[FFmpeg-devel] [PATCH 07/37] avformat/matroskadec: Set offset of first cluster

[Andreas Rheinhardt]

By default, the data_offset member of the AVFormatInternal of the
AVFormatContext associated with the MatroskaDemuxContext has not been
initialized explicitly by any Matroska-specific function, so that it was
initialized by default to the offset at the end of matroska_read_header,
i.e. usually to the offset of the length field of the first encountered
cluster. This meant that in case that the Matroska-specific seek-code
fails because there are no index entries for the target track a seek to
data_offset would be performed and ordinary parsing would start from
there which is nonsense: The length field would be treated as EBML ID and
(if the length field is not longer than four bytes (EBML numbers that
long are rejected as invalid EBML IDs)) whatever comes next would be
treated as its EBML size although it simply isn't.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt at gmail.com>
---
 libavformat/matroskadec.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/libavformat/matroskadec.c b/libavformat/matroskadec.c
index 927462d3ad..1cda5d5b76 100644
--- a/libavformat/matroskadec.c
+++ b/libavformat/matroskadec.c
@@ -2669,6 +2669,9 @@ static int matroska_read_header(AVFormatContext *s)
         pos = avio_tell(matroska->ctx->pb);
         res = ebml_parse(matroska, matroska_segment, matroska);
     }
+    /* Set data_offset as it might be needed later by seek_frame_generic. */
+    if (matroska->current_id == MATROSKA_ID_CLUSTER)
+        s->internal->data_offset = avio_tell(matroska->ctx->pb) - 4;
     matroska_execute_seekhead(matroska);
 
     if (!matroska->time_scale)
-- 
2.21.0