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

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Date:	Sat, 16 Feb 2008 14:48:17 +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:

> > 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.

> > > Indeed it does fail with an Oops on s390 since we enable low address
> > > protection in the kernel so we get an exception if something within the
> > > kernel writes to the first 512 bytes of the kernel address space.
> > > Otherwise it would have silently passed the test...
> > 
> > NULL pointer dereferencing faults on all architectures, at least it
> > should, but we explicitely disable pagefaults and recover via the
> > extable fixup, which is in S390 as well. That returns -EFAULT and
> > signals that there is a working implementation, while those which have
> > no support return -ENOSYS, which keeps the robust/pi stuff disabled.
> 
> ...one of our exception table entries has an off-by-one bug.
> Never mind, I'll go and fix our own stuff instead ;)

Maybe we should do such tests on all exception table protected
assembler constructs :)

Thanks,

	tglx
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