linux-kernel - Re: [PATCH V2 9/9] x86/speculation: Remove all ANNOTATE_NOSPEC

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Date:   Tue, 7 Apr 2020 19:26:30 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Alexandre Chartre <alexandre.chartre@...cle.com>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, jthierry@...hat.com,
        tglx@...utronix.de
Subject: Re: [PATCH V2 9/9] x86/speculation: Remove all
 ANNOTATE_NOSPEC_ALTERNATIVE directives

On Tue, Apr 07, 2020 at 07:01:43PM +0200, Alexandre Chartre wrote:

> I think that's the case with smap_save:
> 
> static __always_inline unsigned long smap_save(void)
> {
>         unsigned long flags;
> 
>         asm volatile (ALTERNATIVE("", "pushf; pop %0; " __ASM_CLAC,
>                                   X86_FEATURE_SMAP)
>                       : "=rm" (flags) : : "memory", "cc");
> 
>         return flags;
> }
> 
> The alternative does change the stack but objtool won't complain
> because it handles the pushf and pop instruction.

Oh bugger; indeed! That'll wreck unwinding.