On Wed, Jun 8, 2011 at 10:32 PM, Ian Lance Taylor <i...@google.com> wrote: > asharif tools <asharif.to...@gmail.com> writes: > >> function: >> call __i686.get_pc_thunk.bx >> addl $_GLOBAL_OFFSET_TABLE_, %ebx >> movl %gs:20, %eax # Stack-guard init >> movl %eax, -12(%ebp) # Stack-guard init > >> Now, what I want to do is move stack guard initialization part >> (consisting of the two instructions I have commented as "Stack-guard >> init" into get_pc_thunk.bx for those functions that have both the >> stack guard and a call to get_pc_thunk.bx. The compiler should >> generate a "stack_guarded_get_pc_thunk.bx" that will do move the >> %gs:20 value to the correction location on the stack instead of >> executing nops. In this way some useful work can be done instead of >> nops. > > I don't understand how you can do that. The offset from %ebp will be > different in different functions. When optimizing, it is likely to be > an offset from %esp instead. The scratch register used may also be > different; consider functions with __attribute__ ((regparm (2))), or the > use of -mregparm=2.
I see. Would it be possible for the caller of stack_protected_get_pc_thunk to pass in this offset from gs in the return register (ebx in this case) in all the cases you described? > > >> As far as I understand, the stack guard part is done in function.c >> (called by cfgexpand.c) which adds the prologue and epilogue to the >> protected functions. At this point, we are still target-independent. >> >> The get_pc_thunk.bx is generated by the target-specific >> config/i386/i386.c part (ix86_file_end). >> >> How should I go about creating such a patch that should: >> >> 1. Check if a function has both stack guard as well as access to a >> global (which would result in having the get_pc_thunk call). >> 2. Remove the stack guard prologue instructions. >> 3. Replace calls to get_pc_thunk.bx in that function to >> stack_guarded_get_pc_thunk. >> >> Specifically, what phase should this be done in? > > If you can do this, do it at final, in the expansion for the stack_set > and set_got insns. > > Ian >
