decoding: decoder max should match MAX_INSTR_LENGTH

From derek.br...@gmail.com on February 14, 2009 10:10:53

Today we use a MAX_INSTR_LENGTH define of 17 in various places yet our own decoder does not stop at 17. The two should match so we can allocate a buffer and decode from it. Since it is a conservative estimate the maximum should probably be higher since some processors (at list historically) have allowed 18, 20, and even (so I've heard, never seen) 24-byte instructions. The AMD manual claims 15 is the limit, and 17 used to be the typically assumed limit, but the actual limit at which a processor raises #GP is variable. We shouldn't make it too high though since we don't want to read off onto the next page when not necessary.

Example of our decoder on a 23-byte instr (this raises #GP on my Q9300):

0x080494a8 66 66 66 66 66 66 66 data16 nop 0x00000000(%eax,%eax,1) 66 66 66 66 66 66 66 66 0f 1f 84 00 00 00 00 00

Original issue: http://code.google.com/p/dynamorio/issues/detail?id=9