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SubjectRe: kfree(0) - ok?
On Sat, 18 Aug 2007, Pekka Enberg wrote:

> Agreed, especially as we have real zero-sized objects returned from
> kmalloc() et al now.



Slab allocators: Fail if ksize is called with a NULL parameter

A NULL pointer means that the object was not allocated. One cannot
determine the size of an object that has not been allocated. Currently
we return 0 but we really should BUG() on attempts to determine the size
of something nonexistent.

krealloc() interprets NULL to mean a zero sized object. Handle that
separately in krealloc().

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

Index: linux-2.6/mm/slab.c
===================================================================
--- linux-2.6.orig/mm/slab.c 2007-08-17 16:17:41.000000000 -0700
+++ linux-2.6/mm/slab.c 2007-08-17 16:18:15.000000000 -0700
@@ -4436,7 +4436,8 @@ const struct seq_operations slabstats_op
*/
size_t ksize(const void *objp)
{
- if (unlikely(ZERO_OR_NULL_PTR(objp)))
+ BUG_ON(!objp);
+ if (unlikely(objp == ZERO_SIZE_PTR))
return 0;

return obj_size(virt_to_cache(objp));
Index: linux-2.6/mm/slob.c
===================================================================
--- linux-2.6.orig/mm/slob.c 2007-08-17 16:18:19.000000000 -0700
+++ linux-2.6/mm/slob.c 2007-08-17 16:18:40.000000000 -0700
@@ -484,7 +484,8 @@ size_t ksize(const void *block)
{
struct slob_page *sp;

- if (ZERO_OR_NULL_PTR(block))
+ BUG_ON(!block);
+ if (block == ZERO_SIZE_PTR)
return 0;

sp = (struct slob_page *)virt_to_page(block);
Index: linux-2.6/mm/slub.c
===================================================================
--- linux-2.6.orig/mm/slub.c 2007-08-17 16:16:36.000000000 -0700
+++ linux-2.6/mm/slub.c 2007-08-17 16:17:36.000000000 -0700
@@ -2426,7 +2426,8 @@ size_t ksize(const void *object)
struct page *page;
struct kmem_cache *s;

- if (ZERO_OR_NULL_PTR(object))
+ BUG_ON(!object);
+ if (object == ZERO_SIZE_PTR)
return 0;

page = get_object_page(object);
Index: linux-2.6/mm/util.c
===================================================================
--- linux-2.6.orig/mm/util.c 2007-08-17 16:16:29.000000000 -0700
+++ linux-2.6/mm/util.c 2007-08-17 16:16:32.000000000 -0700
@@ -81,14 +81,16 @@ EXPORT_SYMBOL(kmemdup);
void *krealloc(const void *p, size_t new_size, gfp_t flags)
{
void *ret;
- size_t ks;
+ size_t ks = 0;

if (unlikely(!new_size)) {
kfree(p);
return ZERO_SIZE_PTR;
}

- ks = ksize(p);
+ if (p)
+ ks = ksize(p);
+
if (ks >= new_size)
return (void *)p;

-
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