linux-kernel - RE: [PATCH 05/29] x86: Base IBT bits

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Date:   Fri, 18 Feb 2022 21:11:25 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Andrew Cooper' <Andrew.Cooper3@...rix.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "joao@...rdrivepizza.com" <joao@...rdrivepizza.com>,
        "hjl.tools@...il.com" <hjl.tools@...il.com>,
        "jpoimboe@...hat.com" <jpoimboe@...hat.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "ndesaulniers@...gle.com" <ndesaulniers@...gle.com>,
        "keescook@...omium.org" <keescook@...omium.org>,
        "samitolvanen@...gle.com" <samitolvanen@...gle.com>,
        "mark.rutland@....com" <mark.rutland@....com>,
        "alyssa.milburn@...el.com" <alyssa.milburn@...el.com>
Subject: RE: [PATCH 05/29] x86: Base IBT bits

From: Andrew Cooper
> Sent: 18 February 2022 20:50
> 
> On 18/02/2022 16:49, Peter Zijlstra wrote:
> > +/*
> > + * A bit convoluted, but matches both endbr32 and endbr64 without
> > + * having either as literal in the text.
> > + */
> > +static inline bool is_endbr(const void *addr)
> > +{
> > +	unsigned int val = ~*(unsigned int *)addr;
> > +	val |= 0x01000000U;
> > +	return val == ~0xfa1e0ff3;
> > +}
> 
> At this point, I feel I've earned an "I told you so". :)
> 
> Clang 13 sees straight through the trickery and generates:
> 
> is_endbr:                               # @is_endbr
>         movl    $-16777217, %eax                # imm = 0xFEFFFFFF
>         andl    (%rdi), %eax
>         cmpl    $-98693133, %eax                # imm = 0xFA1E0FF3
>         sete    %al
>         retq

I think it is enough to add:
	asm("", "=r" (val));
somewhere in the middle.
(I think that is right for asm with input and output in the same
register.)
There might be a HIDE_FOR_OPTIMISER() define that does that.

	David

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