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

[Peter Zijlstra]

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@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> > 
> > 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.