[FFmpeg-devel] [PATCH 03/10] avformat/matroskadec: Don't make unnecessary assumptions

[Andreas Rheinhardt]

regarding the length of a cluster's size field.

The earlier code relied on the length of clusters always being coded on
eight bytes (as is current Matroska muxer behaviour). But there is no
need to rely on this and this commit changes it.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt at googlemail.com>
---
 libavformat/matroskadec.c | 13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/libavformat/matroskadec.c b/libavformat/matroskadec.c
index 8a14764d1a..88e80b2fda 100644
--- a/libavformat/matroskadec.c
+++ b/libavformat/matroskadec.c
@@ -3695,14 +3695,16 @@ static int webm_clusters_start_with_keyframe(AVFormatContext *s)
     before_pos = avio_tell(s->pb);
     while (1) {
         int64_t cluster_id = 0, cluster_length = 0;
+        int read;
         AVPacket *pkt;
         avio_seek(s->pb, cluster_pos, SEEK_SET);
         // read cluster id and length
-        ebml_read_num(matroska, matroska->ctx->pb, 4, &cluster_id);
-        ebml_read_length(matroska, matroska->ctx->pb, &cluster_length);
-        if (cluster_id != 0xF43B675) { // done with all clusters
+        read = ebml_read_num(matroska, matroska->ctx->pb, 4, &cluster_id);
+        if (read < 0 || cluster_id != 0xF43B675) // done with all clusters
+            break;
+        read = ebml_read_length(matroska, matroska->ctx->pb, &cluster_length);
+        if (read < 0)
             break;
-        }
         avio_seek(s->pb, cluster_pos, SEEK_SET);
         matroska->current_id = 0;
         matroska_clear_queue(matroska);
@@ -3711,7 +3713,8 @@ static int webm_clusters_start_with_keyframe(AVFormatContext *s)
             break;
         }
         pkt = &matroska->queue->pkt;
-        cluster_pos += cluster_length + 12; // 12 is the offset of the cluster id and length.
+        // 4 + read is the length of the cluster id and the cluster length field.
+        cluster_pos += 4 + read + cluster_length;
         if (!(pkt->flags & AV_PKT_FLAG_KEY)) {
             rv = 0;
             break;
-- 
2.19.2