[PATCH] lkdtm: use SLAB_NO_MERGE instead of an empty constructor

[PATCH] lkdtm: use SLAB_NO_MERGE instead of an empty constructor

[Harry Yoo]

Use SLAB_NO_MERGE flag to prevent merging instead of providing an
empty constructor. Using an empty constructor in this manner is an abuse
of slab interface.

The SLAB_NO_MERGE flag should be used with caution, but in this case,
it is acceptable as the cache is intended soley for debugging purposes.

No functional changes intended.

Signed-off-by: Harry Yoo <harry.yoo@oracle.com>
---
 drivers/misc/lkdtm/heap.c | 17 +++--------------
 1 file changed, 3 insertions(+), 14 deletions(-)

diff --git a/drivers/misc/lkdtm/heap.c b/drivers/misc/lkdtm/heap.c
index b1b316f99703..c1a05b935894 100644
--- a/drivers/misc/lkdtm/heap.c
+++ b/drivers/misc/lkdtm/heap.c
@@ -355,23 +355,12 @@ static void lkdtm_SLAB_FREE_PAGE(void)
 	free_page(p);
 }
 
-/*
- * We have constructors to keep the caches distinctly separated without
- * needing to boot with "slab_nomerge".
- */
-static void ctor_double_free(void *region)
-{ }
-static void ctor_a(void *region)
-{ }
-static void ctor_b(void *region)
-{ }
-
 void __init lkdtm_heap_init(void)
 {
 	double_free_cache = kmem_cache_create("lkdtm-heap-double_free",
-					      64, 0, 0, ctor_double_free);
-	a_cache = kmem_cache_create("lkdtm-heap-a", 64, 0, 0, ctor_a);
-	b_cache = kmem_cache_create("lkdtm-heap-b", 64, 0, 0, ctor_b);
+					      64, 0, SLAB_NO_MERGE, NULL);
+	a_cache = kmem_cache_create("lkdtm-heap-a", 64, 0, SLAB_NO_MERGE, NULL);
+	b_cache = kmem_cache_create("lkdtm-heap-b", 64, 0, SLAB_NO_MERGE, NULL);
 }
 
 void __exit lkdtm_heap_exit(void)
-- 
2.43.0

Re: [PATCH] lkdtm: use SLAB_NO_MERGE instead of an empty constructor

[Harry Yoo]

On Tue, Mar 18, 2025 at 10:45:33AM +0900, Harry Yoo wrote:
> Use SLAB_NO_MERGE flag to prevent merging instead of providing an
> empty constructor. Using an empty constructor in this manner is an abuse
> of slab interface.
> 
> The SLAB_NO_MERGE flag should be used with caution, but in this case,
> it is acceptable as the cache is intended soley for debugging purposes.
> 
> No functional changes intended.
> 
> Signed-off-by: Harry Yoo <harry.yoo@oracle.com>

Kindly ping :)

-- 
Cheers,
Harry / Hyeonggon

> ---
>  drivers/misc/lkdtm/heap.c | 17 +++--------------
>  1 file changed, 3 insertions(+), 14 deletions(-)
> 
> diff --git a/drivers/misc/lkdtm/heap.c b/drivers/misc/lkdtm/heap.c
> index b1b316f99703..c1a05b935894 100644
> --- a/drivers/misc/lkdtm/heap.c
> +++ b/drivers/misc/lkdtm/heap.c
> @@ -355,23 +355,12 @@ static void lkdtm_SLAB_FREE_PAGE(void)
>  	free_page(p);
>  }
>  
> -/*
> - * We have constructors to keep the caches distinctly separated without
> - * needing to boot with "slab_nomerge".
> - */
> -static void ctor_double_free(void *region)
> -{ }
> -static void ctor_a(void *region)
> -{ }
> -static void ctor_b(void *region)
> -{ }
> -
>  void __init lkdtm_heap_init(void)
>  {
>  	double_free_cache = kmem_cache_create("lkdtm-heap-double_free",
> -					      64, 0, 0, ctor_double_free);
> -	a_cache = kmem_cache_create("lkdtm-heap-a", 64, 0, 0, ctor_a);
> -	b_cache = kmem_cache_create("lkdtm-heap-b", 64, 0, 0, ctor_b);
> +					      64, 0, SLAB_NO_MERGE, NULL);
> +	a_cache = kmem_cache_create("lkdtm-heap-a", 64, 0, SLAB_NO_MERGE, NULL);
> +	b_cache = kmem_cache_create("lkdtm-heap-b", 64, 0, SLAB_NO_MERGE, NULL);
>  }
>  
>  void __exit lkdtm_heap_exit(void)
> -- 
> 2.43.0
> 

Re: [PATCH] lkdtm: use SLAB_NO_MERGE instead of an empty constructor

[Kees Cook]

On Tue, 18 Mar 2025 10:45:33 +0900, Harry Yoo wrote:
> Use SLAB_NO_MERGE flag to prevent merging instead of providing an
> empty constructor. Using an empty constructor in this manner is an abuse
> of slab interface.
> 
> The SLAB_NO_MERGE flag should be used with caution, but in this case,
> it is acceptable as the cache is intended soley for debugging purposes.
> 
> [...]

Applied to for-next/hardening, thanks!

[1/1] lkdtm: use SLAB_NO_MERGE instead of an empty constructor
      https://git.kernel.org/kees/c/9891398efae8

Take care,

-- 
Kees Cook