Re: [PATCH v2 4/4] x86/static_call: Add inline static call implementation for x86-64

Peter Zijlstra
2018-11-29T22:02:13Z

On Thu, Nov 29, 2018 at 11:10:50AM -0600, Josh Poimboeuf wrote:
> On Thu, Nov 29, 2018 at 08:59:31AM -0800, Andy Lutomirski wrote:

> > (like pointing IP at a stub that retpolines to the target by reading
> > the function pointer, a la the unoptimizable version), then okay, I
> > guess, with only a small amount of grumbling.
>
> I tried that in v2, but Peter pointed out it's racy:
>
> https://lkml.kernel.org/r/20181126160217.GR2113@hirez.programming.kicks-ass.net

Ah, but that is because it is a global shared trampoline.

Each static_call has it's own trampoline; which currently reads
something like:

RETPOLINE_SAFE
JMP *key

which you then 'defuse' by writing an UD2 on. _However_, if you write
that trampoline like:

1: RETPOLINE_SAFE
JMP *key
2: CALL_NOSPEC *key
RET

and have the text_poke_bp() handler jump to 2 (a location you'll never
reach when you enter at 1), it will in fact work I think. The trampoline
is never modified and not shared between different static_call's.