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>

include/linux/slub_def.h

@@ -70,11 +70,8 @@ extern struct kmem_cache kmalloc_caches[KMALLOC_SHIFT_HIGH + 1];
  */
 static inline int kmalloc_index(size_t size)
 {
-	/*
-	 * We should return 0 if size == 0 but we use the smallest object
-	 * here for SLAB legacy reasons.
-	 */
-	WARN_ON_ONCE(size == 0);
+	if (!size)
+		return 0;
 
 	if (size > KMALLOC_MAX_SIZE)
 		return -1;
@@ -153,13 +150,25 @@ static inline struct kmem_cache *kmalloc_slab(size_t size)
 #define SLUB_DMA 0
 #endif
 
+
+/*
+ * ZERO_SIZE_PTR will be returned for zero sized kmalloc requests.
+ *
+ * Dereferencing ZERO_SIZE_PTR will lead to a distinct access fault.
+ *
+ * ZERO_SIZE_PTR can be passed to kfree though in the same way that NULL can.
+ * Both make kfree a no-op.
+ */
+#define ZERO_SIZE_PTR ((void *)16)
+
+
 static inline void *kmalloc(size_t size, gfp_t flags)
 {
 	if (__builtin_constant_p(size) && !(flags & SLUB_DMA)) {
 		struct kmem_cache *s = kmalloc_slab(size);
 
 		if (!s)
-			return NULL;
+			return ZERO_SIZE_PTR;
 
 		return kmem_cache_alloc(s, flags);
 	} else
@@ -172,7 +181,7 @@ static inline void *kzalloc(size_t size, gfp_t flags)
 		struct kmem_cache *s = kmalloc_slab(size);
 
 		if (!s)
-			return NULL;
+			return ZERO_SIZE_PTR;
 
 		return kmem_cache_zalloc(s, flags);
 	} else
@@ -188,7 +197,7 @@ static inline void *kmalloc_node(size_t size, gfp_t flags, int node)
 		struct kmem_cache *s = kmalloc_slab(size);
 
 		if (!s)
-			return NULL;
+			return ZERO_SIZE_PTR;
 
 		return kmem_cache_alloc_node(s, flags, node);
 	} else

mm/slub.c

@@ -2241,7 +2241,7 @@ void *__kmalloc(size_t size, gfp_t flags)
 
 	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 @@ void *__kmalloc_node(size_t size, gfp_t flags, int node)
 
 	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 @@ void kfree(const void *x)
 	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 *krealloc(const void *p, size_t new_size, gfp_t flags)
 	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 @@ void *__kmalloc_track_caller(size_t size, gfp_t gfpflags, void *caller)
 	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 @@ void *__kmalloc_node_track_caller(size_t size, gfp_t gfpflags,
 	struct kmem_cache *s = get_slab(size, gfpflags);
 
 	if (!s)
-		return NULL;
+		return ZERO_SIZE_PTR;
 
 	return slab_alloc(s, gfpflags, node, caller);
 }