linux-kernel - Re: [PATCH 05/20] x86/uaccess/xen: Suppress SMAP warnings

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Date:   Fri, 8 Mar 2019 20:50:12 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     torvalds@...ux-foundation.org, tglx@...utronix.de, hpa@...or.com,
        julien.thierry@....com, will.deacon@....com, luto@...capital.net,
        mingo@...nel.org, catalin.marinas@....com, james.morse@....com,
        valentin.schneider@....com, brgerst@...il.com, luto@...nel.org,
        bp@...en8.de, dvlasenk@...hat.com, linux-kernel@...r.kernel.org,
        dvyukov@...gle.com, rostedt@...dmis.org, andrew.cooper3@...rix.com
Subject: Re: [PATCH 05/20] x86/uaccess/xen: Suppress SMAP warnings

On Fri, Mar 08, 2019 at 01:00:38PM -0600, Josh Poimboeuf wrote:
> On Thu, Mar 07, 2019 at 12:45:16PM +0100, Peter Zijlstra wrote:
> > drivers/xen/privcmd.o: warning: objtool: privcmd_ioctl()+0x1414: call to hypercall_page() with UACCESS enabled
> > 
> > Xen needs to do HV calls with AC=1 for hysterical raisins. Make the
> > warning go away.
> > 
> > XXX: arguably we should rename ANNOTATE_NOSPEC_ALTERNATIVE to
> >      ANNOTATE_IGNORE_ALTERNATIVE.
> > 
> > Cc: andrew.cooper3@...rix.com
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> > ---
> >  arch/x86/include/asm/xen/hypercall.h |   26 ++++++++++++++++++++++----
> >  1 file changed, 22 insertions(+), 4 deletions(-)
> > 
> > --- a/arch/x86/include/asm/xen/hypercall.h
> > +++ b/arch/x86/include/asm/xen/hypercall.h
> > @@ -214,6 +214,24 @@ xen_single_call(unsigned int call,
> >  	return (long)__res;
> >  }
> >  
> > +static __always_inline void __xen_stac(void)
> > +{
> > +	/*
> > +	 * This is just about as horrible as this interface; we abuse the
> > +	 * nospec alternative annotation to supress objtool seeing the
> > +	 * STAC/CLAC and getting confused about it calling random code with
> > +	 * AC=1.
> > +	 */
> > +	asm volatile(ANNOTATE_NOSPEC_ALTERNATIVE
> > +		     ASM_STAC ::: "memory", "flags");
> > +}
> > +
> > +static __always_inline void __xen_clac(void)
> > +{
> > +	asm volatile(ANNOTATE_NOSPEC_ALTERNATIVE
> > +		     ASM_CLAC ::: "memory", "flags");
> > +}
> > +
> 
> Shouldn't these be using SMAP-based alternatives like stac()/clac() do?

They are, __ASM_{ST,CL}AC are the raw instructions ASM_{ST,CL}AC are the
alternatives.