linux-kernel - Re: ARM unaligned MMIO access with attribute((packed))

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Date:	Wed, 2 Feb 2011 17:09:59 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Richard Guenther <richard.guenther@...il.com>
Cc:	Arnd Bergmann <arnd@...db.de>,
	Peter Maydell <peter.maydell@...aro.org>, gcc@....gnu.org,
	linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
	Ulrich Weigand <Ulrich.Weigand@...ibm.com>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: ARM unaligned MMIO access with attribute((packed))

On Wed, Feb 02, 2011 at 05:51:27PM +0100, Richard Guenther wrote:
> > I would suggest fixing this by:
> >
> > 1. auditing all uses of __attribute__((packed)) in the Linux USB code
> > and other drivers, removing the ones that are potentially harmful.
> >
> > 2. Changing the ARM MMIO functions to use inline assembly instead of
> > direct pointer dereference.
> >
> > 3. Documenting the gcc behavior as undefined.
> 
> The pointer conversions already invoke undefined behavior as specified by the
> C standard (6.3.2.3/7).

Just to be clear: you are not saying that the ARM implementation is
undefined.

What you're saying is that converting from a pointer with less strict
alignment requirements to a pointer with more strict alignment
requirements is undefined.

IOW:

unsigned long *blah(unsigned char *c)
{
	return (unsigned long *)c;
}

would be undefined, but:

unsigned char *blah(unsigned long *c)
{
	return (unsigned char *)c;
}

would not be.

If you're saying something else, please explain with reference to the
point in the C standard you quote above.
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