From mboxrd@z Thu Jan  1 00:00:00 1970
From: jamie@shareable.org (Jamie Lokier)
Date: Thu, 17 Dec 2009 19:38:26 +0000
Subject: [PATCH] ARM: Convert BUG() to use unreachable()
In-Reply-To: <20091217191401.GD10707@synopsys.com>
References: <1260266138-17684-1-git-send-email-u.kleine-koenig@pengutronix.de>
	<20091217150120.GD24967@shareable.org>
	<4B2A65C6.7080009@caviumnetworks.com>
	<84fc9c000912170917y4b2772d0n47bb4dabc74b91ef@mail.gmail.com>
	<20091217181718.GA4440@n2100.arm.linux.org.uk>
	<20091217183517.GC10707@synopsys.com>
	<20091217190613.GB4440@n2100.arm.linux.org.uk>
	<20091217191401.GD10707@synopsys.com>
Message-ID: <20091217193826.GE362@shareable.org>
To: linux-arm-kernel@lists.infradead.org
List-Id: linux-arm-kernel.lists.infradead.org

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 don't understand, though, why you would want to implement a generally
> useful facility (make the kernel trap so you can do a post-mortem
> analysis) in a way that's only safe for the ARM port.

The patch under discussion, which led to these questions and Cc
gcc at gcc.gnu.org, is specific to the ARM architecture and that is
Russell's focus, as ARM kernel maintainer.

But yes, the whole principle of how to trap and whether it's safe to
follow a null pointer dereference with __builtin_unreachable() applies
to all the other architectures too.

-- Jamie