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

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Message-ID: <20170426193147.GQ128305@google.com>
Date:   Wed, 26 Apr 2017 12:31:47 -0700
From:   Matthias Kaehlcke <mka@...omium.org>
To:     Kees Cook <keescook@...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

Hi Kees,

El Wed, Apr 26, 2017 at 12:09:13PM -0700 Kees Cook ha dit:

> 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;
> >
> 
> 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" ...

Sounds very reasonable. Thanks for the suggestion, I'll rework the patch.

Matthias