linux-kernel - [PATCH 5/5] [DEBUG] slab: Report number of NULLings

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Message-Id: <20250321204105.1898507-5-kees@kernel.org>
Date: Fri, 21 Mar 2025 13:41:01 -0700
From: Kees Cook <kees@...nel.org>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: Kees Cook <kees@...nel.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,
	Miguel Ojeda <ojeda@...nel.org>,
	Nathan Chancellor <nathan@...nel.org>,
	Marco Elver <elver@...gle.com>,
	Nick Desaulniers <ndesaulniers@...gle.com>,
	Przemek Kitszel <przemyslaw.kitszel@...el.com>,
	linux-kernel@...r.kernel.org,
	linux-hardening@...r.kernel.org
Subject: [PATCH 5/5] [DEBUG] slab: Report number of NULLings

Just to get a sense of what's happening, report the number of NULL
assignments that have been done. After booting an otherwise standard
Ubuntu image, this shows about 240,000 NULLifications have been performed.

Signed-off-by: Kees Cook <kees@...nel.org>
---
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: Vlastimil Babka <vbabka@...e.cz>
Cc: Roman Gushchin <roman.gushchin@...ux.dev>
Cc: Hyeonggon Yoo <42.hyeyoo@...il.com>
Cc: linux-mm@...ck.org
---
 include/linux/slab.h | 3 +++
 mm/slab_common.c     | 4 ++++
 2 files changed, 7 insertions(+)

diff --git a/include/linux/slab.h b/include/linux/slab.h
index 2717ad238fa2..a4740c8b6ccb 100644
--- a/include/linux/slab.h
+++ b/include/linux/slab.h
@@ -469,6 +469,8 @@ void __kfree(const void *objp);
 void __kfree_sensitive(const void *objp);
 size_t __ksize(const void *objp);
 
+extern atomic_t count_nulled;
+
 static inline void kfree_and_null(void **ptr)
 {
 	__kfree(*ptr);
@@ -487,6 +489,7 @@ static inline void kfree_sensitive_and_null(void **ptr)
 ({					\
 	typeof(x) *__ptr = &(x);	\
 	__how ## _and_null((void **)__ptr);	\
+	atomic_inc(&count_nulled);	\
 })
 #define __free_and_maybe_null(__how, x)	\
 	__builtin_choose_expr(__is_lvalue(x), \
diff --git a/mm/slab_common.c b/mm/slab_common.c
index 9a82952ec266..0412cbab81f9 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -42,6 +42,9 @@ LIST_HEAD(slab_caches);
 DEFINE_MUTEX(slab_mutex);
 struct kmem_cache *kmem_cache;
 
+atomic_t count_nulled = ATOMIC_INIT(0);
+EXPORT_SYMBOL(count_nulled);
+
 /*
  * Set of flags that will prevent slab merging
  */
@@ -1084,6 +1087,7 @@ static void print_slabinfo_header(struct seq_file *m)
 	 * without _too_ many complaints.
 	 */
 	seq_puts(m, "slabinfo - version: 2.1\n");
+	seq_printf(m, "# nulled: %d\n", atomic_read(&count_nulled));
 	seq_puts(m, "# name            <active_objs> <num_objs> <objsize> <objperslab> <pagesperslab>");
 	seq_puts(m, " : tunables <limit> <batchcount> <sharedfactor>");
 	seq_puts(m, " : slabdata <active_slabs> <num_slabs> <sharedavail>");
-- 
2.34.1