Re: [RFC 00/15] x86_64: Optimize percpu accesses

[ebiederm@xmission.com (Eric W. Biederman)]

Jeremy Fitzhardinge <jeremy@goop.org> writes:

> It's just the stack canary.  It isn't library accesses; it's the code gcc
> generates:
>
> foo:	subq	$152, %rsp
>        movq    %gs:40, %rax
>        movq    %rax, 136(%rsp)
> ...
>        movq    136(%rsp), %rdx
>        xorq    %gs:40, %rdx
>        je      .L3
>        call    __stack_chk_fail
> .L3:
>        addq    $152, %rsp
>        .p2align 4,,4
>        ret
>
>
> There are two irritating things here:
>
> One is that the kernel supports -fstack-protector for x86-64, which forces us
> into all these contortions in the first place.  We don't support stack-protector
> for 32-bit (gcc does), and things are much easier.

How does gcc know to use %gs instead of the usual %fs for accessing
the stack protector variable?  My older gcc-4.1.x on ubuntu always uses %fs.

> The other somewhat orthogonal irritation is the fixed "40".  If they'd generated
> %gs:__gcc_stack_canary, then we could alias that to a per-cpu variable like
> anything else and the whole problem would go away - and we could support
> stack-protector on 32-bit with no problems (and normal usermode could define
> __gcc_stack_canary to be a weak symbol with value "40" (20 on 32-bit) for
> backwards compatibility).
>
> I'm close to proposing that we run a post-processor over the generated assembly
> to perform the %gs:40 -> %gs:__gcc_stack_canary transformation and deal with it
> that way.

Or we could do something completely evil.  And use the other segment
register for the stack canary.

I think the unification is valid and useful, and that trying to keep
that stupid stack canary working is currently more trouble then it is
worth.

Eric