On Mon, Mar 5, 2012 at 4:21 PM, Kevin O'Connor kevin@koconnor.net wrote:
On Mon, Mar 05, 2012 at 04:05:11PM +0000, Julian Pidancet wrote:
Well, it is not a very elegant solution, but it seems to be the best plan we have so far.
I can see two problems:
- If you look at the patch I tried to submit to xorg-devel. Other
instructions are concerned, in particular some forms of call (opcode 0xFF). Which means that if we decide to write a postprocess tool, we'll have to check that the ROM doesn't use those instructions.
Agreed. If it's just "calll *%ereg" then it can probably just be replaced with "pushw $0 ; callw *%reg".
These forms of the call instruction must also be taken care of:
66 ff 16 34 12 calll *0x1234 2e 66 ff 16 34 12 calll *%cs:0x1234
- Replacing instructions in the binary is simple, as long as the new
instruction is the same size as the replaced instruction.
66 c3 retl (2 bytes) c2 02 00 ret $0x2 (3 bytes)
66 c9 leavel (2 bytes) 66 89 ec mov %ebp,%esp (3 bytes) 66 5d pop %ebp (1 bytes)
Replacing instructions and handling displacement is probably going to be a huge pain.
I don't think that will be an issue. One can tell gcc to generate assembler and then post-process that. The gcc created assembler is still label based so no positional issues should arise.
Yes you're right. Post-processing the intermediate assembly will definitely be a huge win.