linux-kernel - Re: [PATCH] ARM: futex: fix clobbering oldval

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Date:	Fri, 23 Sep 2011 09:18:49 +0900
From:	mhban <mhban@...sung.com>
To:	Will Deacon <will.deacon@....com>
Cc:	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	DavidHowells <dhowells@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Michel Lespinasse <walken@...gle.com>,
	Russell King <linux@....linux.org.uk>,
	Chris Metcalf <cmetcalf@...era.com>
Subject: Re: [PATCH] ARM: futex: fix clobbering oldval

On Thu, 2011-09-22 at 18:26 +0100, Will Deacon wrote:
> Hi,
> 
> On Thu, Sep 22, 2011 at 03:53:35AM +0100, mhban wrote:
> > This patch fixes clobbering oldval bug. oldval should be preserved for next
> > compare operation.
> > 
> > Change-Id: I2a63bc1bdb8de330eb9e1ac02d7da1f77e6e8c3c
> > Signed-off-by: Minho Ban <mhban@...sung.com>
> 
> It would have been nice to have been CC'd on this...

Will not miss next time. Thanks.

> 
> I ran LTP tests on this, so I'm surprised that this was broken (the tests
> passed). Well spotted anyway!
> 
> > ---
> >  arch/arm/include/asm/futex.h |    6 +++---
> >  1 files changed, 3 insertions(+), 3 deletions(-)
> > 
> > diff --git a/arch/arm/include/asm/futex.h b/arch/arm/include/asm/futex.h
> > index d2d733c..b0f2e8e 100644
> > --- a/arch/arm/include/asm/futex.h
> > +++ b/arch/arm/include/asm/futex.h
> > @@ -30,14 +30,14 @@
> >  	__asm__ __volatile__(					\
> >  	"1:	ldrex	%1, [%2]\n"				\
> >  	"	" insn "\n"					\
> > -	"2:	strex	%1, %0, [%2]\n"				\
> > -	"	teq	%1, #0\n"				\
> > +	"2:	strex	r5, %0, [%2]\n"				\
> > +	"	teq	r5, #0\n"				\
> >  	"	bne	1b\n"					\
> >  	"	mov	%0, #0\n"				\
> >  	__futex_atomic_ex_table("%4")				\
> >  	: "=&r" (ret), "=&r" (oldval)				\
> >  	: "r" (uaddr), "r" (oparg), "Ir" (-EFAULT)		\
> > -	: "cc", "memory")
> > +	: "r5", "cc", "memory")
> 
> You shouldn't reference r5 directly here, but due to the way the futex code
> is laid out, you can't add an extra output operand without converting the
> code to use named arguments.
> 
> I'll post a patch to do that.
> 
> Will

I'm not familiar with gcc inline, thanks for pointing it out.

BTW, my last name is Ban not Ben.


Minho


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