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

From: Josh Poimboeuf
Date: Thu Nov 29 2018 - 12:11:01 EST

On Thu, Nov 29, 2018 at 08:59:31AM -0800, Andy Lutomirski wrote:
> > On Nov 29, 2018, at 8:49 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> >
> > On Thu, Nov 29, 2018 at 10:33:42AM -0600, Josh Poimboeuf wrote:
> >>> can't we 'fix' that again? The alternative is moving that IRET-frame and
> >>> fixing everything up, which is going to be fragile, ugly and such
> >>> things more.
> >
> >> This seems to work...
> >
> > That's almost too easy... nice!
> It is indeed too easy: youâre putting pt_regs in the wrong place for
> int3 from user mode, which is probably a root hole if you arrange for
> a ptraced process to do int3 and try to write to whatever register
> aliases CS.
> If you make it conditional on CPL, do it for 32-bit as well, add
> comments convince yourself that there isnât a better solution

I could do that - but why subject 32-bit to it? I was going to make it
conditional on CONFIG_HAVE_STATIC_CALL_INLINE which is 64-bit only.

> (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: