Re: Adding fstack-protector prologue to get_pc_thunk for targets with TARGET_PAD_SHORT_FUNCTION

[Ian Lance Taylor <iant at google dot com>]

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.


> 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