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SubjectRe: SLUB: Return ZERO_SIZE_PTR for kmalloc(0)
On Fri, 1 Jun 2007, Linus Torvalds wrote:

> So when I suggested the uglier
>
> if ((unsigned long)x <= 16)
> return;
>
> I really did mean to use that ugly cast.. Yours is prettier, but sadly,
> yours is simply not safe: a signed comparison might end up making _all_
> kernel pointers trigger that test.

Maybe we can have a compromise? Lets at least keep the ZERO_SIZE_PTR
reference in there.


SLUB: Make sure that the comparision with ZERO_SIZE_PTR is unsigned

Signed-off-by: Christoph Lameter <clameter@sgi.com>


---
mm/slub.c | 8 +++++++-
1 file changed, 7 insertions(+), 1 deletion(-)
Index: slub/mm/slub.c
===================================================================
--- slub.orig/mm/slub.c 2007-06-01 20:00:56.000000000 -0700
+++ slub/mm/slub.c 2007-06-01 20:04:26.000000000 -0700
@@ -2338,7 +2338,13 @@ void kfree(const void *x)
struct kmem_cache *s;
struct page *page;

- if (x <= ZERO_SIZE_PTR)
+ /*
+ * This has to be an unsigned comparison. According to Linus
+ * some gcc version tread a pointer as a signed entity. Then
+ * this comparison would be true for all "negative" pointers
+ * (which would cover the whole upper half of the address space).
+ */
+ if ((unsigned long)x <= (unsigned long)ZERO_SIZE_PTR)
return;

page = virt_to_head_page(x);
-
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