Re: [FFmpeg-devel] [PATCH] avcodec/mpeg4_unpack_bframes_bsf: Improve DivX userdata check
Michael Niedermayer: > On Mon, Mar 11, 2019 at 12:36:08PM +0100, Andreas Rheinhardt wrote: >> 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 >> --- >> 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. > > Is this based on anything from the spec ? No. According to the spec, a user_data ends upon the next occurence of a byte-aligned 0x01 (and it may not contain a non-byte-aligned 0x01), so it is allowed to have a binary zero in there. But I thought that a user_data for indicating a packed bitstream is a string, so shouldn't contain a binary zero. I can modify this if desired. (If so, does a "p\0" where the 'p' is part of the user_data be taken to indicate a packed bitstream or does the 'p' have to be the last nonzero character of the user_data?) (And shouldn't the same logic also be applied to the decoder (that currently requires the "DivX" to conclude that a stream is a packed bitstream)?) ___ ffmpeg-devel mailing list ffmpeg-devel@ffmpeg.org https://ffmpeg.org/mailman/listinfo/ffmpeg-devel
Re: [FFmpeg-devel] [PATCH] avcodec/mpeg4_unpack_bframes_bsf: Improve DivX userdata check
On Mon, Mar 11, 2019 at 12:36:08PM +0100, Andreas Rheinhardt wrote: > 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 > --- > 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. Is this based on anything from the spec ? [...] -- Michael GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB Its not that you shouldnt use gotos but rather that you should write readable code and code with gotos often but not always is less readable signature.asc Description: PGP signature ___ ffmpeg-devel mailing list ffmpeg-devel@ffmpeg.org https://ffmpeg.org/mailman/listinfo/ffmpeg-devel
[FFmpeg-devel] [PATCH] avcodec/mpeg4_unpack_bframes_bsf: Improve DivX userdata check
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
---
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
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