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

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Date:   Wed, 26 Apr 2017 14:24:25 -0700
From:   hpa@...or.com
To:     Kees Cook <keescook@...omium.org>
CC:     Matthias Kaehlcke <mka@...omium.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "x86@...nel.org" <x86@...nel.org>,
        LKML <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 2:21:25 PM PDT, Kees Cook <keescook@...omium.org> wrote:
>On Wed, Apr 26, 2017 at 2:00 PM,  <hpa@...or.com> wrote:
>> 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.
>
>We already use the other forms, what's so bad about adding mul too?
>And if this lets us build under clang, all the better.
>
>-Kees

It's not bad per se, but if this doesn't eventually gets fixed in clang we'll have no end of this crap.
-- 
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