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 23:01:48 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Andy Lutomirski <luto@...capital.net>,
        Andy Lutomirski <luto@...nel.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 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.