Re: [RFC] [PATCH] cpuset operations causes Badness at mm/slab.c:777 warning

[Linus Torvalds <torvalds@linux-foundation.org>]

On Fri, 1 Jun 2007, Christoph Lameter wrote:
> 
> Right it could catch a lot of other bugs as well.

I'd actually prefer "malloc(0)" to _not_ return NULL, but some known 
(non-NULL) bogus pointer.

Why?

Because it's quite sane to have simple logic like

	ptr = malloc(size);
	if (!ptr)
		return -ENOMEM;

and writing it as

	if (size && !ptr)
		return -ENOMEM;

is just annoying.

Also, NULL is _special_. There are absolutely tons of code in the kernel 
(and elsewhere) that just does something *different* from NULL pointers, 
and that totally breaks the whole notion of "you can allocate a zero-sized 
allocation, you just must not dereference it". If people special-case 
NULL as something else, they won't even go through the normal code-path.

So for *both* of the above reasons, it's actually stupid to return NULL 
for a zero-sized allocation. It would be much better to return another 
pointer that will trap on access. A good candidate might be to return

	#define BADPTR ((void *)16)

which is a portable-enough (where "portable-enough" is "against strict 
ANSI C rules, but works in practice on all architectures") way to return 
something that will cause the same page fault behaviour as NULL, but will 
_not_ trigger the "NULL is special" code.

(Of course, you then need to teach "kfree()" to accept it as another 
pointer to be ignored, that's fine).

I bet you'd find *more* problems that way than by returning NULL, and 
you'd also avoid the whole problem with "if (!ptr) return -ENOMEM".

		Linus