Invalid ID3v2.4.0 frames without synchsafe integers are generated

[Urs Fleisch]

 lrcShow-X_2_1_1.tar.bz2
Ubuntu 13.10
Python 2.7.5+ (default, Feb 27 2014, 19:39:55)
audacious 3.4-1

First thanks for lrcShow-X, finally an application for synchronized lyrics which works on Linux!
lrcShow-X is generating invalid ID3v2.4.0 tags. ID3v2.4.0 stores the frame size as a 32 bit synchsafe integer. ID3v2.3.0 does not use synchsafe integers. lrcShow-X does not seem to use synchsafe integers.

How to reproduce:

# Create an MP3 with silence
dd if=/dev/zero bs=1M count=4 | lame -r - - >test.mp3
# Add an ID3v2.4.0 tag with artist and title
kid3-cli -c "set artist 'Def Leppard'" -c "set title 'Love Bites'" -c save -c to24 test.mp3

Now start lrcShow-X.py, select audacious, play test.mp3, searching on tuneWiki, wait until LRC is found, use "Other functions/Write lrc into media file" to create a SYLT frame.

Test the modified file using "MP3 Diags":
Invalid ID3V2.4.0 frame. Frame size is supposed to be stored as a synchsafe integer, which uses only 7 bits in a byte, but the size uses all 8 bits, as in ID3V2.3.0. This will confuse some applications

xxd test.mp3 | head
0000000: 4944 3304 0000 0000 176d 5450 4531 0000 ID3......mTPE1..
0000010: 000d 0000 0044 6566 204c 6570 7061 7264 .....Def Leppard
0000020: 0054 4954 3200 0000 0c00 0000 4c6f 7665 .TIT2.......Love
0000030: 2042 6974 6573 0053 594c 5400 0009 b600 Bites.SYLT.....
0000040: 0001 656e 6702 01ff fe53 0079 006c 0074 ..eng....S.y.l.t
0000050: 0020 0077 0072 0069 0074 0074 0065 006e . .w.r.i.t.t.e.n
0000060: 0020 0062 0079 0020 006c 0072 0063 0053 . .b.y. .l.r.c.S
0000070: 0068 006f 0077 002d 0049 0049 0000 0049 .h.o.w.-.I.I...I
0000080: 0066 0020 0059 006f 0075 0027 0076 0065 .f. .Y.o.u.'.v.e
0000090: 0020 0047 006f 0074 0020 004c 006f 0076 . .G.o.t. .L.o.v

Here the frame size bytes are 00 00 09 b6, which is not a synchsafe integer, see http://id3.org/id3v2.4.0-structure, sections 4 and 6.2. The correct synchsafe encoding would be 00 00 13 36.

After changing these bytes using a hex editor, "MP3 Diags" does no longer complain.

xxd test-correct.mp3 | head
0000000: 4944 3304 0000 0000 176d 5450 4531 0000 ID3......mTPE1..
0000010: 000d 0000 0044 6566 204c 6570 7061 7264 .....Def Leppard
0000020: 0054 4954 3200 0000 0c00 0000 4c6f 7665 .TIT2.......Love
0000030: 2042 6974 6573 0053 594c 5400 0013 3600 Bites.SYLT...6.
0000040: 0001 656e 6702 01ff fe53 0079 006c 0074 ..eng....S.y.l.t
0000050: 0020 0077 0072 0069 0074 0074 0065 006e . .w.r.i.t.t.e.n
0000060: 0020 0062 0079 0020 006c 0072 0063 0053 . .b.y. .l.r.c.S
0000070: 0068 006f 0077 002d 0049 0049 0000 0049 .h.o.w.-.I.I...I
0000080: 0066 0020 0059 006f 0075 0027 0076 0065 .f. .Y.o.u.'.v.e
0000090: 0020 0047 006f 0074 0020 004c 006f 0076 . .G.o.t. .L.o.v