kasan: only define kasan_never_merge for Generic mode

KASAN prevents merging of slab caches whose objects have per-object
metadata stored in redzones.

As now only the Generic mode uses per-object metadata, define
kasan_never_merge() only for this mode.

Link: https://lkml.kernel.org/r/81ed01f29ff3443580b7e2fe362a8b47b1e8006d.1662411799.git.andreyknvl@google.comSigned-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

@@ -103,14 +103,6 @@ struct kasan_cache {
 	bool is_kmalloc;
 };
 
-slab_flags_t __kasan_never_merge(void);
-static __always_inline slab_flags_t kasan_never_merge(void)
-{
-	if (kasan_enabled())
-		return __kasan_never_merge();
-	return 0;
-}
-
 void __kasan_unpoison_range(const void *addr, size_t size);
 static __always_inline void kasan_unpoison_range(const void *addr, size_t size)
 {
@@ -261,10 +253,6 @@ static __always_inline bool kasan_check_byte(const void *addr)
 
 #else /* CONFIG_KASAN */
 
-static inline slab_flags_t kasan_never_merge(void)
-{
-	return 0;
-}
 static inline void kasan_unpoison_range(const void *address, size_t size) {}
 static inline void kasan_poison_pages(struct page *page, unsigned int order,
 				      bool init) {}
@@ -325,6 +313,7 @@ static inline void kasan_unpoison_task_stack(struct task_struct *task) {}
 #ifdef CONFIG_KASAN_GENERIC
 
 size_t kasan_metadata_size(struct kmem_cache *cache);
+slab_flags_t kasan_never_merge(void);
 
 void kasan_cache_shrink(struct kmem_cache *cache);
 void kasan_cache_shutdown(struct kmem_cache *cache);
@@ -338,6 +327,11 @@ static inline size_t kasan_metadata_size(struct kmem_cache *cache)
 {
 	return 0;
 }
+/* And thus nothing prevents cache merging. */
+static inline slab_flags_t kasan_never_merge(void)
+{
+	return 0;
+}
 
 static inline void kasan_cache_shrink(struct kmem_cache *cache) {}
 static inline void kasan_cache_shutdown(struct kmem_cache *cache) {}

mm/kasan/common.c

@@ -88,14 +88,6 @@ asmlinkage void kasan_unpoison_task_stack_below(const void *watermark)
 }
 #endif /* CONFIG_KASAN_STACK */
 
-/* Only allow cache merging when no per-object metadata is present. */
-slab_flags_t __kasan_never_merge(void)
-{
-	if (kasan_requires_meta())
-		return SLAB_KASAN;
-	return 0;
-}
-
 void __kasan_unpoison_pages(struct page *page, unsigned int order, bool init)
 {
 	u8 tag;

mm/kasan/generic.c

@@ -328,6 +328,14 @@ DEFINE_ASAN_SET_SHADOW(f3);
 DEFINE_ASAN_SET_SHADOW(f5);
 DEFINE_ASAN_SET_SHADOW(f8);
 
+/* Only allow cache merging when no per-object metadata is present. */
+slab_flags_t kasan_never_merge(void)
+{
+	if (!kasan_requires_meta())
+		return 0;
+	return SLAB_KASAN;
+}
+
 /*
  * Adaptive redzone policy taken from the userspace AddressSanitizer runtime.
  * For larger allocations larger redzones are used.