SLUB: return ZERO_SIZE_PTR for kmalloc(0)
Instead of returning the smallest available object return ZERO_SIZE_PTR.
A ZERO_SIZE_PTR can be legitimately used as an object pointer as long as it
is not deferenced. The dereference of ZERO_SIZE_PTR causes a distinctive
fault. kfree can handle a ZERO_SIZE_PTR in the same way as NULL.
This enables functions to use zero sized object. e.g. n = number of objects.
objects = kmalloc(n * sizeof(object));
for (i = 0; i < n; i++)
objects[i].x = y;
kfree(objects);
Signed-off-by: Christoph Lameter <clameter@sgi.com>
Acked-by: Pekka Enberg <penberg@cs.helsinki.fi>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/mm/slub.c b/mm/slub.c
index 51663a3..c9ab688 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -2241,7 +2241,7 @@
if (s)
return slab_alloc(s, flags, -1, __builtin_return_address(0));
- return NULL;
+ return ZERO_SIZE_PTR;
}
EXPORT_SYMBOL(__kmalloc);
@@ -2252,16 +2252,20 @@
if (s)
return slab_alloc(s, flags, node, __builtin_return_address(0));
- return NULL;
+ return ZERO_SIZE_PTR;
}
EXPORT_SYMBOL(__kmalloc_node);
#endif
size_t ksize(const void *object)
{
- struct page *page = get_object_page(object);
+ struct page *page;
struct kmem_cache *s;
+ if (object == ZERO_SIZE_PTR)
+ return 0;
+
+ page = get_object_page(object);
BUG_ON(!page);
s = page->slab;
BUG_ON(!s);
@@ -2293,7 +2297,13 @@
struct kmem_cache *s;
struct page *page;
- if (!x)
+ /*
+ * This has to be an unsigned comparison. According to Linus
+ * some gcc version treat 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);
@@ -2398,12 +2408,12 @@
void *ret;
size_t ks;
- if (unlikely(!p))
+ if (unlikely(!p || p == ZERO_SIZE_PTR))
return kmalloc(new_size, flags);
if (unlikely(!new_size)) {
kfree(p);
- return NULL;
+ return ZERO_SIZE_PTR;
}
ks = ksize(p);
@@ -2652,7 +2662,7 @@
struct kmem_cache *s = get_slab(size, gfpflags);
if (!s)
- return NULL;
+ return ZERO_SIZE_PTR;
return slab_alloc(s, gfpflags, -1, caller);
}
@@ -2663,7 +2673,7 @@
struct kmem_cache *s = get_slab(size, gfpflags);
if (!s)
- return NULL;
+ return ZERO_SIZE_PTR;
return slab_alloc(s, gfpflags, node, caller);
}