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

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Date:   Wed, 26 Apr 2017 14:21:25 -0700
From:   Kees Cook <keescook@...omium.org>
To:     "H. Peter Anvin" <hpa@...or.com>
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 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

-- 
Kees Cook
Pixel Security