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Subject[PATCH 2/6] slab: Remove special-casing of const 0 size allocations
Date
Passing a constant-0 size allocation into kmalloc() or kmalloc_node()
does not need to be a fast-path operation, so the static return value
can be removed entirely. This is in preparation for making sure that
all paths through the inlines result in a full extern function call,
where __alloc_size() hints will actually be seen[1] by GCC. (A constant
return value of 0 means the "0" allocation size won't be propagated by
the inline.)

[1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96503

Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Christoph Lameter <cl@linux.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Cc: linux-mm@kvack.org
Signed-off-by: Kees Cook <keescook@chromium.org>
---
include/linux/slab.h | 12 ++----------
1 file changed, 2 insertions(+), 10 deletions(-)

diff --git a/include/linux/slab.h b/include/linux/slab.h
index e08fe7978b5c..970e9504949e 100644
--- a/include/linux/slab.h
+++ b/include/linux/slab.h
@@ -562,17 +562,13 @@ void *kmalloc_large_node(size_t size, gfp_t flags, int node) __assume_page_align
#ifndef CONFIG_SLOB
static __always_inline __alloc_size(1) void *kmalloc(size_t size, gfp_t flags)
{
- if (__builtin_constant_p(size)) {
+ if (__builtin_constant_p(size) && size) {
unsigned int index;

if (size > KMALLOC_MAX_CACHE_SIZE)
return kmalloc_large(size, flags);

index = kmalloc_index(size);
-
- if (!index)
- return ZERO_SIZE_PTR;
-
return kmalloc_trace(
kmalloc_caches[kmalloc_type(flags)][index],
flags, size);
@@ -592,17 +588,13 @@ static __always_inline __alloc_size(1) void *kmalloc(size_t size, gfp_t flags)
#ifndef CONFIG_SLOB
static __always_inline __alloc_size(1) void *kmalloc_node(size_t size, gfp_t flags, int node)
{
- if (__builtin_constant_p(size)) {
+ if (__builtin_constant_p(size) && size) {
unsigned int index;

if (size > KMALLOC_MAX_CACHE_SIZE)
return kmalloc_large_node(size, flags, node);

index = kmalloc_index(size);
-
- if (!index)
- return ZERO_SIZE_PTR;
-
return kmalloc_node_trace(
kmalloc_caches[kmalloc_type(flags)][index],
flags, node, size);
--
2.34.1
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