linux-kernel - Re: SLUB: Return ZERO_SIZE

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Date:	Fri, 1 Jun 2007 20:06:25 -0700 (PDT)
From:	Christoph Lameter <clameter@....com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
cc:	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	Jeremy Fitzhardinge <jeremy@...p.org>
Subject: Re: 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@....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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