[FFmpeg-devel] [PATCH] avcodec/mpeg4_unpack_bframes_bsf: Improve DivX userdata check

[Andreas Rheinhardt]

The earlier version didn't really check that the 'p' of a "p\0" is
actually part of a user_data section, instead it treated the first
"p\0" after the start of a user_data section as end of a user_data
section if it is close enough to the beginning of the user_data section;
it actually needn't be part of a user_data section at all.

Furthermore, the code worked under the assumption that there is a 0x00
after the 'p' although this might not be true for extradata if the
user_data unit is at the end of the extradata.

Both of these flaws have been fixed.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt at googlemail.com>
---
The earlier version was inspired by the check for packed bitstreams in
decode_user_data() in mpeg4videodec.c where the "DivX" is mandatory,
too.
Note that this new version relies on there being no binary zero inside
the user data.
 libavcodec/mpeg4_unpack_bframes_bsf.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/libavcodec/mpeg4_unpack_bframes_bsf.c b/libavcodec/mpeg4_unpack_bframes_bsf.c
index 1daf133ce5..37bb0b20e3 100644
--- a/libavcodec/mpeg4_unpack_bframes_bsf.c
+++ b/libavcodec/mpeg4_unpack_bframes_bsf.c
@@ -41,11 +41,12 @@ static void scan_buffer(const uint8_t *buf, int buf_size,
 
         if (startcode == USER_DATA_STARTCODE && pos_p) {
             /* check if the (DivX) userdata string ends with 'p' (packed) */
-            for (int i = 0; i < 255 && pos + i + 1 < end; i++) {
-                if (pos[i] == 'p' && pos[i + 1] == '\0') {
+            for (int i = 0; i < 255 && pos + i < end; i++) {
+                if (pos[i] == 'p' && (pos + i + 1 == end || pos[i + 1] == '\0')) {
                     *pos_p = pos + i - buf;
                     break;
-                }
+                } else if (pos[i] == 0)
+                    break;
             }
         } else if (startcode == VOP_STARTCODE && nb_vop) {
             *nb_vop += 1;
-- 
2.19.2