linux-kernel - [PATCH v2 2/2] slab: Remove special-casing of const 0 size allocations

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Message-Id: <20221118035200.1269184-2-keescook@chromium.org>
Date:   Thu, 17 Nov 2022 19:51:59 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Kees Cook <keescook@...omium.org>,
        Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Hyeonggon Yoo <42.hyeyoo@...il.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: [PATCH v2 2/2] slab: Remove special-casing of const 0 size allocations

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@...e.cz>
Cc: Christoph Lameter <cl@...ux.com>
Cc: Pekka Enberg <penberg@...nel.org>
Cc: David Rientjes <rientjes@...gle.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@....com>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Roman Gushchin <roman.gushchin@...ux.dev>
Cc: Hyeonggon Yoo <42.hyeyoo@...il.com>
Cc: linux-mm@...ck.org
Signed-off-by: Kees Cook <keescook@...omium.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 9033937c758e..84be05208418 100644
--- a/include/linux/slab.h
+++ b/include/linux/slab.h
@@ -561,17 +561,13 @@ 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);
@@ -591,17 +587,13 @@ 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