syntax of stts sample_delta has been unchanged since the start.
I see  'unsigned int(32) sample_delta;' in the 2005 2nd ed.
Same in the 2015 5th ed.

> FFABS(delta_magnitude) is also undefined for INT_MIN

Will add a check for that.

> also please select the default so that it works with all real world files

No single value can all work for all files. Hence the option.

The standard says this,

"The time to sample boxes must give non-zero durations for all samples 
with the possible exception of the last
one. Durations in the ‘stts’ box are strictly positive (non-zero), 
except for the very last entry, which may be zero"

This part is normative. mp4-negative-stts-problem is out-of-spec by 
using signed stts deltas. It uses 'mp42' for ftyp and 'isom' as a 
compatible brand.

The default value emulates pre-patch handling. So my in-spec file 
wouldn't parse well, but users would have the ability to allow it.
FATE passes.

My file had slightly different offsets of 7 hrs in both audio and video 
tracks with a time scale of 90000 in both tracks, recorded using Wowza.

(The stts delta is not a media packet duration. It is a decoding interval.
"The decoding time is defined in the Decoding Time to Sample Box, giving 
time deltas between successive decoding times.")

This is not an academic exercise.

Regards,
Gyan