On Fri, Oct 14, 2005 at 01:43:03PM -0700, Kean Johnston wrote: > Also, when you say "stack going into main is 16 byte aligned", > what specifically do you mean? that its 16-byte aligned before > the call to main() itself? That at the first insn in main, most > likely a push %ebp, its 16-byte aligned (i.e does the call > to main from crt1.o have to take the push of the return address > into account)?
The stack alignment is computed before the saved-EIP is pushed on the stack by a CALL instruction. So on function entry, the ESP has already been decremented by 4 off of its 16-byte alignment. Conventioanlly the EBP is pushed, making the ESP 8 bytes off its 16-byte alignment. If your ABI does not require 16-byte stack frame alignment, aligning it correctly in main() will not fix all the problems unless you can recompile all of the code (and fix all the hand-written assembly) on the entire system. If you're 16-byte aligned and you call into a library that only requires 4-byte alignment (the traditional SysV x86 ABI--Pinski says it's been updated to require 16-byte alignment but I don't know when that happened) and that library function calls into a newly-gcc-recompiled function, you can crash over there because of a misaligned operation. J
