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
operand.

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@chromium.org>
Cc: Grant Grundler <grundler@chromium.org>
Cc: Greg Hackmann <ghackmann@google.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Michael Davidson <md@google.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20170501224741.133938-1-mka@chromium.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

2 files changed, 3 insertions, 1 deletions

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 5761a4f19455..ab2d1d73e9e7 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;