linux-kernel - Re: [patch] use __asm__ and __volatile__ in asm-x86

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Date:	Mon, 18 Jun 2007 10:02:21 -0400
From:	Mike Frysinger <vapier@...too.org>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org, ak@...e.de
Subject: Re: [patch] use __asm__ and __volatile__ in asm-x86_64/msr.h

On Monday 18 June 2007, H. Peter Anvin wrote:
> Mike Frysinger wrote:
> > Most of the macros in msr.h already use __asm__ and __volatile__, this
> > patch brings the rest into line.  This is needed since these get exported
> > into userspace (need to use strict __asm__ vs asm when gcc compiles with
> > GNU extensions turned off).
> > ---
> > diff --git a/include/asm-x86_64/msr.h b/include/asm-x86_64/msr.h
> > index d5c55b8..ed15d2b 100644
> > --- a/include/asm-x86_64/msr.h
> > +++ b/include/asm-x86_64/msr.h
> > @@ -33,7 +33,7 @@
> >
> >  /* wrmsr with exception handling */
> >  #define wrmsr_safe(msr,a,b) ({ int ret__;			\
> > -	asm volatile("2: wrmsr ; xorl %0,%0\n"			\
> > +	__asm__ __volatile__("2: wrmsr ; xorl %0,%0\n"			\
> >  		     "1:\n\t"					\
> >  		     ".section .fixup,\"ax\"\n\t"		\
> >  		     "3:  movl %4,%0 ; jmp 1b\n\t"		\
> > @@ -50,7 +50,7 @@
> >
> >  #define rdmsr_safe(msr,a,b) \
> >  	({ int ret__;						\
> > -	  asm volatile ("1:       rdmsr\n"			\
> > +	  __asm__ __volatile__ ("1:       rdmsr\n"			\
> >                        "2:\n"					\
> >                        ".section .fixup,\"ax\"\n"		\
> >                        "3:       movl %4,%0\n"			\
>
> NAK.
>
> This is wrong for two reasons:
>
> - MSRs are not accessible in user space.
> - .section .fixup is not applicable to user space.
>
> Anything MSR-related should be protected by __KERNEL__ instead.
>
> This NAK doesn't apply to rdtsc* and rdpmc* changes, however, as those
> instructions are legitimate in userspace.

then someone needs to audit msr.h for this because currently it lacks 
__KERNEL__ of any kind yet is exported completely to userspace ... i dont 
know anything about this instructions so that someone != me

this is similar to what i heard last time i tried to fixup msr.h for 
userspace, but the file still hasnt been updated ...
-mike

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