ARM unaligned MMIO access with attribute((packed))

On Wed, Feb 02, 2011 at 05:51:27PM +0100, Richard Guenther wrote:

quoted

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.