[FFmpeg-cvslog] avformat/flvdec: check FLVHeader PreviousTagSize0

[Steven Liu]

ffmpeg | branch: master | Steven Liu <lq at chinaffmpeg.org> | Tue Mar 28 16:51:28 2017 +0800| [c0628919b8c5761d64b1169e8de7584544d15ebf] | committer: Steven Liu

avformat/flvdec: check FLVHeader PreviousTagSize0

refer to SPEC:
Annex E. The FLV File Format said:
E.3 TheFLVFileBody have a table:
Field            Type    Comment
PreviousTagSize0 UI32    Always 0

Reviewed-by: Bela Bodecs <bodecsb at vivanet.hu>
Reviewed-by: Hendrik Leppkes <h.leppkes at gmail.com>
Signed-off-by: Steven Liu <lq at chinaffmpeg.org>

> http://git.videolan.org/gitweb.cgi/ffmpeg.git/?a=commit;h=c0628919b8c5761d64b1169e8de7584544d15ebf
---

 libavformat/flvdec.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/libavformat/flvdec.c b/libavformat/flvdec.c
index cdcfb9c..94c9e28 100644
--- a/libavformat/flvdec.c
+++ b/libavformat/flvdec.c
@@ -709,6 +709,7 @@ static int flv_read_header(AVFormatContext *s)
     int flags;
     FLVContext *flv = s->priv_data;
     int offset;
+    int pre_tag_size = 0;
 
     avio_skip(s->pb, 4);
     flags = avio_r8(s->pb);
@@ -719,7 +720,16 @@ static int flv_read_header(AVFormatContext *s)
 
     offset = avio_rb32(s->pb);
     avio_seek(s->pb, offset, SEEK_SET);
-    avio_skip(s->pb, 4);
+
+    /* Annex E. The FLV File Format
+     * E.3 TheFLVFileBody
+     *     Field               Type    Comment
+     *     PreviousTagSize0    UI32    Always 0
+     * */
+    pre_tag_size = avio_rb32(s->pb);
+    if (pre_tag_size) {
+        av_log(s, AV_LOG_WARNING, "Read FLV header error, input file is not a standard flv format, first PreviousTagSize0 always is 0\n");
+    }
 
     s->start_time = 0;
     flv->sum_flv_tag_size = 0;