The patch titled
SLUB: return ZERO_SIZE_PTR for kmalloc(0)
has been added to the -mm tree. Its filename is
slub-return-zero_size_ptr-for-kmalloc0.patch
*** Remember to use Documentation/SubmitChecklist when testing your code ***
See http://www.zip.com.au/~akpm/linux/patches/stuff/added-to-mm.txt to find
out what to do about this
------------------------------------------------------
Subject: SLUB: return ZERO_SIZE_PTR for kmalloc(0)
From: Christoph Lameter <[EMAIL PROTECTED]>
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 <[EMAIL PROTECTED]>
Signed-off-by: Andrew Morton <[EMAIL PROTECTED]>
---
include/linux/slub_def.h | 25 +++++++++++++++++--------
mm/slub.c | 26 ++++++++++++++++++--------
2 files changed, 35 insertions(+), 16 deletions(-)
diff -puN include/linux/slub_def.h~slub-return-zero_size_ptr-for-kmalloc0
include/linux/slub_def.h
--- a/include/linux/slub_def.h~slub-return-zero_size_ptr-for-kmalloc0
+++ a/include/linux/slub_def.h
@@ -70,11 +70,8 @@ extern struct kmem_cache kmalloc_caches[
*/
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
#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,
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
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 -puN mm/slub.c~slub-return-zero_size_ptr-for-kmalloc0 mm/slub.c
--- a/mm/slub.c~slub-return-zero_size_ptr-for-kmalloc0
+++ a/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
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
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
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
struct kmem_cache *s = get_slab(size, gfpflags);
if (!s)
- return NULL;
+ return ZERO_SIZE_PTR;
return slab_alloc(s, gfpflags, node, caller);
}
_
Patches currently in -mm which might be from [EMAIL PROTECTED] are
origin.patch
mount-t-tmpfs-o-mpol=-check-nodes-online.patch
slab-fix-alien-cache-handling.patch
slub-return-zero_size_ptr-for-kmalloc0.patch
fix-gregkh-driver-dmi-based-module-autoloading.patch
git-ubi.patch
quicklist-support-for-x86_64.patch
change-zonelist-order-zonelist-order-selection-logic.patch
change-zonelist-order-zonelist-order-selection-logic-add-check_highest_zone-to-build_zonelists_in_zone_order.patch
change-zonelist-order-v6-zonelist-fix.patch
change-zonelist-order-auto-configuration.patch
change-zonelist-order-documentaion.patch
make-proc-slabinfo-use-seq_list_xxx-helpers.patch
make-proc-slabinfo-use-seq_list_xxx-helpers-fix.patch
remove-the-deprecated-kmem_cache_t-typedef-from-slabh.patch
slub-use-ilog2-instead-of-series-of-constant-comparisons.patch
slub-support-slub_debug-on-by-default.patch
slub-support-slub_debug-on-by-default-tidy.patch
add-__gfp_movable-for-callers-to-flag-allocations-from-high-memory-that-may-be-migrated.patch
group-short-lived-and-reclaimable-kernel-allocations.patch
fix-calculation-in-move_freepages_block-for-counting-pages.patch
breakout-page_order-to-internalh-to-avoid-special-knowledge-of-the-buddy-allocator.patch
do-not-depend-on-max_order-when-grouping-pages-by-mobility.patch
print-out-statistics-in-relation-to-fragmentation-avoidance-to-proc-pagetypeinfo.patch
have-kswapd-keep-a-minimum-order-free-other-than-order-0.patch
have-kswapd-keep-a-minimum-order-free-other-than-order-0-fix.patch
only-check-absolute-watermarks-for-alloc_high-and-alloc_harder-allocations.patch
slub-mm-only-make-slub-the-default-slab-allocator.patch
slub-exploit-page-mobility-to-increase-allocation-order.patch
slub-reduce-antifrag-max-order.patch
slub-reduce-antifrag-max-order-use-antifrag-constant-instead-of-hardcoding-page-order.patch
slub-change-error-reporting-format-to-follow-lockdep-loosely.patch
slub-change-error-reporting-format-to-follow-lockdep-loosely-fix.patch
hexdump-more-output-formatting.patch
define-config_bounce-to-avoid-useless-inclusion-of-bounce-buffer.patch
define-new-percpu-interface-for-shared-data.patch
use-the-new-percpu-interface-for-shared-data.patch
revoke-core-code.patch
mm-implement-swap-prefetching.patch
rename-gfp_high_movable-to-gfp_highuser_movable-prefetch.patch
print-out-page_owner-statistics-in-relation-to-fragmentation-avoidance.patch
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