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

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

[Michael Niedermayer]

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On Thu, Nov 25, 2021 at 06:03:18PM +0100, Michael Niedermayer wrote:
> On Thu, Nov 25, 2021 at 10:51:15AM +0530, Gyan Doshi wrote:
> > 
> > 
> > On 2021-11-25 12:56 am, Michael Niedermayer wrote:
> > > On Wed, Nov 24, 2021 at 10:58:00AM +0530, Gyan Doshi wrote:
> > > > 
> > > > 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).
> > > > > > +
> > > > > > +@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.
> > > > 
> > > If iam not mistaken there is code that adds the values up to initilize some
> > > duration. this works with 64bit so the small negative values interpreted as
> > > unsigned will result in a wrong result i would expect
> > > that may very well not be the only issue
> > 
> > Yeah, I see the stream duration being set in mov_read_stts().
> > Interestingly,  after 153639cb9cf, the unadjusted deltas were being added,
> > which would have included all negative deltas, small or large.
> 
> did you check which variant leads to the correct duration ?
> i naively thought that as it was before was correct but i didnt check
> that so i can very well be wrong
> 
> 
> > 
> > 153639cb9cf  shifted the original negative delta adjustment code from
> > mov_read_stts to mov_build_index. Was that necessary?
> 
> thats andreas commit, 5 years ago, i cannot awnser this 
> 
> 
> > 
> > 
> > > > > the cast is also fragile as it will break when someone tries to change it
> > > > > to int64
> > > > In what circumstances would that happen?
> > > when someone tries to change the code.
> > > having both signed and unsigned 32bit in a 32bit element screams
> > 
> > Originally, negative values never left mov_read_stts. Maybe that's best.
> > 
> > Anyway, FATE passes, so it may only affect edge cases if it does. But I will
> > walk through the code to check.
> > 
> > 
> > > [...]
> > > > > 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.
> > > but what i wrote was about all real world files.
> > > Sure you can make a file that fails but so far the files i have seen and
> > > the ones you told me about all would work with 49% of all possibly defaults
> > > why do you pick a default that doesnt work with them and then argue about
> > > that ?
> > > Lets try to fix the code not win the argument please
> > 
> > I picked the default to keep the old behaviour. 99% of files I see have all
> > valid offsets.
> > Among the rest, large values are mostly errors. Very few are intended.
> > What default do you suggest?
> 
> I would suggest something that treats small negative ones as negative
> signed but leaves all the rest as unsigned. I dont really know what
> "small" should be here but what the file here used was around -10ms
> so if we take a common timebase and 10seconds that should hopefully
> catch all these negative cases and leave >99% of the space for unsigned

Whats the status of this ?
Do you have time and interrest to work on a fix ?
If not this should be reverted until someone has time
I think we should neither break one file to fix one nor should
a per file user action like tuning an option be needed for normal
non crafted files.

Thanks


[...]

-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Concerning the gods, I have no means of knowing whether they exist or not
or of what sort they may be, because of the obscurity of the subject, and
the brevity of human life -- Protagoras

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

[Gyan Doshi]

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.
Values greater than the limit if negative when cast to int32 are used to adjust onward dts.

Unit is the track time scale. Default is UINT_MAX - 48000*10 which
allows upto a 10 second dts correction for 48 kHz audio streams while
accommodating 99.9% of uint32 range.
---

v2 changes:

mp4-negative-stts-problem.mp4 plays in sync with the default value
chosen.
stts adjustment shifted to mov_read_stts so that any downstream code
that references raw stts does not have to contend with possibly changed
stts values.

 libavformat/isom.h |  1 +
 libavformat/mov.c  | 64 +++++++++++++++++++++++++++-------------------
 2 files changed, 39 insertions(+), 26 deletions(-)

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 2aed6e80ef..a36752c6a4 100644
--- a/libavformat/mov.c
+++ b/libavformat/mov.c
@@ -2925,6 +2925,9 @@ static int mov_read_stts(MOVContext *c, AVIOContext *pb, MOVAtom atom)
     unsigned int i, entries, alloc_size = 0;
     int64_t duration = 0;
     int64_t total_sample_count = 0;
+    int64_t current_dts = 0;
+    int64_t last_dts = 0;
+    int64_t dts_correction = 0;
 
     if (c->fc->nb_streams < 1)
         return 0;
@@ -2948,6 +2951,7 @@ static int mov_read_stts(MOVContext *c, AVIOContext *pb, MOVAtom atom)
     for (i = 0; i < entries && !pb->eof_reached; i++) {
         unsigned int sample_duration;
         unsigned int sample_count;
+        unsigned int stts_warn = 0;
         unsigned int min_entries = FFMIN(FFMAX(i + 1, 1024 * 1024), entries);
         MOVStts *stts_data = av_fast_realloc(sc->stts_data, &alloc_size,
                                              min_entries * sizeof(*sc->stts_data));
@@ -2965,13 +2969,41 @@ static int mov_read_stts(MOVContext *c, AVIOContext *pb, MOVAtom atom)
         sc->stts_data[i].count= sample_count;
         sc->stts_data[i].duration= sample_duration;
 
-        av_log(c->fc, AV_LOG_TRACE, "sample_count=%d, sample_duration=%d\n",
+        av_log(c->fc, AV_LOG_TRACE, "sample_count=%u, sample_duration=%u\n",
                 sample_count, sample_duration);
 
-        duration+=(int64_t)sample_duration*(uint64_t)sample_count;
-        total_sample_count+=sample_count;
-    }
+        for (int j = 0; j < sample_count; j++) {
+            /* 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 (sample_duration > c->max_stts_delta) {
+                // assume high delta is a negative correction if greater than c->max_stts_delta
+                int32_t delta_magnitude = *((int32_t *)&sample_duration);
+                av_log_once(c->fc, AV_LOG_WARNING, AV_LOG_DEBUG, &stts_warn,
+                       "Too large sample offset %u in stts entry %u with count %u in st:%d. Clipping to 1.\n",
+                       sample_duration, i, sample_count, st->index);
+                sc->stts_data[i].duration = 1;
+                dts_correction += (delta_magnitude < 0 ? delta_magnitude - 1 : 0);
+            }
+            current_dts += sc->stts_data[i].duration;
+
+            if (!dts_correction || current_dts + dts_correction > last_dts) {
+                current_dts += dts_correction;
+                if (!j)
+                    sc->stts_data[i].duration += dts_correction/sample_count;
+                dts_correction = 0;
+            } else {
+                /* Avoid creating non-monotonous DTS */
+                dts_correction += current_dts - last_dts - 1;
+                current_dts = last_dts + 1;
+            }
+            last_dts = current_dts;
+        }
 
+        duration+=(int64_t)sc->stts_data[i].duration*(uint64_t)sc->stts_data[i].count;
+        total_sample_count+=sc->stts_data[i].count;
+
+    }
     sc->stts_count = i;
 
     if (duration > 0 &&
@@ -3856,13 +3888,10 @@ static void mov_build_index(MOVContext *mov, AVStream *st)
         unsigned int distance = 0;
         unsigned int rap_group_index = 0;
         unsigned int rap_group_sample = 0;
-        int64_t last_dts = 0;
-        int64_t dts_correction = 0;
         int rap_group_present = sc->rap_group_count && sc->rap_group;
         int key_off = (sc->keyframe_count && sc->keyframes[0] > 0) || (sc->stps_count && sc->stps_data[0] > 0);
 
         current_dts -= sc->dts_shift;
-        last_dts     = current_dts;
 
         if (!sc->sample_count || sti->nb_index_entries)
             return;
@@ -3973,26 +4002,8 @@ 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) {
-                    av_log(mov->fc, AV_LOG_WARNING,
-                           "Invalid SampleDelta %d 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;
-                    sc->stts_data[stts_index].duration = 1;
-                }
                 current_dts += sc->stts_data[stts_index].duration;
-                if (!dts_correction || current_dts + dts_correction > last_dts) {
-                    current_dts += dts_correction;
-                    dts_correction = 0;
-                } else {
-                    /* Avoid creating non-monotonous DTS */
-                    dts_correction += current_dts - last_dts - 1;
-                    current_dts = last_dts + 1;
-                }
-                last_dts = current_dts;
+
                 distance++;
                 stts_sample++;
                 current_sample++;
@@ -8577,6 +8588,7 @@ 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 = UINT_MAX-48000*10 }, 0, UINT_MAX, .flags = AV_OPT_FLAG_DECODING_PARAM },
 
     { NULL },
 };
-- 
2.33.0

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

[Michael Niedermayer]

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On Mon, Dec 20, 2021 at 02:01:05PM +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.
> Values greater than the limit if negative when cast to int32 are used to adjust onward dts.
> 
> Unit is the track time scale. Default is UINT_MAX - 48000*10 which
> allows upto a 10 second dts correction for 48 kHz audio streams while
> accommodating 99.9% of uint32 range.
> ---
> 
> v2 changes:
> 
> mp4-negative-stts-problem.mp4 plays in sync with the default value
> chosen.
> stts adjustment shifted to mov_read_stts so that any downstream code
> that references raw stts does not have to contend with possibly changed
> stts values.
> 
>  libavformat/isom.h |  1 +
>  libavformat/mov.c  | 64 +++++++++++++++++++++++++++-------------------
>  2 files changed, 39 insertions(+), 26 deletions(-)

Thanks for looking into this, this patch seems to add another bug though
which is the newly added loop.
Its a few million times too slow.
A sample file which takes a fraction of a second before has not finished
demuxing after an hour
ill mail you the sample privatly


[...]


> +                int32_t delta_magnitude = *((int32_t *)&sample_duration);

a plain cast should be fine with no pointers


[...]

thx

-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Never trust a computer, one day, it may think you are the virus. -- Compn

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