[FFmpeg-devel] [PATCH v4] avformat/mov: add option max_stts_delta

[Gyan Doshi <ffmpeg@gyani.pro>]

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.
---
v4 change:
  avoid division by 0

 doc/demuxers.texi  |  9 ++++++++
 libavformat/isom.h |  1 +
 libavformat/mov.c  | 55 +++++++++++++++++++++++++---------------------
 3 files changed, 40 insertions(+), 25 deletions(-)

diff --git a/doc/demuxers.texi b/doc/demuxers.texi
index cab8a7072c..99d4ab7bc5 100644
--- a/doc/demuxers.texi
+++ b/doc/demuxers.texi
@@ -713,6 +713,15 @@ 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
+Very high sample deltas written in a trak's stts box 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. Range is 0 to UINT_MAX. Default is @code{UINT_MAX - 48000*10} which allows upto
+a 10 second dts correction for 48 kHz audio streams while accommodating 99.9% of @code{uint32} range.
 @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 2aed6e80ef..351ecde770 100644
--- a/libavformat/mov.c
+++ b/libavformat/mov.c
@@ -2925,6 +2925,8 @@ 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 corrected_dts = 0;
 
     if (c->fc->nb_streams < 1)
         return 0;
@@ -2965,11 +2967,34 @@ 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;
+        /* 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 correction if negative when cast as int32
+            int32_t delta_magnitude = (int32_t)sample_duration;
+            av_log(c->fc, AV_LOG_WARNING, "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;
+            corrected_dts += (delta_magnitude < 0 ? (int64_t)delta_magnitude : 1) * sample_count;
+        } else {
+            corrected_dts += sample_duration * sample_count;
+        }
+
+        current_dts += sc->stts_data[i].duration * sample_count;
+
+        if (current_dts > corrected_dts) {
+            int64_t drift = (current_dts - corrected_dts)/FFMAX(sample_count, 1);
+            uint32_t correction = (sc->stts_data[i].duration > drift) ? drift : sc->stts_data[i].duration - 1;
+            current_dts -= correction * sample_count;
+            sc->stts_data[i].duration -= correction;
+        }
+
+        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;
@@ -3856,13 +3881,10 @@ 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 +3995,8 @@ 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 +8581,7 @@ 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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