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.gr2...@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.