[tip:x86/urgent] x86/mm/kaslr: Use the _ASM_MUL macro for multiplication to work around Clang incompatibility

From: tip-bot for Matthias Kaehlcke
Date: Fri May 05 2017 - 04:21:09 EST

Commit-ID: 121843eb02a6e2fa30aefab64bfe183c97230c75
Gitweb: http://git.kernel.org/tip/121843eb02a6e2fa30aefab64bfe183c97230c75
Author: Matthias Kaehlcke <mka@xxxxxxxxxxxx>
AuthorDate: Mon, 1 May 2017 15:47:41 -0700
Committer: Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Fri, 5 May 2017 08:31:05 +0200

x86/mm/kaslr: Use the _ASM_MUL macro for multiplication to work around Clang incompatibility

The constraint "rm" allows the compiler to put mix_const into memory.
When the input operand is a memory location then MUL needs an operand
size suffix, since Clang can't infer the multiplication width from the

Add and use the _ASM_MUL macro which determines the operand size and
resolves to the NUL 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@xxxxxxxxxxxx>
Cc: Grant Grundler <grundler@xxxxxxxxxxxx>
Cc: Greg Hackmann <ghackmann@xxxxxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Michael Davidson <md@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Link: http://lkml.kernel.org/r/20170501224741.133938-1-mka@xxxxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
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 7acb51c..7a9df3b 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 5761a4f..ab2d1d7 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;