linux-kernel - Re: [PATCH] ARM: Convert BUG() to use unreachable()

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Message-ID: <20091217194837.GD4440@n2100.arm.linux.org.uk>
Date:	Thu, 17 Dec 2009 19:48:37 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Jamie Lokier <jamie@...reable.org>
Cc:	Joe Buck <Joe.Buck@...opsys.COM>,
	Richard Guenther <richard.guenther@...il.com>,
	David Daney <ddaney@...iumnetworks.com>,
	"gcc@....gnu.org" <gcc@....gnu.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>
Subject: Re: [PATCH] ARM: Convert BUG() to use unreachable()

On Thu, Dec 17, 2009 at 07:38:26PM +0000, Jamie Lokier wrote:
> Joe Buck wrote:
> > On Thu, Dec 17, 2009 at 11:06:13AM -0800, Russell King - ARM Linux wrote:
> > > On Thu, Dec 17, 2009 at 10:35:17AM -0800, Joe Buck wrote:
> > > > Besides, didn't I see a whole bunch of kernel security patches related
> > > > to null pointer dereferences lately?  If page 0 can be mapped, you
> > > > suddenly won't get your trap.
> > > 
> > > Page 0 can not be mapped on ARM kernels since the late 1990s, and this
> > > protection is independent of the generic kernel.
> > > 
> > > Milage may vary on other architectures, but that's not a concern here.
> 
> It does not trap on at least one ARM-nommu kernel...

I was going to say the following in a different reply but discarded it
because it wasn't relevant to the GCC list.

I regard ARM nommu as highly experimental, especially as the maintainer
vanished half way through merging it into mainline.  I know that there
are some parts of ARM nommu that are highly buggy - such as ARM940
support invalidating the entire data cache on any DMA transaction...
say goodbye stacked return addresses.

As such, I would not be surprised if the ARM nommu kernel has _lots_ of
weird and wonderful bugs.  I am not surprised that NULL pointer dereferences
don't work - its actually something I'd expect given that they have a
protection unit which the kernel doesn't apparently touch.

Maybe the protection unit code never got merged?  I've no idea.  As I
say, uclinux support got as far as being half merged and that's roughly
the state it's remained in ever since.

We don't even have any no-MMU configurations which the kernel builder
automatically tests for us.

Given the lack of progress/bug reporting on ARM uclinux, the lack of
platform support and the lack of configurations, my view is that there
is no one actually using it.  I know that I don't particularly take any
care with respect to uclinux when making changes to the MMU based kernels.
Why bother if apparantly no one's using it?
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