Commit 272c1d21 authored by Christoph Lameter's avatar Christoph Lameter Committed by Linus Torvalds
Browse files

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: default avatarChristoph Lameter <clameter@sgi.com>
Acked-by: default avatarPekka Enberg <penberg@cs.helsinki.fi>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent a17627ef

include/linux/slub_def.h

+17 −8
Original line number Diff line number Diff line
@@ -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

+18 −8
Original line number Diff line number Diff line
@@ -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);

}

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