linux-kernel - Re: [PATCH v2] x86/mm/kaslr: Use _ASM

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Date:   Wed, 26 Apr 2017 14:00:37 -0700
From:   hpa@...or.com
To:     Matthias Kaehlcke <mka@...omium.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Kees Cook <keescook@...omium.org>
CC:     x86@...nel.org, linux-kernel@...r.kernel.org,
        Grant Grundler <grundler@...omium.org>,
        Greg Hackmann <ghackmann@...gle.com>,
        Michael Davidson <md@...gle.com>
Subject: Re: [PATCH v2] x86/mm/kaslr: Use _ASM_MUL macro for multiplication

On April 26, 2017 1:55:04 PM PDT, Matthias Kaehlcke <mka@...omium.org> wrote:
>In difference to gas clang doesn't seem to infer the size from the
>operands. Add and use the _ASM_MUL macro which determines the operand
>size and resolves to the 'mul' instruction with the corresponding
>suffix.
>
>This fixes the following error when building with clang:
>
>CC      arch/x86/lib/kaslr.o
>/tmp/kaslr-dfe1ad.s: Assembler messages:
>/tmp/kaslr-dfe1ad.s:182: Error: no instruction mnemonic suffix given
>and
>no register operands; can't size instruction
>
>Signed-off-by: Matthias Kaehlcke <mka@...omium.org>
>---
>Changes in v2:
>- use _ASM_MUL instead of #ifdef
>- updated commit message
>
> arch/x86/include/asm/asm.h | 1 +
> arch/x86/lib/kaslr.c       | 3 ++-
> 2 files changed, 3 insertions(+), 1 deletion(-)
>
>diff --git a/arch/x86/include/asm/asm.h b/arch/x86/include/asm/asm.h
>index 7acb51c49fec..7a9df3beb89b 100644
>--- a/arch/x86/include/asm/asm.h
>+++ b/arch/x86/include/asm/asm.h
>@@ -32,6 +32,7 @@
> #define _ASM_ADD	__ASM_SIZE(add)
> #define _ASM_SUB	__ASM_SIZE(sub)
> #define _ASM_XADD	__ASM_SIZE(xadd)
>+#define _ASM_MUL	__ASM_SIZE(mul)
> 
> #define _ASM_AX		__ASM_REG(ax)
> #define _ASM_BX		__ASM_REG(bx)
>diff --git a/arch/x86/lib/kaslr.c b/arch/x86/lib/kaslr.c
>index 121f59c6ee54..0c7fe444dcdd 100644
>--- a/arch/x86/lib/kaslr.c
>+++ b/arch/x86/lib/kaslr.c
>@@ -5,6 +5,7 @@
>  * kernel starts. This file is included in the compressed kernel and
>  * normally linked in the regular.
>  */
>+#include <asm/asm.h>
> #include <asm/kaslr.h>
> #include <asm/msr.h>
> #include <asm/archrandom.h>
>@@ -79,7 +80,7 @@ unsigned long kaslr_get_random_long(const char
>*purpose)
> 	}
> 
> 	/* Circular multiply for better bit diffusion */
>-	asm("mul %3"
>+	asm(_ASM_MUL "%3"
> 	    : "=a" (random), "=d" (raw)
> 	    : "a" (random), "rm" (mix_const));
> 	random += raw;

This really feels like a "fix your compiler" issue.
-- 
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