Re: comparison of safe pointers

[Al Viro <viro@ftp.linux.org.uk>]

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...