Bug Hunter Diary

Step 1: Find a Sample 4X Movie File with a Valid strk ChunkI used the following to get a sample file from http://samples.mplayerhq.hu/.linux$ wget -q http://samples.mplayerhq.hu/game-formats/4xm/TimeGatep01s01n01a02_2.4xm→After downloading the file, I renamed it original.4xm.Step 2: Learn About the Layout of the strk ChunkAccording to the 4X movie file format description, a strk chunk hasthe following structure:bytes 0-3 fourcc: 'strk'bytes 4-7 length of strk structure (40 or 0x28 bytes)bytes 8-11 track numberbytes 12-15 audio type: 0 = PCM, 1 = 4X IMA ADPCMbytes 16-35 unknownbytes 36-39 number of audio channelsbytes 40-43 audio sample ratebytes 44-47 audio sample resolution (8 or 16 bits)The strk chunk of the downloaded sample file starts at file offset0x1a6, as shown in Figure 4-4:&header[i](1) (2) (3)000001a0h: 32 2E 77 61 76 00 73 74 72 6B 28 00 00 00 00 00 ; 2.wav.strk(.....000001b0h: 00 00 00 00 00 00 00 00 04 00 D1 07 00 00 2F 00 ; ..........Ñ.../.(4)Figure 4-4: A strk chunk from the 4X movie sample file I downloaded. The numbersshown are referenced in Table 4-1.Table 4-1 describes the layout of the strk chunk illustrated inFigure 4-4.Table 4-1: Components of strk Chunk Layout Shown in Figure 4-4Reference Header offset Description(1) &header[i] fourcc: 'strk'(2) &header[i+4] length of strk structure (0x28 bytes)(3) &header[i+8] track number (this is the current_track variable fromFFmpeg source code)(4) &header[i+12] audio type (this is the value that gets written at thefirst dereferenced memory location)NULL Pointer FTW 57