bpf: add hashtable type of eBPF maps

add new map type BPF_MAP_TYPE_HASH and its implementation

- maps are created/destroyed by userspace. Both userspace and eBPF programs
  can lookup/update/delete elements from the map

- eBPF programs can be called in_irq(), so use spin_lock_irqsave() mechanism
  for concurrent updates

- key/value are opaque range of bytes (aligned to 8 bytes)

- user space provides 3 configuration attributes via BPF syscall:
  key_size, value_size, max_entries

- map takes care of allocating/freeing key/value pairs

- map_update_elem() must fail to insert new element when max_entries
  limit is reached to make sure that eBPF programs cannot exhaust memory

- map_update_elem() replaces elements in an atomic way

- optimized for speed of lookup() which can be called multiple times from
  eBPF program which itself is triggered by high volume of events
  . in the future JIT compiler may recognize lookup() call and optimize it
    further, since key_size is constant for life of eBPF program

Signed-off-by: Alexei Starovoitov <[email protected]>
Signed-off-by: David S. Miller <[email protected]>

Author: Alexei Starovoitov

include/uapi/linux/bpf.h

@@ -111,6 +111,7 @@ enum bpf_cmd {
 
 enum bpf_map_type {
 	BPF_MAP_TYPE_UNSPEC,
+	BPF_MAP_TYPE_HASH,
 };
 
 enum bpf_prog_type {

kernel/bpf/Makefile

@@ -1,5 +1,5 @@
 obj-y := core.o
-obj-$(CONFIG_BPF_SYSCALL) += syscall.o verifier.o
+obj-$(CONFIG_BPF_SYSCALL) += syscall.o verifier.o hashtab.o
 ifdef CONFIG_TEST_BPF
 obj-$(CONFIG_BPF_SYSCALL) += test_stub.o
 endif

kernel/bpf/hashtab.c

@@ -0,0 +1,362 @@
+/* Copyright (c) 2011-2014 PLUMgrid, http://plumgrid.com
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of version 2 of the GNU General Public
+ * License as published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+ * General Public License for more details.
+ */
+#include <linux/bpf.h>
+#include <linux/jhash.h>
+#include <linux/filter.h>
+#include <linux/vmalloc.h>
+
+struct bpf_htab {
+	struct bpf_map map;
+	struct hlist_head *buckets;
+	spinlock_t lock;
+	u32 count;	/* number of elements in this hashtable */
+	u32 n_buckets;	/* number of hash buckets */
+	u32 elem_size;	/* size of each element in bytes */
+};
+
+/* each htab element is struct htab_elem + key + value */
+struct htab_elem {
+	struct hlist_node hash_node;
+	struct rcu_head rcu;
+	u32 hash;
+	char key[0] __aligned(8);
+};
+
+/* Called from syscall */
+static struct bpf_map *htab_map_alloc(union bpf_attr *attr)
+{
+	struct bpf_htab *htab;
+	int err, i;
+
+	htab = kzalloc(sizeof(*htab), GFP_USER);
+	if (!htab)
+		return ERR_PTR(-ENOMEM);
+
+	/* mandatory map attributes */
+	htab->map.key_size = attr->key_size;
+	htab->map.value_size = attr->value_size;
+	htab->map.max_entries = attr->max_entries;
+
+	/* check sanity of attributes.
+	 * value_size == 0 may be allowed in the future to use map as a set
+	 */
+	err = -EINVAL;
+	if (htab->map.max_entries == 0 || htab->map.key_size == 0 ||
+	    htab->map.value_size == 0)
+		goto free_htab;
+
+	/* hash table size must be power of 2 */
+	htab->n_buckets = roundup_pow_of_two(htab->map.max_entries);
+
+	err = -E2BIG;
+	if (htab->map.key_size > MAX_BPF_STACK)
+		/* eBPF programs initialize keys on stack, so they cannot be
+		 * larger than max stack size
+		 */
+		goto free_htab;
+
+	err = -ENOMEM;
+	htab->buckets = kmalloc_array(htab->n_buckets, sizeof(struct hlist_head),
+				      GFP_USER | __GFP_NOWARN);
+
+	if (!htab->buckets) {
+		htab->buckets = vmalloc(htab->n_buckets * sizeof(struct hlist_head));
+		if (!htab->buckets)
+			goto free_htab;
+	}
+
+	for (i = 0; i < htab->n_buckets; i++)
+		INIT_HLIST_HEAD(&htab->buckets[i]);
+
+	spin_lock_init(&htab->lock);
+	htab->count = 0;
+
+	htab->elem_size = sizeof(struct htab_elem) +
+			  round_up(htab->map.key_size, 8) +
+			  htab->map.value_size;
+	return &htab->map;
+
+free_htab:
+	kfree(htab);
+	return ERR_PTR(err);
+}
+
+static inline u32 htab_map_hash(const void *key, u32 key_len)
+{
+	return jhash(key, key_len, 0);
+}
+
+static inline struct hlist_head *select_bucket(struct bpf_htab *htab, u32 hash)
+{
+	return &htab->buckets[hash & (htab->n_buckets - 1)];
+}
+
+static struct htab_elem *lookup_elem_raw(struct hlist_head *head, u32 hash,
+					 void *key, u32 key_size)
+{
+	struct htab_elem *l;
+
+	hlist_for_each_entry_rcu(l, head, hash_node)
+		if (l->hash == hash && !memcmp(&l->key, key, key_size))
+			return l;
+
+	return NULL;
+}
+
+/* Called from syscall or from eBPF program */
+static void *htab_map_lookup_elem(struct bpf_map *map, void *key)
+{
+	struct bpf_htab *htab = container_of(map, struct bpf_htab, map);
+	struct hlist_head *head;
+	struct htab_elem *l;
+	u32 hash, key_size;
+
+	/* Must be called with rcu_read_lock. */
+	WARN_ON_ONCE(!rcu_read_lock_held());
+
+	key_size = map->key_size;
+
+	hash = htab_map_hash(key, key_size);
+
+	head = select_bucket(htab, hash);
+
+	l = lookup_elem_raw(head, hash, key, key_size);
+
+	if (l)
+		return l->key + round_up(map->key_size, 8);
+
+	return NULL;
+}
+
+/* Called from syscall */
+static int htab_map_get_next_key(struct bpf_map *map, void *key, void *next_key)
+{
+	struct bpf_htab *htab = container_of(map, struct bpf_htab, map);
+	struct hlist_head *head;
+	struct htab_elem *l, *next_l;
+	u32 hash, key_size;
+	int i;
+
+	WARN_ON_ONCE(!rcu_read_lock_held());
+
+	key_size = map->key_size;
+
+	hash = htab_map_hash(key, key_size);
+
+	head = select_bucket(htab, hash);
+
+	/* lookup the key */
+	l = lookup_elem_raw(head, hash, key, key_size);
+
+	if (!l) {
+		i = 0;
+		goto find_first_elem;
+	}
+
+	/* key was found, get next key in the same bucket */
+	next_l = hlist_entry_safe(rcu_dereference_raw(hlist_next_rcu(&l->hash_node)),
+				  struct htab_elem, hash_node);
+
+	if (next_l) {
+		/* if next elem in this hash list is non-zero, just return it */
+		memcpy(next_key, next_l->key, key_size);
+		return 0;
+	}
+
+	/* no more elements in this hash list, go to the next bucket */
+	i = hash & (htab->n_buckets - 1);
+	i++;
+
+find_first_elem:
+	/* iterate over buckets */
+	for (; i < htab->n_buckets; i++) {
+		head = select_bucket(htab, i);
+
+		/* pick first element in the bucket */
+		next_l = hlist_entry_safe(rcu_dereference_raw(hlist_first_rcu(head)),
+					  struct htab_elem, hash_node);
+		if (next_l) {
+			/* if it's not empty, just return it */
+			memcpy(next_key, next_l->key, key_size);
+			return 0;
+		}
+	}
+
+	/* itereated over all buckets and all elements */
+	return -ENOENT;
+}
+
+/* Called from syscall or from eBPF program */
+static int htab_map_update_elem(struct bpf_map *map, void *key, void *value,
+				u64 map_flags)
+{
+	struct bpf_htab *htab = container_of(map, struct bpf_htab, map);
+	struct htab_elem *l_new, *l_old;
+	struct hlist_head *head;
+	unsigned long flags;
+	u32 key_size;
+	int ret;
+
+	if (map_flags > BPF_EXIST)
+		/* unknown flags */
+		return -EINVAL;
+
+	WARN_ON_ONCE(!rcu_read_lock_held());
+
+	/* allocate new element outside of lock */
+	l_new = kmalloc(htab->elem_size, GFP_ATOMIC);
+	if (!l_new)
+		return -ENOMEM;
+
+	key_size = map->key_size;
+
+	memcpy(l_new->key, key, key_size);
+	memcpy(l_new->key + round_up(key_size, 8), value, map->value_size);
+
+	l_new->hash = htab_map_hash(l_new->key, key_size);
+
+	/* bpf_map_update_elem() can be called in_irq() */
+	spin_lock_irqsave(&htab->lock, flags);
+
+	head = select_bucket(htab, l_new->hash);
+
+	l_old = lookup_elem_raw(head, l_new->hash, key, key_size);
+
+	if (!l_old && unlikely(htab->count >= map->max_entries)) {
+		/* if elem with this 'key' doesn't exist and we've reached
+		 * max_entries limit, fail insertion of new elem
+		 */
+		ret = -E2BIG;
+		goto err;
+	}
+
+	if (l_old && map_flags == BPF_NOEXIST) {
+		/* elem already exists */
+		ret = -EEXIST;
+		goto err;
+	}
+
+	if (!l_old && map_flags == BPF_EXIST) {
+		/* elem doesn't exist, cannot update it */
+		ret = -ENOENT;
+		goto err;
+	}
+
+	/* add new element to the head of the list, so that concurrent
+	 * search will find it before old elem
+	 */
+	hlist_add_head_rcu(&l_new->hash_node, head);
+	if (l_old) {
+		hlist_del_rcu(&l_old->hash_node);
+		kfree_rcu(l_old, rcu);
+	} else {
+		htab->count++;
+	}
+	spin_unlock_irqrestore(&htab->lock, flags);
+
+	return 0;
+err:
+	spin_unlock_irqrestore(&htab->lock, flags);
+	kfree(l_new);
+	return ret;
+}
+
+/* Called from syscall or from eBPF program */
+static int htab_map_delete_elem(struct bpf_map *map, void *key)
+{
+	struct bpf_htab *htab = container_of(map, struct bpf_htab, map);
+	struct hlist_head *head;
+	struct htab_elem *l;
+	unsigned long flags;
+	u32 hash, key_size;
+	int ret = -ENOENT;
+
+	WARN_ON_ONCE(!rcu_read_lock_held());
+
+	key_size = map->key_size;
+
+	hash = htab_map_hash(key, key_size);
+
+	spin_lock_irqsave(&htab->lock, flags);
+
+	head = select_bucket(htab, hash);
+
+	l = lookup_elem_raw(head, hash, key, key_size);
+
+	if (l) {
+		hlist_del_rcu(&l->hash_node);
+		htab->count--;
+		kfree_rcu(l, rcu);
+		ret = 0;
+	}
+
+	spin_unlock_irqrestore(&htab->lock, flags);
+	return ret;
+}
+
+static void delete_all_elements(struct bpf_htab *htab)
+{
+	int i;
+
+	for (i = 0; i < htab->n_buckets; i++) {
+		struct hlist_head *head = select_bucket(htab, i);
+		struct hlist_node *n;
+		struct htab_elem *l;
+
+		hlist_for_each_entry_safe(l, n, head, hash_node) {
+			hlist_del_rcu(&l->hash_node);
+			htab->count--;
+			kfree(l);
+		}
+	}
+}
+
+/* Called when map->refcnt goes to zero, either from workqueue or from syscall */
+static void htab_map_free(struct bpf_map *map)
+{
+	struct bpf_htab *htab = container_of(map, struct bpf_htab, map);
+
+	/* at this point bpf_prog->aux->refcnt == 0 and this map->refcnt == 0,
+	 * so the programs (can be more than one that used this map) were
+	 * disconnected from events. Wait for outstanding critical sections in
+	 * these programs to complete
+	 */
+	synchronize_rcu();
+
+	/* some of kfree_rcu() callbacks for elements of this map may not have
+	 * executed. It's ok. Proceed to free residual elements and map itself
+	 */
+	delete_all_elements(htab);
+	kvfree(htab->buckets);
+	kfree(htab);
+}
+
+static struct bpf_map_ops htab_ops = {
+	.map_alloc = htab_map_alloc,
+	.map_free = htab_map_free,
+	.map_get_next_key = htab_map_get_next_key,
+	.map_lookup_elem = htab_map_lookup_elem,
+	.map_update_elem = htab_map_update_elem,
+	.map_delete_elem = htab_map_delete_elem,
+};
+
+static struct bpf_map_type_list tl = {
+	.ops = &htab_ops,
+	.type = BPF_MAP_TYPE_HASH,
+};
+
+static int __init register_htab_map(void)
+{
+	bpf_register_map_type(&tl);
+	return 0;
+}
+late_initcall(register_htab_map);