kasan: only define kasan_cache_create for Generic mode

Right now, kasan_cache_create() assigns SLAB_KASAN for all KASAN modes and
then sets up metadata-related cache parameters for the Generic mode.

SLAB_KASAN is used in two places:

1. In slab_ksize() to account for per-object metadata when
   calculating the size of the accessible memory within the object.
2. In slab_common.c via kasan_never_merge() to prevent merging of
   caches with per-object metadata.

Both cases are only relevant when per-object metadata is present, which is
only the case with the Generic mode.

Thus, assign SLAB_KASAN and define kasan_cache_create() only for the
Generic mode.

Also update the SLAB_KASAN-related comment.

Link: https://lkml.kernel.org/r/61faa2aa1906e2d02c97d00ddf99ce8911dda095.1662411799.git.andreyknvl@google.com
Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
Reviewed-by: Marco Elver <elver@google.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Evgenii Stepanov <eugenis@google.com>
Cc: Peter Collingbourne <pcc@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

include/linux/kasan.h

@@ -128,15 +128,6 @@ static __always_inline void kasan_unpoison_pages(struct page *page,
 		__kasan_unpoison_pages(page, order, init);
 }
 
-void __kasan_cache_create(struct kmem_cache *cache, unsigned int *size,
-				slab_flags_t *flags);
-static __always_inline void kasan_cache_create(struct kmem_cache *cache,
-				unsigned int *size, slab_flags_t *flags)
-{
-	if (kasan_enabled())
-		__kasan_cache_create(cache, size, flags);
-}
-
 void __kasan_cache_create_kmalloc(struct kmem_cache *cache);
 static __always_inline void kasan_cache_create_kmalloc(struct kmem_cache *cache)
 {
@@ -260,9 +251,6 @@ static inline void kasan_poison_pages(struct page *page, unsigned int order,
 				      bool init) {}
 static inline void kasan_unpoison_pages(struct page *page, unsigned int order,
 					bool init) {}
-static inline void kasan_cache_create(struct kmem_cache *cache,
-				      unsigned int *size,
-				      slab_flags_t *flags) {}
 static inline void kasan_cache_create_kmalloc(struct kmem_cache *cache) {}
 static inline void kasan_poison_slab(struct slab *slab) {}
 static inline void kasan_unpoison_object_data(struct kmem_cache *cache,
@@ -316,6 +304,8 @@ static inline void kasan_unpoison_task_stack(struct task_struct *task) {}
 
 size_t kasan_metadata_size(struct kmem_cache *cache);
 slab_flags_t kasan_never_merge(void);
+void kasan_cache_create(struct kmem_cache *cache, unsigned int *size,
+			slab_flags_t *flags);
 
 void kasan_cache_shrink(struct kmem_cache *cache);
 void kasan_cache_shutdown(struct kmem_cache *cache);
@@ -334,6 +324,10 @@ static inline slab_flags_t kasan_never_merge(void)
 {
 	return 0;
 }
+/* And no cache-related metadata initialization is required. */
+static inline void kasan_cache_create(struct kmem_cache *cache,
+				      unsigned int *size,
+				      slab_flags_t *flags) {}
 
 static inline void kasan_cache_shrink(struct kmem_cache *cache) {}
 static inline void kasan_cache_shutdown(struct kmem_cache *cache) {}

include/linux/slab.h

@@ -106,7 +106,7 @@
 # define SLAB_ACCOUNT		0
 #endif
 
-#ifdef CONFIG_KASAN
+#ifdef CONFIG_KASAN_GENERIC
 #define SLAB_KASAN		((slab_flags_t __force)0x08000000U)
 #else
 #define SLAB_KASAN		0

mm/kasan/common.c

@@ -110,22 +110,6 @@ void __kasan_poison_pages(struct page *page, unsigned int order, bool init)
 			     KASAN_PAGE_FREE, init);
 }
 
-void __kasan_cache_create(struct kmem_cache *cache, unsigned int *size,
-			  slab_flags_t *flags)
-{
-	/*
-	 * SLAB_KASAN is used to mark caches as ones that are sanitized by
-	 * KASAN. Currently this flag is used in two places:
-	 * 1. In slab_ksize() when calculating the size of the accessible
-	 *    memory within the object.
-	 * 2. In slab_common.c to prevent merging of sanitized caches.
-	 */
-	*flags |= SLAB_KASAN;
-
-	if (kasan_requires_meta())
-		kasan_init_cache_meta(cache, size);
-}
-
 void __kasan_cache_create_kmalloc(struct kmem_cache *cache)
 {
 	cache->kasan_info.is_kmalloc = true;

mm/kasan/generic.c

@@ -352,11 +352,26 @@ static inline unsigned int optimal_redzone(unsigned int object_size)
 		object_size <= (1 << 16) - 1024 ? 1024 : 2048;
 }
 
-void kasan_init_cache_meta(struct kmem_cache *cache, unsigned int *size)
+void kasan_cache_create(struct kmem_cache *cache, unsigned int *size,
+			  slab_flags_t *flags)
 {
 	unsigned int ok_size;
 	unsigned int optimal_size;
 
+	if (!kasan_requires_meta())
+		return;
+
+	/*
+	 * SLAB_KASAN is used to mark caches that are sanitized by KASAN
+	 * and that thus have per-object metadata.
+	 * Currently this flag is used in two places:
+	 * 1. In slab_ksize() to account for per-object metadata when
+	 *    calculating the size of the accessible memory within the object.
+	 * 2. In slab_common.c via kasan_never_merge() to prevent merging of
+	 *    caches with per-object metadata.
+	 */
+	*flags |= SLAB_KASAN;
+
 	ok_size = *size;
 
 	/* Add alloc meta into redzone. */