This is the mail archive of the
gcc@gcc.gnu.org
mailing list for the GCC project.
Re: Adding fstack-protector prologue to get_pc_thunk for targets with TARGET_PAD_SHORT_FUNCTION
On Wed, Jun 8, 2011 at 10:32 PM, Ian Lance Taylor <iant@google.com> wrote:
> asharif tools <asharif.tools@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
>