linux-kernel - Re: [PATCH] ARM: alignment: setup alignment handler earlier

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Message-ID: <20110907162857.GB2327@n2100.arm.linux.org.uk>
Date:	Wed, 7 Sep 2011 17:28:58 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Catalin Marinas <catalin.marinas@....com>
Cc:	John Ogness <john.ogness@...utronix.de>,
	Nicolas Pitre <nicolas.pitre@...aro.org>,
	Will Deacon <Will.Deacon@....com>,
	Grant Likely <grant.likely@...retlab.ca>,
	Dave Martin <dave.martin@...aro.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ARM: alignment: setup alignment handler earlier

On Wed, Sep 07, 2011 at 03:40:16PM +0100, Catalin Marinas wrote:
> On Wed, Sep 07, 2011 at 02:35:04PM +0100, John Ogness wrote:
> > From 6f3f381800367127dc6430d9b9fa9bd6fc6d8ed0 Mon Sep 17 00:00:00 2001
> > From: John Ogness <john.ogness@...utronix.de>
> > 
> > The alignment exception handler is setup fairly late in
> > the boot process (fs_initcall). However, with newer gcc
> > versions and the compiler option -fconserve-stack, code
> > accessing unaligned data is generated in functions that
> > are called earlier, for example pcpu_dump_alloc_info().
> > This results in unhandled alignment exceptions during
> > boot. By setting up the exception handler sooner, we
> > reduce the window where a compiler may generate code
> > accessing unaligned data.
> 
> While this reduces the window and fixes this particular case, it still
> doesn't solve the problem. We never know when some unaligned access
> would be generated for some earlier code.

Is the problem even solvable?  There are instructions on ARMv6+ which
always produce an alignment fault (eg, ldrd) irrespective of strict
alignment checking.

There will always be a window for those - when we don't have the
vectors setup - where we potentically could take such a fault and end
up crashing.  So I'm not sure that the right answer is what's being
proposed.

What it's saying to me is that building the kernel in a way that gcc
intentionally generates misaligned accesses _will_ bite us in random
unknown ways sooner or later.

I don't think its feasible to build some of the kernel with alignment
faults enabled and other parts with it disabled - that's going to be
very fragile and probably be hell to identify which parts should and
should not.

So I think where we're heading is to need gcc _not_ to create any code
what so ever which would create a misalignment fault.
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