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SubjectRe: [PATCH v2] x86/mm/kaslr: Use _ASM_MUL macro for multiplication
On Wed, Apr 26, 2017 at 2:00 PM,  <hpa@zytor.com> wrote:
> On April 26, 2017 1:55:04 PM PDT, Matthias Kaehlcke <mka@chromium.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@chromium.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

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