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

Josh Poimboeuf
2018-11-29T22:30:23Z

On Thu, Nov 29, 2018 at 02:25:33PM -0800, Andy Lutomirski wrote:
> On Thu, Nov 29, 2018 at 2:22 PM Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > On Thu, Nov 29, 2018 at 04:14:46PM -0600, Josh Poimboeuf wrote:
> > > On Thu, Nov 29, 2018 at 11:01:48PM +0100, Peter Zijlstra wrote:
> > > > 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.
> > >
> > > But after returning from the function to the trampoline, how does it
> > > return from the trampoline to the call site? At that point there is no
> > > return address on the stack.
> >
> > Oh, right, so that RET don't work. ARGH. Time to go sleep I suppose.
>
> I assume I'm missing something, but can't it just be JMP_NOSPEC *key?
> The code would call the trampoline just like any other function and,
> if the alignment is bad, we can skip patching it. And, if we want the
> performance back, maybe some day we can find a clean way to patch
> those misaligned callers, too.

Yeah, this is currently the leading contender, though I believe it will
use a direct jump like the current out-of-line implementation.

--
Josh