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.
