[PATCH v6 net-next 6/6] bpf: add hashtable type of BPF maps

From: Alexei Starovoitov
Date: Mon Aug 25 2014 - 21:02:02 EST


add new map type BPF_MAP_TYPE_HASH and its implementation

- key/value are opaque range of bytes

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

- if value_size == 0, the map is used as a set

- map_update_elem() must fail to insert new element when max_entries
limit is reached

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

- update/lookup/delete methods may be called from eBPF program attached
to kprobes, so use spin_lock_irqsave() mechanism for concurrent updates

- 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 <ast@xxxxxxxxxxxx>
---

Note, lib/rhashtable.c couldn't be reused here, since it's not managing the
objects, shrink/expand is not supported out of irq context, overhead of
generic key/head_offsets and hashfn is too high.
In the future if/when rhashtable gains shrink/expand as a separate thread,
we may add another BPF_MAP_TYPE_ for it, but security implications of
dynamically resizeable maps would need to be analyzed.

include/uapi/linux/bpf.h | 1 +
kernel/bpf/Makefile | 2 +-
kernel/bpf/hashtab.c | 373 ++++++++++++++++++++++++++++++++++++++++++++++
3 files changed, 375 insertions(+), 1 deletion(-)
create mode 100644 kernel/bpf/hashtab.c

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index f68edb2681f8..8069ab7b64cf 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -365,6 +365,7 @@ enum bpf_map_attributes {

enum bpf_map_type {
BPF_MAP_TYPE_UNSPEC,
+ BPF_MAP_TYPE_HASH,
};

#endif /* _UAPI__LINUX_BPF_H__ */
diff --git a/kernel/bpf/Makefile b/kernel/bpf/Makefile
index e9f7334ed07a..558e12712ebc 100644
--- a/kernel/bpf/Makefile
+++ b/kernel/bpf/Makefile
@@ -1 +1 @@
-obj-y := core.o syscall.o
+obj-y := core.o syscall.o hashtab.o
diff --git a/kernel/bpf/hashtab.c b/kernel/bpf/hashtab.c
new file mode 100644
index 000000000000..3178a6746b92
--- /dev/null
+++ b/kernel/bpf/hashtab.c
@@ -0,0 +1,373 @@
+/* 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 <net/netlink.h>
+#include <linux/jhash.h>
+
+struct bpf_htab {
+ struct bpf_map map;
+ struct hlist_head *buckets;
+ struct kmem_cache *elem_cache;
+ 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;
+ struct bpf_htab *htab;
+ u32 hash;
+ u32 pad;
+ char key[0];
+};
+
+#define BPF_MAP_MAX_KEY_SIZE 256
+static struct bpf_map *htab_map_alloc(struct nlattr *attr[BPF_MAP_ATTR_MAX + 1])
+{
+ struct bpf_htab *htab;
+ int err, i;
+
+ htab = kzalloc(sizeof(*htab), GFP_USER);
+ if (!htab)
+ return ERR_PTR(-ENOMEM);
+
+ /* look for mandatory map attributes */
+ err = -EINVAL;
+ if (!attr[BPF_MAP_KEY_SIZE])
+ goto free_htab;
+ htab->map.key_size = nla_get_u32(attr[BPF_MAP_KEY_SIZE]);
+
+ if (!attr[BPF_MAP_VALUE_SIZE])
+ goto free_htab;
+ htab->map.value_size = nla_get_u32(attr[BPF_MAP_VALUE_SIZE]);
+
+ if (!attr[BPF_MAP_MAX_ENTRIES])
+ goto free_htab;
+ htab->map.max_entries = nla_get_u32(attr[BPF_MAP_MAX_ENTRIES]);
+
+ /* check sanity of attributes.
+ * value_size == 0 is allowed, in this case map is used as a set
+ */
+ if (htab->map.max_entries == 0 || htab->map.key_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 > BPF_MAP_MAX_KEY_SIZE)
+ goto free_htab;
+
+ err = -ENOMEM;
+ htab->buckets = kmalloc_array(htab->n_buckets,
+ sizeof(struct hlist_head), GFP_USER);
+
+ 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;
+
+ htab->elem_cache = kmem_cache_create("bpf_htab", htab->elem_size, 0, 0,
+ NULL);
+ if (!htab->elem_cache)
+ goto free_buckets;
+
+ return &htab->map;
+
+free_buckets:
+ kfree(htab->buckets);
+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;
+}
+
+/* Must be called with rcu_read_lock. */
+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;
+
+ 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;
+}
+
+/* Must be called with rcu_read_lock. */
+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;
+}
+
+static struct htab_elem *htab_alloc_elem(struct bpf_htab *htab)
+{
+ void *l;
+
+ l = kmem_cache_alloc(htab->elem_cache, GFP_ATOMIC);
+ if (!l)
+ return ERR_PTR(-ENOMEM);
+ return l;
+}
+
+static void free_htab_elem_rcu(struct rcu_head *rcu)
+{
+ struct htab_elem *l = container_of(rcu, struct htab_elem, rcu);
+
+ kmem_cache_free(l->htab->elem_cache, l);
+}
+
+static void release_htab_elem(struct bpf_htab *htab, struct htab_elem *l)
+{
+ l->htab = htab;
+ call_rcu(&l->rcu, free_htab_elem_rcu);
+}
+
+/* Must be called with rcu_read_lock. */
+static int htab_map_update_elem(struct bpf_map *map, void *key, void *value)
+{
+ 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;
+
+ WARN_ON_ONCE(!rcu_read_lock_held());
+
+ l_new = htab_alloc_elem(htab);
+ if (IS_ERR(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() as well, so
+ * spin_lock() or spin_lock_bh() cannot be used
+ */
+ 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
+ */
+ spin_unlock_irqrestore(&htab->lock, flags);
+ kmem_cache_free(htab->elem_cache, l_new);
+ return -EFBIG;
+ }
+
+ /* 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);
+ release_htab_elem(htab, l_old);
+ } else {
+ htab->count++;
+ }
+ spin_unlock_irqrestore(&htab->lock, flags);
+
+ return 0;
+}
+
+/* Must be called with rcu_read_lock. */
+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 = -ESRCH;
+
+ 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--;
+ release_htab_elem(htab, l);
+ 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--;
+ kmem_cache_free(htab->elem_cache, l);
+ }
+ }
+}
+
+/* called when map->refcnt goes to zero */
+static void htab_map_free(struct bpf_map *map)
+{
+ struct bpf_htab *htab = container_of(map, struct bpf_htab, map);
+
+ /* wait for all outstanding updates to complete */
+ synchronize_rcu();
+
+ /* kmem_cache_free all htab elements */
+ delete_all_elements(htab);
+
+ /* and destroy cache, which might sleep */
+ kmem_cache_destroy(htab->elem_cache);
+
+ kfree(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);
--
1.7.9.5

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