[PATCH 1/2] mm/slub: remove redundant kasan_reset_tag() from freelist_ptr calculations

[Vlastimil Babka]

Commit d36a63a943e3 ("kasan, slub: fix more conflicts with
CONFIG_SLAB_FREELIST_HARDENED") has introduced kasan_reset_tags() to
freelist_ptr() encoding/decoding when CONFIG_SLAB_FREELIST_HARDENED is
enabled to resolve issues when passing tagged or untagged pointers
inconsistently would lead to incorrect calculations.

Later, commit aa1ef4d7b3f6 ("kasan, mm: reset tags when accessing
metadata") made sure all pointers have tags reset regardless of
CONFIG_SLAB_FREELIST_HARDENED, because there was no other way to access
the freepointer metadata safely with hw tag-based KASAN.

Therefore the kasan_reset_tag() usage in freelist_ptr_encode()/decode()
is now redundant, as all callers use kasan_reset_tag() unconditionally
when constructing ptr_addr. Remove the redundant calls and simplify the
code and remove obsolete comments.

Also in freelist_ptr_encode() introduce an 'encoded' variable to make
the lines shorter and make it similar to the _decode() one.

Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
---
These 2 patches build on top of:
https://lore.kernel.org/all/20230704135834.3884421-1-matteorizzo@xxxxxxxxxx/

 mm/slub.c | 22 ++++++----------------
 1 file changed, 6 insertions(+), 16 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index f8cc47eff742..07edad305512 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -374,22 +374,14 @@ typedef struct { unsigned long v; } freeptr_t;
 static inline freeptr_t freelist_ptr_encode(const struct kmem_cache *s,
 					    void *ptr, unsigned long ptr_addr)
 {
+	unsigned long encoded;
+
 #ifdef CONFIG_SLAB_FREELIST_HARDENED
-	/*
-	 * When CONFIG_KASAN_SW/HW_TAGS is enabled, ptr_addr might be tagged.
-	 * Normally, this doesn't cause any issues, as both set_freepointer()
-	 * and get_freepointer() are called with a pointer with the same tag.
-	 * However, there are some issues with CONFIG_SLUB_DEBUG code. For
-	 * example, when __free_slub() iterates over objects in a cache, it
-	 * passes untagged pointers to check_object(). check_object() in turns
-	 * calls get_freepointer() with an untagged pointer, which causes the
-	 * freepointer to be restored incorrectly.
-	 */
-	return (freeptr_t){.v = (unsigned long)ptr ^ s->random ^
-			swab((unsigned long)kasan_reset_tag((void *)ptr_addr))};
+	encoded = (unsigned long)ptr ^ s->random ^ swab(ptr_addr);
 #else
-	return (freeptr_t){.v = (unsigned long)ptr};
+	encoded = (unsigned long)ptr;
 #endif
+	return (freeptr_t){.v = encoded};
 }
 
 static inline void *freelist_ptr_decode(const struct kmem_cache *s,
@@ -398,9 +390,7 @@ static inline void *freelist_ptr_decode(const struct kmem_cache *s,
 	void *decoded;
 
 #ifdef CONFIG_SLAB_FREELIST_HARDENED
-	/* See the comment in freelist_ptr_encode */
-	decoded = (void *)(ptr.v ^ s->random ^
-		swab((unsigned long)kasan_reset_tag((void *)ptr_addr)));
+	decoded = (void *)(ptr.v ^ s->random ^ swab(ptr_addr));
 #else
 	decoded = (void *)ptr.v;
 #endif
-- 
2.41.0