Re: [PATCH 2/5] gcc-plugins/stackleak: Use asm instrumentation to avoid useless register saving

Kees Cook
2020-06-09T18:46:43Z

On Thu, Jun 04, 2020 at 04:49:54PM +0300, Alexander Popov wrote:
> Let's improve the instrumentation to avoid this:
>
> 1. Make stackleak_track_stack() save all register that it works with.
> Use no_caller_saved_registers attribute for that function. This attribute
> is available for x86_64 and i386 starting from gcc-7.
>
> 2. Insert calling stackleak_track_stack() in asm:
> asm volatile("call stackleak_track_stack" :: "r" (current_stack_pointer))
> Here we use ASM_CALL_CONSTRAINT trick from arch/x86/include/asm/asm.h.
> The input constraint is taken into account during gcc shrink-wrapping
> optimization. It is needed to be sure that stackleak_track_stack() call is
> inserted after the prologue of the containing function, when the stack
> frame is prepared.

Very cool; nice work!

> +static void add_stack_tracking(gimple_stmt_iterator *gsi)
> +{
> + /*
> + * The 'no_caller_saved_registers' attribute is used for
> + * stackleak_track_stack(). If the compiler supports this attribute for
> + * the target arch, we can add calling stackleak_track_stack() in asm.
> + * That improves performance: we avoid useless operations with the
> + * caller-saved registers in the functions from which we will remove
> + * stackleak_track_stack() call during the stackleak_cleanup pass.
> + */
> + if (lookup_attribute_spec(get_identifier("no_caller_saved_registers")))
> + add_stack_tracking_gasm(gsi);
> + else
> + add_stack_tracking_gcall(gsi);
> +}

The build_for_x86 flag is only ever used as an assert() test against
no_caller_saved_registers, but we're able to test for that separately.
Why does the architecture need to be tested? (i.e. when this flag
becomes supported o other architectures, why must it still be x86-only?)

--
Kees Cook