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>

2 files changed, 35 insertions, 16 deletions

diff --git a/include/linux/slub_def.h b/include/linux/slub_def.h
index 0764c829d967..a0ad37463d62 100644
--- a/include/linux/slub_def.h
+++ b/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
diff --git a/mm/slub.c b/mm/slub.c
index 51663a3c3c24..c9ab68881b43 100644
--- a/mm/slub.c
+++ b/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);
 }