linux-kernel - Re: [PATCH] x86/build: Move '-mindirect-branch-cs-prefix' out of GCC-only block

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Message-ID: <Yv3xu7f3NDC1is6o@worktop.programming.kicks-ass.net>
Date:   Thu, 18 Aug 2022 10:00:59 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Nathan Chancellor <nathan@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, llvm@...ts.linux.dev
Subject: Re: [PATCH] x86/build: Move '-mindirect-branch-cs-prefix' out of
 GCC-only block

On Thu, Aug 18, 2022 at 09:45:39AM +0200, Peter Zijlstra wrote:

> The thing to do is boot with (warning, *very* verbose):
> 
> 	"spectre_v2=retpoline,lfence debug-alternative"
> 
> and observe that the retpoline sites all replace:
> 
> 	"cs call __x86_indirect_thunk_r11" (6 bytes)
> 
> with:
> 
> 	"lfence; jmp *%r11" (6 bytes)

		^^ call, ofc, unless the original was jmp then jmp...

/me goes get more morning juice in an attempt to wake up.

> 
> 
> This being clang, you'll ofcourse still have a few weird:
> 
> 	"Jcc.d32 __x86_indirect_thunk_r11"
> 
> sites that will not be able to be replaced, but there's nothing I can do
> about that other than to continue to encourage y'all to stop emitting
> them ;-)