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    Subject[PATCH AUTOSEL for 4.9 006/281] x86/mm/kaslr: Use the _ASM_MUL macro for multiplication to work around Clang incompatibility
    Date
    From: Matthias Kaehlcke <mka@chromium.org>

    [ Upstream commit 121843eb02a6e2fa30aefab64bfe183c97230c75 ]

    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>
    Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
    ---
    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 f35f246a26bf..8d8c24f3a963 100644
    --- a/arch/x86/include/asm/asm.h
    +++ b/arch/x86/include/asm/asm.h
    @@ -34,6 +34,7 @@
    #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;
    --
    2.14.1
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