What does "__attribute__((noderef, address_space(2)))" mean?

moo-cow · 01-30-2014, 03:41 AM

I'm trying to figure out what this definition in the Linux kernel means:

Code:

# define __iomem        __attribute__((noderef, address_space(2)))

It's found in include/linux/compiler.h. I understand that __attribute__ is a GCC extension, but the documentation does not mention attributes named "noderef" or "address_space()". What do they mean?

moo-cow · 01-30-2014, 03:55 AM

Got it!

We're dealing with annotations for a static code checking tool called Sparse. Linus Torvalds explains this on the Linux kernel mailing list:

Quote:

This is important to remember: for gcc, the sparse annotations are
meaningless. They can still be useful just to tell the _programmer_
that "hey, that pointer you got wasn't a normal pointer" in a fairly
readable manner, but in the end, unless you use sparse, they don't
actually _do_ anything.

HOWEVER. When you _do_ use parse, it is another matter entirely. For
"sparse", that "__iomem" has lots of meaning:

# define __iomem __attribute__((noderef, address_space(2)))

ie "iomem" means two separate things: it means that sparse should complain
if the pointer is ever dereferenced (it's a "noderef" pointer) directly,
and it's in "address space 2" as opposed to the normal address space (0).

Now, that means that _sparse_ will complain if such a pointer is ever
passed into a function that wants a regular pointer (because it is _not_ a
normal pointer, and you obviously shouldn't do things like "strcmp()" etc
on it), and sparse will also complain if you try to cast it to another
pointer in another address space.