[FFmpeg-devel] [PATCH v3] avformat/rtpdec_rfc4175: Fix incorrect copy_offset calculation

Jacob Siddall kobe at live.com.au
Thu Jun 27 09:06:22 EEST 2019

The previous calculation code did not account for the fact that the
copy_offset for the start of the frame array is at index 0, yet the
scan line number from the rfc4175 RTP header starts at 1.
This caused 2 issues to appear:
- The first scan line was being copied into the array where the second
  scan line should be. This caused the resulting video to have a green
  line at the top of it.
- Since the packet containing the last scan line would fail the
  calculation, the packet with the RTP marker would not be processed
  which caused a log message saying "Missed previous RTP marker" to be
  outputted for each frame.

Signed-off-by: Jacob Siddall <kobe at live.com.au>
Changes in v2:
  - Don't handle packet if the line number is less than 1

Section 12 in the VSF technical recommendation TR-03 specifies that the
video scan line numbers should start at 1.

Changes in v3:
  - Changed the commit hash abbreviation in the patch file diff to be 10
    characters in length rather than 7. This was causing the patch file
    to fail when it was applied. 

 libavformat/rtpdec_rfc4175.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/libavformat/rtpdec_rfc4175.c b/libavformat/rtpdec_rfc4175.c
index e9c62c1389..490db87520 100644
--- a/libavformat/rtpdec_rfc4175.c
+++ b/libavformat/rtpdec_rfc4175.c
@@ -205,8 +205,11 @@ static int rfc4175_handle_packet(AVFormatContext *ctx, PayloadContext *data,
         if (length > payload_len)
             length = payload_len;
+        if (line < 1)
+            return AVERROR_INVALIDDATA;
         /* prevent ill-formed packets to write after buffer's end */
-        copy_offset = (line * data->width + offset) * data->pgroup / data->xinc;
+        copy_offset = ((line - 1) * data->width + offset) * data->pgroup / data->xinc;
         if (copy_offset + length > data->frame_size)
             return AVERROR_INVALIDDATA;

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