comparison of safe pointers

comparison of safe pointers

[Al Viro]

	Why do we generate a warning when we compare two pointers
declared as safe?  I understand why we do that when such beast gets
used as condition (i.e. implicitly compare with NULL), but what's
wrong with

int foo(void __safe *p, void __safe *q)
{
	return p == q;
}

What did you want that check in evaluate_compare() to catch?  Is that
about warning on explicit comparison with NULL?

Al, crawling through evaluate.c and fixing odd cases in typechecking...

Re: comparison of safe pointers

[Linus Torvalds]

On Fri, 25 May 2007, Al Viro wrote:
> 
> What did you want that check in evaluate_compare() to catch?  Is that
> about warning on explicit comparison with NULL?

Yes. It was explicitly meant to be a "anybody who tests this pointer 
missed the 'safe' part of it".

But I never used it, so it was really meant as a concept thing, and I 
don't think it's wrong to test against non-null.

		Linus

Re: comparison of safe pointers

[Al Viro]

On Fri, May 25, 2007 at 08:29:11AM -0700, Linus Torvalds wrote:
> 
> 
> On Fri, 25 May 2007, Al Viro wrote:
> > 
> > What did you want that check in evaluate_compare() to catch?  Is that
> > about warning on explicit comparison with NULL?
> 
> Yes. It was explicitly meant to be a "anybody who tests this pointer 
> missed the 'safe' part of it".
> 
> But I never used it, so it was really meant as a concept thing, and I 
> don't think it's wrong to test against non-null.

OK...  Pointer comparisons care about null pointer constants anyway
(BTW, ordered comparison with NULL should give a warning - it's not
a valid C and it makes no sense whatsoever even as an extension),
so that's not a problem.

Other places that care are ?:, simple assignment when target is a pointer,
passing arguments to function with prototype when parameter type is a pointer
(same logics as with assignment) and cast adding address space (valid for
NULL, not valid for random void *).

AFAICS, that covers everything; definitely all contexts where null pointer
constants are recognized by C and I think that it covers all places where
they matter for sparse extensions...