2022-11-01 22:52:21

by Kees Cook

Subject: [PATCH 2/6] 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 <[email protected]>
Cc: Christoph Lameter <[email protected]>
Cc: Pekka Enberg <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Joonsoo Kim <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Roman Gushchin <[email protected]>
Cc: Hyeonggon Yoo <[email protected]>
Cc: [email protected]
Signed-off-by: Kees Cook <[email protected]>
---
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

2022-11-03 14:29:30

by Hyeonggon Yoo

Subject: Re: [PATCH 2/6] slab: Remove special-casing of const 0 size allocations

On Tue, Nov 01, 2022 at 03:33:10PM -0700, Kees Cook wrote:
> 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 <[email protected]>
> Cc: Christoph Lameter <[email protected]>
> Cc: Pekka Enberg <[email protected]>
> Cc: David Rientjes <[email protected]>
> Cc: Joonsoo Kim <[email protected]>
> Cc: Andrew Morton <[email protected]>
> Cc: Roman Gushchin <[email protected]>
> Cc: Hyeonggon Yoo <[email protected]>
> Cc: [email protected]
> Signed-off-by: Kees Cook <[email protected]>
> ---
> 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

Looks good to me.

Reviewed-by: Hyeonggon Yoo <[email protected]>

--
Thanks,
Hyeonggon