kasan: introduce kasan_requires_meta

Add a kasan_requires_meta() helper that indicates whether the enabled
KASAN mode requires per-object metadata and use this helper in the common
code.

Also hide kasan_init_object_meta() under CONFIG_KASAN_GENERIC ifdef check,
as Generic is the only mode that uses per-object metadata.

To allow for a potential future change that makes Generic KASAN support
the kasan.stacktrace command-line parameter, let kasan_requires_meta()
return kasan_stack_collection_enabled() instead of simply returning true.

Link: https://lkml.kernel.org/r/cf837e9996246aaaeebf704ccf8ec26a34fcf64f.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>

mm/kasan/common.c

@@ -88,13 +88,10 @@ asmlinkage void kasan_unpoison_task_stack_below(const void *watermark)
 }
 #endif /* CONFIG_KASAN_STACK */
 
-/*
- * Only allow cache merging when stack collection is disabled and no metadata
- * is present.
- */
+/* Only allow cache merging when no per-object metadata is present. */
 slab_flags_t __kasan_never_merge(void)
 {
-	if (kasan_stack_collection_enabled())
+	if (kasan_requires_meta())
 		return SLAB_KASAN;
 	return 0;
 }
@@ -152,7 +149,7 @@ void __kasan_cache_create(struct kmem_cache *cache, unsigned int *size,
 	 */
 	*flags |= SLAB_KASAN;
 
-	if (!kasan_stack_collection_enabled())
+	if (!kasan_requires_meta())
 		return;
 
 	ok_size = *size;
@@ -220,7 +217,7 @@ void __kasan_cache_create_kmalloc(struct kmem_cache *cache)
 
 size_t __kasan_metadata_size(struct kmem_cache *cache)
 {
-	if (!kasan_stack_collection_enabled())
+	if (!kasan_requires_meta())
 		return 0;
 	return (cache->kasan_info.alloc_meta_offset ?
 		sizeof(struct kasan_alloc_meta) : 0) +
@@ -295,7 +292,7 @@ void * __must_check __kasan_init_slab_obj(struct kmem_cache *cache,
 						const void *object)
 {
 	/* Initialize per-object metadata if it is present. */
-	if (kasan_stack_collection_enabled())
+	if (kasan_requires_meta())
 		kasan_init_object_meta(cache, object);
 
 	/* Tag is ignored in set_tag() without CONFIG_KASAN_SW/HW_TAGS */

mm/kasan/kasan.h

@@ -43,7 +43,7 @@ static inline bool kasan_sync_fault_possible(void)
 	return kasan_mode == KASAN_MODE_SYNC || kasan_mode == KASAN_MODE_ASYMM;
 }
 
-#else
+#else /* CONFIG_KASAN_HW_TAGS */
 
 static inline bool kasan_stack_collection_enabled(void)
 {
@@ -60,7 +60,31 @@ static inline bool kasan_sync_fault_possible(void)
 	return true;
 }
 
-#endif
+#endif /* CONFIG_KASAN_HW_TAGS */
+
+#ifdef CONFIG_KASAN_GENERIC
+
+/* Generic KASAN uses per-object metadata to store stack traces. */
+static inline bool kasan_requires_meta(void)
+{
+	/*
+	 * Technically, Generic KASAN always collects stack traces right now.
+	 * However, let's use kasan_stack_collection_enabled() in case the
+	 * kasan.stacktrace command-line argument is changed to affect
+	 * Generic KASAN.
+	 */
+	return kasan_stack_collection_enabled();
+}
+
+#else /* CONFIG_KASAN_GENERIC */
+
+/* Tag-based KASAN modes do not use per-object metadata. */
+static inline bool kasan_requires_meta(void)
+{
+	return false;
+}
+
+#endif /* CONFIG_KASAN_GENERIC */
 
 #if defined(CONFIG_KASAN_GENERIC) || defined(CONFIG_KASAN_SW_TAGS)
 #define KASAN_GRANULE_SIZE	(1UL << KASAN_SHADOW_SCALE_SHIFT)
@@ -272,13 +296,14 @@ void kasan_report_invalid_free(void *object, unsigned long ip, enum kasan_report
 struct page *kasan_addr_to_page(const void *addr);
 struct slab *kasan_addr_to_slab(const void *addr);
 
-void kasan_init_object_meta(struct kmem_cache *cache, const void *object);
-
 #ifdef CONFIG_KASAN_GENERIC
+void kasan_init_object_meta(struct kmem_cache *cache, const void *object);
 struct kasan_alloc_meta *kasan_get_alloc_meta(struct kmem_cache *cache,
 						const void *object);
 struct kasan_free_meta *kasan_get_free_meta(struct kmem_cache *cache,
 						const void *object);
+#else
+static inline void kasan_init_object_meta(struct kmem_cache *cache, const void *object) { }
 #endif
 
 depot_stack_handle_t kasan_save_stack(gfp_t flags, bool can_alloc);

mm/kasan/tags.c

@@ -17,10 +17,6 @@
 
 #include "kasan.h"
 
-void kasan_init_object_meta(struct kmem_cache *cache, const void *object)
-{
-}
-
 void kasan_save_alloc_info(struct kmem_cache *cache, void *object, gfp_t flags)
 {
 }