ARM unaligned MMIO access with attribute((packed))
Russell King - ARM Linux
linux at arm.linux.org.uk
Wed Feb 2 12:09:59 EST 2011
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 (18.104.22.168/7).
Just to be clear: you are not saying that the ARM implementation is
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.
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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