linux-kernel - Re: [PATCH] x86/mm/kaslr: Add operand size suffix to 'mul' instruction

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Date:   Wed, 26 Apr 2017 12:09:13 -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] x86/mm/kaslr: Add operand size suffix to 'mul' instruction

On Wed, Apr 26, 2017 at 12:01 PM, Matthias Kaehlcke <mka@...omium.org> wrote:
> In difference to gas clang doesn't seem to infer the size from the
> operands. Adding the suffix 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>
> ---
>  arch/x86/lib/kaslr.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/lib/kaslr.c b/arch/x86/lib/kaslr.c
> index 121f59c6ee54..947d4aa92ff7 100644
> --- a/arch/x86/lib/kaslr.c
> +++ b/arch/x86/lib/kaslr.c
> @@ -79,7 +79,11 @@ unsigned long kaslr_get_random_long(const char *purpose)
>         }
>
>         /* Circular multiply for better bit diffusion */
> -       asm("mul %3"
> +#ifdef CONFIG_X86_64
> +       asm("mulq %3"
> +#else
> +       asm("mull %3"
> +#endif
>             : "=a" (random), "=d" (raw)
>             : "a" (random), "rm" (mix_const));
>         random += raw;
> --
> 2.13.0.rc0.306.g87b477812d-goog
>

Since there may be more of these and we try to avoid #ifdef in .c
files, I think we should create an isns helper macro and use it here
instead. Like:

#ifdef CONFIG_X86_64
# define ASM_MUL "mulq"
#else
# define ASM_MUL "mull"
#endif

...


asm(ASM_MUL " %3" ...

-Kees

-- 
Kees Cook
Pixel Security