2022-11-18 04:07:00

by Kees Cook

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 <[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 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

2022-11-18 11:55:05

by Hyeonggon Yoo

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

On Thu, Nov 17, 2022 at 07:51:59PM -0800, 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 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

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

--
Thanks,
Hyeonggon

2022-11-18 12:28:07

by Vlastimil Babka

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

On 11/18/22 04:51, 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,

So with the kmalloc_trace() already solved, we could now say it's not "in
preparation", but simply "makes sure", right? I can correct that while
picking this patch.

> 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 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);

2022-11-18 18:52:55

by Kees Cook

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

On Fri, Nov 18, 2022 at 12:34:01PM +0100, Vlastimil Babka wrote:
> On 11/18/22 04:51, 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,
>
> So with the kmalloc_trace() already solved, we could now say it's not "in
> preparation", but simply "makes sure", right? I can correct that while
> picking this patch.

Yeah, good point. I missed this when updating the commit logs. Thanks!

--
Kees Cook

2022-11-21 09:43:25

by Vlastimil Babka

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

On 11/18/22 18:06, Kees Cook wrote:
> On Fri, Nov 18, 2022 at 12:34:01PM +0100, Vlastimil Babka wrote:
>> On 11/18/22 04:51, 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,
>>
>> So with the kmalloc_trace() already solved, we could now say it's not "in
>> preparation", but simply "makes sure", right? I can correct that while
>> picking this patch.
>
> Yeah, good point. I missed this when updating the commit logs. Thanks!

Great, pushed to slab/for-6.2/alloc_size