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