Merge branch 'slab/for-6.12/rcu_barriers' into slab/for-next

Merge most of SLUB feature work for 6.12:

- Barrier for pending kfree_rcu() in kmem_cache_destroy() and associated
  refactoring of the destroy path (Vlastimil Babka)
- CONFIG_SLUB_RCU_DEBUG to allow KASAN catching UAF bugs in
  SLAB_TYPESAFE_BY_RCU caches (Jann Horn)
- kmem_cache_charge() for delayed kmemcg charging (Shakeel Butt)

6 files changed, 300 insertions, 113 deletions

diff --git a/mm/Kconfig.debug b/mm/Kconfig.debug
index afc72fde0f03..41a58536531d 100644
--- a/mm/Kconfig.debug
+++ b/mm/Kconfig.debug
@@ -70,6 +70,38 @@ config SLUB_DEBUG_ON
 	  off in a kernel built with CONFIG_SLUB_DEBUG_ON by specifying
 	  "slab_debug=-".
 
+config SLUB_RCU_DEBUG
+	bool "Enable UAF detection in TYPESAFE_BY_RCU caches (for KASAN)"
+	depends on SLUB_DEBUG
+	# SLUB_RCU_DEBUG should build fine without KASAN, but is currently useless
+	# without KASAN, so mark it as a dependency of KASAN for now.
+	depends on KASAN
+	default KASAN_GENERIC || KASAN_SW_TAGS
+	help
+	  Make SLAB_TYPESAFE_BY_RCU caches behave approximately as if the cache
+	  was not marked as SLAB_TYPESAFE_BY_RCU and every caller used
+	  kfree_rcu() instead.
+
+	  This is intended for use in combination with KASAN, to enable KASAN to
+	  detect use-after-free accesses in such caches.
+	  (KFENCE is able to do that independent of this flag.)
+
+	  This might degrade performance.
+	  Unfortunately this also prevents a very specific bug pattern from
+	  triggering (insufficient checks against an object being recycled
+	  within the RCU grace period); so this option can be turned off even on
+	  KASAN builds, in case you want to test for such a bug.
+
+	  If you're using this for testing bugs / fuzzing and care about
+	  catching all the bugs WAY more than performance, you might want to
+	  also turn on CONFIG_RCU_STRICT_GRACE_PERIOD.
+
+	  WARNING:
+	  This is designed as a debugging feature, not a security feature.
+	  Objects are sometimes recycled without RCU delay under memory pressure.
+
+	  If unsure, say N.
+
 config PAGE_OWNER
 	bool "Track page owner"
 	depends on DEBUG_KERNEL && STACKTRACE_SUPPORT
diff --git a/mm/kasan/common.c b/mm/kasan/common.c
index 85e7c6b4575c..ed4873e18c75 100644
--- a/mm/kasan/common.c
+++ b/mm/kasan/common.c
@@ -208,15 +208,12 @@ void * __must_check __kasan_init_slab_obj(struct kmem_cache *cache,
 	return (void *)object;
 }
 
-static inline bool poison_slab_object(struct kmem_cache *cache, void *object,
-				      unsigned long ip, bool init)
+/* Returns true when freeing the object is not safe. */
+static bool check_slab_allocation(struct kmem_cache *cache, void *object,
+				  unsigned long ip)
 {
-	void *tagged_object;
-
-	if (!kasan_arch_is_ready())
-		return false;
+	void *tagged_object = object;
 
-	tagged_object = object;
 	object = kasan_reset_tag(object);
 
 	if (unlikely(nearest_obj(cache, virt_to_slab(object), object) != object)) {
@@ -224,37 +221,47 @@ static inline bool poison_slab_object(struct kmem_cache *cache, void *object,
 		return true;
 	}
 
-	/* RCU slabs could be legally used after free within the RCU period. */
-	if (unlikely(cache->flags & SLAB_TYPESAFE_BY_RCU))
-		return false;
-
 	if (!kasan_byte_accessible(tagged_object)) {
 		kasan_report_invalid_free(tagged_object, ip, KASAN_REPORT_DOUBLE_FREE);
 		return true;
 	}
 
+	return false;
+}
+
+static inline void poison_slab_object(struct kmem_cache *cache, void *object,
+				      bool init, bool still_accessible)
+{
+	void *tagged_object = object;
+
+	object = kasan_reset_tag(object);
+
+	/* RCU slabs could be legally used after free within the RCU period. */
+	if (unlikely(still_accessible))
+		return;
+
 	kasan_poison(object, round_up(cache->object_size, KASAN_GRANULE_SIZE),
 			KASAN_SLAB_FREE, init);
 
 	if (kasan_stack_collection_enabled())
 		kasan_save_free_info(cache, tagged_object);
+}
 
-	return false;
+bool __kasan_slab_pre_free(struct kmem_cache *cache, void *object,
+				unsigned long ip)
+{
+	if (!kasan_arch_is_ready() || is_kfence_address(object))
+		return false;
+	return check_slab_allocation(cache, object, ip);
 }
 
-bool __kasan_slab_free(struct kmem_cache *cache, void *object,
-				unsigned long ip, bool init)
+bool __kasan_slab_free(struct kmem_cache *cache, void *object, bool init,
+		       bool still_accessible)
 {
-	if (is_kfence_address(object))
+	if (!kasan_arch_is_ready() || is_kfence_address(object))
 		return false;
 
-	/*
-	 * If the object is buggy, do not let slab put the object onto the
-	 * freelist. The object will thus never be allocated again and its
-	 * metadata will never get released.
-	 */
-	if (poison_slab_object(cache, object, ip, init))
-		return true;
+	poison_slab_object(cache, object, init, still_accessible);
 
 	/*
 	 * If the object is put into quarantine, do not let slab put the object
@@ -504,11 +511,16 @@ bool __kasan_mempool_poison_object(void *ptr, unsigned long ip)
 		return true;
 	}
 
-	if (is_kfence_address(ptr))
-		return false;
+	if (is_kfence_address(ptr) || !kasan_arch_is_ready())
+		return true;
 
 	slab = folio_slab(folio);
-	return !poison_slab_object(slab->slab_cache, ptr, ip, false);
+
+	if (check_slab_allocation(slab->slab_cache, ptr, ip))
+		return false;
+
+	poison_slab_object(slab->slab_cache, ptr, false, false);
+	return true;
 }
 
 void __kasan_mempool_unpoison_object(void *ptr, size_t size, unsigned long ip)
diff --git a/mm/kasan/kasan_test.c b/mm/kasan/kasan_test.c
index 7b32be2a3cf0..567d33b493e2 100644
--- a/mm/kasan/kasan_test.c
+++ b/mm/kasan/kasan_test.c
@@ -996,6 +996,51 @@ static void kmem_cache_invalid_free(struct kunit *test)
 	kmem_cache_destroy(cache);
 }
 
+static void kmem_cache_rcu_uaf(struct kunit *test)
+{
+	char *p;
+	size_t size = 200;
+	struct kmem_cache *cache;
+
+	KASAN_TEST_NEEDS_CONFIG_ON(test, CONFIG_SLUB_RCU_DEBUG);
+
+	cache = kmem_cache_create("test_cache", size, 0, SLAB_TYPESAFE_BY_RCU,
+				  NULL);
+	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, cache);
+
+	p = kmem_cache_alloc(cache, GFP_KERNEL);
+	if (!p) {
+		kunit_err(test, "Allocation failed: %s\n", __func__);
+		kmem_cache_destroy(cache);
+		return;
+	}
+	*p = 1;
+
+	rcu_read_lock();
+
+	/* Free the object - this will internally schedule an RCU callback. */
+	kmem_cache_free(cache, p);
+
+	/*
+	 * We should still be allowed to access the object at this point because
+	 * the cache is SLAB_TYPESAFE_BY_RCU and we've been in an RCU read-side
+	 * critical section since before the kmem_cache_free().
+	 */
+	READ_ONCE(*p);
+
+	rcu_read_unlock();
+
+	/*
+	 * Wait for the RCU callback to execute; after this, the object should
+	 * have actually been freed from KASAN's perspective.
+	 */
+	rcu_barrier();
+
+	KUNIT_EXPECT_KASAN_FAIL(test, READ_ONCE(*p));
+
+	kmem_cache_destroy(cache);
+}
+
 static void empty_cache_ctor(void *object) { }
 
 static void kmem_cache_double_destroy(struct kunit *test)
@@ -1937,6 +1982,7 @@ static struct kunit_case kasan_kunit_test_cases[] = {
 	KUNIT_CASE(kmem_cache_oob),
 	KUNIT_CASE(kmem_cache_double_free),
 	KUNIT_CASE(kmem_cache_invalid_free),
+	KUNIT_CASE(kmem_cache_rcu_uaf),
 	KUNIT_CASE(kmem_cache_double_destroy),
 	KUNIT_CASE(kmem_cache_accounted),
 	KUNIT_CASE(kmem_cache_bulk),
diff --git a/mm/slab.h b/mm/slab.h
index dcdb56b8e7f5..9f907e930609 100644
--- a/mm/slab.h
+++ b/mm/slab.h
@@ -443,6 +443,13 @@ static inline bool is_kmalloc_cache(struct kmem_cache *s)
 	return (s->flags & SLAB_KMALLOC);
 }
 
+static inline bool is_kmalloc_normal(struct kmem_cache *s)
+{
+	if (!is_kmalloc_cache(s))
+		return false;
+	return !(s->flags & (SLAB_CACHE_DMA|SLAB_ACCOUNT|SLAB_RECLAIM_ACCOUNT));
+}
+
 /* Legal flag mask for kmem_cache_create(), for various configurations */
 #define SLAB_CORE_FLAGS (SLAB_HWCACHE_ALIGN | SLAB_CACHE_DMA | \
 			 SLAB_CACHE_DMA32 | SLAB_PANIC | \
diff --git a/mm/slab_common.c b/mm/slab_common.c
index 85afeb69b3c0..11ef221bce17 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -40,11 +40,6 @@ LIST_HEAD(slab_caches);
 DEFINE_MUTEX(slab_mutex);
 struct kmem_cache *kmem_cache;
 
-static LIST_HEAD(slab_caches_to_rcu_destroy);
-static void slab_caches_to_rcu_destroy_workfn(struct work_struct *work);
-static DECLARE_WORK(slab_caches_to_rcu_destroy_work,
-		    slab_caches_to_rcu_destroy_workfn);
-
 /*
  * Set of flags that will prevent slab merging
  */
@@ -502,81 +497,19 @@ fail:
 }
 EXPORT_SYMBOL(kmem_buckets_create);
 
-#ifdef SLAB_SUPPORTS_SYSFS
 /*
  * For a given kmem_cache, kmem_cache_destroy() should only be called
  * once or there will be a use-after-free problem. The actual deletion
  * and release of the kobject does not need slab_mutex or cpu_hotplug_lock
  * protection. So they are now done without holding those locks.
- *
- * Note that there will be a slight delay in the deletion of sysfs files
- * if kmem_cache_release() is called indrectly from a work function.
  */
 static void kmem_cache_release(struct kmem_cache *s)
 {
-	if (slab_state >= FULL) {
-		sysfs_slab_unlink(s);
+	kfence_shutdown_cache(s);
+	if (__is_defined(SLAB_SUPPORTS_SYSFS) && slab_state >= FULL)
 		sysfs_slab_release(s);
-	} else {
+	else
 		slab_kmem_cache_release(s);
-	}
-}
-#else
-static void kmem_cache_release(struct kmem_cache *s)
-{
-	slab_kmem_cache_release(s);
-}
-#endif
-
-static void slab_caches_to_rcu_destroy_workfn(struct work_struct *work)
-{
-	LIST_HEAD(to_destroy);
-	struct kmem_cache *s, *s2;
-
-	/*
-	 * On destruction, SLAB_TYPESAFE_BY_RCU kmem_caches are put on the
-	 * @slab_caches_to_rcu_destroy list.  The slab pages are freed
-	 * through RCU and the associated kmem_cache are dereferenced
-	 * while freeing the pages, so the kmem_caches should be freed only
-	 * after the pending RCU operations are finished.  As rcu_barrier()
-	 * is a pretty slow operation, we batch all pending destructions
-	 * asynchronously.
-	 */
-	mutex_lock(&slab_mutex);
-	list_splice_init(&slab_caches_to_rcu_destroy, &to_destroy);
-	mutex_unlock(&slab_mutex);
-
-	if (list_empty(&to_destroy))
-		return;
-
-	rcu_barrier();
-
-	list_for_each_entry_safe(s, s2, &to_destroy, list) {
-		debugfs_slab_release(s);
-		kfence_shutdown_cache(s);
-		kmem_cache_release(s);
-	}
-}
-
-static int shutdown_cache(struct kmem_cache *s)
-{
-	/* free asan quarantined objects */
-	kasan_cache_shutdown(s);
-
-	if (__kmem_cache_shutdown(s) != 0)
-		return -EBUSY;
-
-	list_del(&s->list);
-
-	if (s->flags & SLAB_TYPESAFE_BY_RCU) {
-		list_add_tail(&s->list, &slab_caches_to_rcu_destroy);
-		schedule_work(&slab_caches_to_rcu_destroy_work);
-	} else {
-		kfence_shutdown_cache(s);
-		debugfs_slab_release(s);
-	}
-
-	return 0;
 }
 
 void slab_kmem_cache_release(struct kmem_cache *s)
@@ -588,29 +521,63 @@ void slab_kmem_cache_release(struct kmem_cache *s)
 
 void kmem_cache_destroy(struct kmem_cache *s)
 {
-	int err = -EBUSY;
-	bool rcu_set;
+	int err;
 
 	if (unlikely(!s) || !kasan_check_byte(s))
 		return;
 
+	/* in-flight kfree_rcu()'s may include objects from our cache */
+	kvfree_rcu_barrier();
+
+	if (IS_ENABLED(CONFIG_SLUB_RCU_DEBUG) &&
+	    (s->flags & SLAB_TYPESAFE_BY_RCU)) {
+		/*
+		 * Under CONFIG_SLUB_RCU_DEBUG, when objects in a
+		 * SLAB_TYPESAFE_BY_RCU slab are freed, SLUB will internally
+		 * defer their freeing with call_rcu().
+		 * Wait for such call_rcu() invocations here before actually
+		 * destroying the cache.
+		 *
+		 * It doesn't matter that we haven't looked at the slab refcount
+		 * yet - slabs with SLAB_TYPESAFE_BY_RCU can't be merged, so
+		 * the refcount should be 1 here.
+		 */
+		rcu_barrier();
+	}
+
 	cpus_read_lock();
 	mutex_lock(&slab_mutex);
 
-	rcu_set = s->flags & SLAB_TYPESAFE_BY_RCU;
-
 	s->refcount--;
-	if (s->refcount)
-		goto out_unlock;
+	if (s->refcount) {
+		mutex_unlock(&slab_mutex);
+		cpus_read_unlock();
+		return;
+	}
 
-	err = shutdown_cache(s);
+	/* free asan quarantined objects */
+	kasan_cache_shutdown(s);
+
+	err = __kmem_cache_shutdown(s);
 	WARN(err, "%s %s: Slab cache still has objects when called from %pS",
 	     __func__, s->name, (void *)_RET_IP_);
-out_unlock:
+
+	list_del(&s->list);
+
 	mutex_unlock(&slab_mutex);
 	cpus_read_unlock();
-	if (!err && !rcu_set)
-		kmem_cache_release(s);
+
+	if (slab_state >= FULL)
+		sysfs_slab_unlink(s);
+	debugfs_slab_release(s);
+
+	if (err)
+		return;
+
+	if (s->flags & SLAB_TYPESAFE_BY_RCU)
+		rcu_barrier();
+
+	kmem_cache_release(s);
 }
 EXPORT_SYMBOL(kmem_cache_destroy);
 
diff --git a/mm/slub.c b/mm/slub.c
index d52c88f29f69..81cea762d094 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -2184,6 +2184,45 @@ void memcg_slab_free_hook(struct kmem_cache *s, struct slab *slab, void **p,
 
 	__memcg_slab_free_hook(s, slab, p, objects, obj_exts);
 }
+
+static __fastpath_inline
+bool memcg_slab_post_charge(void *p, gfp_t flags)
+{
+	struct slabobj_ext *slab_exts;
+	struct kmem_cache *s;
+	struct folio *folio;
+	struct slab *slab;
+	unsigned long off;
+
+	folio = virt_to_folio(p);
+	if (!folio_test_slab(folio)) {
+		return folio_memcg_kmem(folio) ||
+			(__memcg_kmem_charge_page(folio_page(folio, 0), flags,
+						  folio_order(folio)) == 0);
+	}
+
+	slab = folio_slab(folio);
+	s = slab->slab_cache;
+
+	/*
+	 * Ignore KMALLOC_NORMAL cache to avoid possible circular dependency
+	 * of slab_obj_exts being allocated from the same slab and thus the slab
+	 * becoming effectively unfreeable.
+	 */
+	if (is_kmalloc_normal(s))
+		return true;
+
+	/* Ignore already charged objects. */
+	slab_exts = slab_obj_exts(slab);
+	if (slab_exts) {
+		off = obj_to_index(s, slab, p);
+		if (unlikely(slab_exts[off].objcg))
+			return true;
+	}
+
+	return __memcg_slab_post_alloc_hook(s, NULL, flags, 1, &p);
+}
+
 #else /* CONFIG_MEMCG */
 static inline bool memcg_slab_post_alloc_hook(struct kmem_cache *s,
 					      struct list_lru *lru,
@@ -2197,18 +2236,37 @@ static inline void memcg_slab_free_hook(struct kmem_cache *s, struct slab *slab,
 					void **p, int objects)
 {
 }
+
+static inline bool memcg_slab_post_charge(void *p, gfp_t flags)
+{
+	return true;
+}
 #endif /* CONFIG_MEMCG */
 
+#ifdef CONFIG_SLUB_RCU_DEBUG
+static void slab_free_after_rcu_debug(struct rcu_head *rcu_head);
+
+struct rcu_delayed_free {
+	struct rcu_head head;
+	void *object;
+};
+#endif
+
 /*
  * Hooks for other subsystems that check memory allocations. In a typical
  * production configuration these hooks all should produce no code at all.
  *
  * Returns true if freeing of the object can proceed, false if its reuse
- * was delayed by KASAN quarantine, or it was returned to KFENCE.
+ * was delayed by CONFIG_SLUB_RCU_DEBUG or KASAN quarantine, or it was returned
+ * to KFENCE.
  */
 static __always_inline
-bool slab_free_hook(struct kmem_cache *s, void *x, bool init)
+bool slab_free_hook(struct kmem_cache *s, void *x, bool init,
+		    bool after_rcu_delay)
 {
+	/* Are the object contents still accessible? */
+	bool still_accessible = (s->flags & SLAB_TYPESAFE_BY_RCU) && !after_rcu_delay;
+
 	kmemleak_free_recursive(x, s->flags);
 	kmsan_slab_free(s, x);
 
@@ -2218,7 +2276,7 @@ bool slab_free_hook(struct kmem_cache *s, void *x, bool init)
 		debug_check_no_obj_freed(x, s->object_size);
 
 	/* Use KCSAN to help debug racy use-after-free. */
-	if (!(s->flags & SLAB_TYPESAFE_BY_RCU))
+	if (!still_accessible)
 		__kcsan_check_access(x, s->object_size,
 				     KCSAN_ACCESS_WRITE | KCSAN_ACCESS_ASSERT);
 
@@ -2226,6 +2284,35 @@ bool slab_free_hook(struct kmem_cache *s, void *x, bool init)
 		return false;
 
 	/*
+	 * Give KASAN a chance to notice an invalid free operation before we
+	 * modify the object.
+	 */
+	if (kasan_slab_pre_free(s, x))
+		return false;
+
+#ifdef CONFIG_SLUB_RCU_DEBUG
+	if (still_accessible) {
+		struct rcu_delayed_free *delayed_free;
+
+		delayed_free = kmalloc(sizeof(*delayed_free), GFP_NOWAIT);
+		if (delayed_free) {
+			/*
+			 * Let KASAN track our call stack as a "related work
+			 * creation", just like if the object had been freed
+			 * normally via kfree_rcu().
+			 * We have to do this manually because the rcu_head is
+			 * not located inside the object.
+			 */
+			kasan_record_aux_stack_noalloc(x);
+
+			delayed_free->object = x;
+			call_rcu(&delayed_free->head, slab_free_after_rcu_debug);
+			return false;
+		}
+	}
+#endif /* CONFIG_SLUB_RCU_DEBUG */
+
+	/*
 	 * As memory initialization might be integrated into KASAN,
 	 * kasan_slab_free and initialization memset's must be
 	 * kept together to avoid discrepancies in behavior.
@@ -2255,7 +2342,7 @@ bool slab_free_hook(struct kmem_cache *s, void *x, bool init)
 
 	}
 	/* KASAN might put x into memory quarantine, delaying its reuse. */
-	return !kasan_slab_free(s, x, init);
+	return !kasan_slab_free(s, x, init, still_accessible);
 }
 
 static __fastpath_inline
@@ -2269,7 +2356,7 @@ bool slab_free_freelist_hook(struct kmem_cache *s, void **head, void **tail,
 	bool init;
 
 	if (is_kfence_address(next)) {
-		slab_free_hook(s, next, false);
+		slab_free_hook(s, next, false, false);
 		return false;
 	}
 
@@ -2284,7 +2371,7 @@ bool slab_free_freelist_hook(struct kmem_cache *s, void **head, void **tail,
 		next = get_freepointer(s, object);
 
 		/* If object's reuse doesn't have to be delayed */
-		if (likely(slab_free_hook(s, object, init))) {
+		if (likely(slab_free_hook(s, object, init, false))) {
 			/* Move object to the new freelist */
 			set_freepointer(s, object, *head);
 			*head = object;
@@ -4073,6 +4160,15 @@ void *kmem_cache_alloc_lru_noprof(struct kmem_cache *s, struct list_lru *lru,
 }
 EXPORT_SYMBOL(kmem_cache_alloc_lru_noprof);
 
+bool kmem_cache_charge(void *objp, gfp_t gfpflags)
+{
+	if (!memcg_kmem_online())
+		return true;
+
+	return memcg_slab_post_charge(objp, gfpflags);
+}
+EXPORT_SYMBOL(kmem_cache_charge);
+
 /**
  * kmem_cache_alloc_node - Allocate an object on the specified node
  * @s: The cache to allocate from.
@@ -4481,7 +4577,7 @@ void slab_free(struct kmem_cache *s, struct slab *slab, void *object,
 	memcg_slab_free_hook(s, slab, &object, 1);
 	alloc_tagging_slab_free_hook(s, slab, &object, 1);
 
-	if (likely(slab_free_hook(s, object, slab_want_init_on_free(s))))
+	if (likely(slab_free_hook(s, object, slab_want_init_on_free(s), false)))
 		do_slab_free(s, slab, object, object, 1, addr);
 }
 
@@ -4490,7 +4586,7 @@ void slab_free(struct kmem_cache *s, struct slab *slab, void *object,
 static noinline
 void memcg_alloc_abort_single(struct kmem_cache *s, void *object)
 {
-	if (likely(slab_free_hook(s, object, slab_want_init_on_free(s))))
+	if (likely(slab_free_hook(s, object, slab_want_init_on_free(s), false)))
 		do_slab_free(s, virt_to_slab(object), object, object, 1, _RET_IP_);
 }
 #endif
@@ -4509,6 +4605,33 @@ void slab_free_bulk(struct kmem_cache *s, struct slab *slab, void *head,
 		do_slab_free(s, slab, head, tail, cnt, addr);
 }
 
+#ifdef CONFIG_SLUB_RCU_DEBUG
+static void slab_free_after_rcu_debug(struct rcu_head *rcu_head)
+{
+	struct rcu_delayed_free *delayed_free =
+			container_of(rcu_head, struct rcu_delayed_free, head);
+	void *object = delayed_free->object;
+	struct slab *slab = virt_to_slab(object);
+	struct kmem_cache *s;
+
+	kfree(delayed_free);
+
+	if (WARN_ON(is_kfence_address(object)))
+		return;
+
+	/* find the object and the cache again */
+	if (WARN_ON(!slab))
+		return;
+	s = slab->slab_cache;
+	if (WARN_ON(!(s->flags & SLAB_TYPESAFE_BY_RCU)))
+		return;
+
+	/* resume freeing */
+	if (slab_free_hook(s, object, slab_want_init_on_free(s), true))
+		do_slab_free(s, slab, object, object, 1, _THIS_IP_);
+}
+#endif /* CONFIG_SLUB_RCU_DEBUG */
+
 #ifdef CONFIG_KASAN_GENERIC
 void ___cache_free(struct kmem_cache *cache, void *x, unsigned long addr)
 {