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

[Andreas Rheinhardt]

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 <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.
> 
> 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 0x000001 (and it may not contain a non-byte-aligned
0x000001), 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)?)