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

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Date:   Thu, 29 Nov 2018 16:30:17 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>, X86 ML <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Jason Baron <jbaron@...mai.com>, Jiri Kosina <jkosina@...e.cz>,
        David Laight <David.Laight@...lab.com>,
        Borislav Petkov <bp@...en8.de>, julia@...com, jeyu@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH v2 4/4] x86/static_call: Add inline static call
 implementation for x86-64

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@...radead.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