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

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Date:   Mon, 1 May 2017 19:08:01 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Matthias Kaehlcke <mka@...omium.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H . Peter Anvin" <hpa@...or.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 Mon, May 1, 2017 at 3:47 PM, Matthias Kaehlcke <mka@...omium.org> wrote:
> The constraint "rm" allows the compiler to put mix_const into memory.
> When the input operand is a memory location mul needs an operand size
> suffix, since it 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 '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>

Acked-by: Kees Cook <keescook@...omium.org>

Thanks for the v2!

-Kees

> ---
> Changes in v2:
> - updated commit message with additional details
>
>  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;
> --
> 2.13.0.rc1.294.g07d810a77f-goog
>



-- 
Kees Cook
Pixel Security