[FFmpeg-devel] [PATCH] MP3: ID3V2 corrupted => bad offset

[Yvan Labadie]

Hi,

According to the ID3v2 standard, frame sizes are stored using 32bit 
SynchSafe Integer format 
(http://www.id3.org/id3v2.4.0-structure?highlight=%28SynchSafe%29).
That's why libavformat/mp3.c uses id3v2_get_size() function.
But I discovered that a lot of mp3 (about a fourth of my mp3s!) use 
classic 32bit unsigned integer!!!
So with these mp3s ID3v2 parse fails and then 
AVFormatContext->data_offset is invalid!

and having a bad data_offset can be very critical for some applications 
(like mine 0:-) ).

For my patch, I use the ID3v2 size extracted from its header to deduce 
the offset.
Then after parsing ID3 I seek to this offset and I made another check by 
verifying that it corresponds to a mp3 frame sync, and if it doesn't I 
look for the first frame sync and made it the offset.

if id3 tag is corrupted, result is :
data_offset is good (so mp3_parse_vbr_tags success)
tag parse still fail but if file doesn't respect standards, we cannot do 
a lot...


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