linux-kernel - Re: + futex-runtime-enable-pi-and-robust-functionality.patch added to -mm tree

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Date:	Sat, 16 Feb 2008 15:29:07 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Heiko Carstens <heiko.carstens@...ibm.com>
cc:	linux-kernel@...r.kernel.org, mm-commits@...r.kernel.org,
	buytenh@...tstofly.org, mingo@...e.hu, riku.voipio@...ial.fi,
	stable@...nel.org, schwidefsky@...ibm.com
Subject: Re: + futex-runtime-enable-pi-and-robust-functionality.patch added
 to -mm tree

On Sat, 16 Feb 2008, Heiko Carstens wrote:

> On Sat, Feb 16, 2008 at 02:48:17PM +0100, Thomas Gleixner wrote:
> > On Sat, 16 Feb 2008, Heiko Carstens wrote:
> > 
> > > > Well, NULL pointer dereferencing is supposed to fail, isn't it ?
> > >
> > > I wasn't sure that this is true for all architectures, but...
> > 
> > It's an requirement for futex support.
> 
> To be more precise: dereferencing alone won't cause an exception for
> NULL pointers on s390. Only writes will do so.
> That is very architecture specific since we cannot unmap page 0,
> it contains per-cpu data -- like exception pointers and all such stuff
> that the cpu needs.
> Just in case there is any code that relies on the fact that also reads
> via a NULL pointer are supposed to failed.

Hmm, not sure whether there is such code, but then it would be not too bad
to add

   if (!p)
      return -EFAULT;

to the S390 implementations which only read data and have an exception
fixup.

Thanks,

	tglx
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