lpm_trie.c

/*
 * Longest prefix match list implementation
 *
 * Copyright (c) 2016,2017 Daniel Mack
 * Copyright (c) 2016 David Herrmann
 *
 * This file is subject to the terms and conditions of version 2 of the GNU
 * General Public License.  See the file COPYING in the main directory of the
 * Linux distribution for more details.
 */

#include <linux/bpf.h>
#include <linux/err.h>
#include <linux/slab.h>
#include <linux/spinlock.h>
#include <linux/vmalloc.h>
#include <net/ipv6.h>

/* Intermediate node */
#define LPM_TREE_NODE_FLAG_IM BIT(0)

struct lpm_trie_node;

struct lpm_trie_node {
	struct rcu_head rcu;
	struct lpm_trie_node __rcu	*child[2];
	u32				prefixlen;
	u32				flags;
	u8				data[0];
};

struct lpm_trie {
	struct bpf_map			map;
	struct lpm_trie_node __rcu	*root;
	size_t				n_entries;
	size_t				max_prefixlen;
	size_t				data_size;
	raw_spinlock_t			lock;
};

/* This trie implements a longest prefix match algorithm that can be used to
 * match IP addresses to a stored set of ranges.
 *
 * Data stored in @data of struct bpf_lpm_key and struct lpm_trie_node is
 * interpreted as big endian, so data[0] stores the most significant byte.
 *
 * Match ranges are internally stored in instances of struct lpm_trie_node
 * which each contain their prefix length as well as two pointers that may
 * lead to more nodes containing more specific matches. Each node also stores
 * a value that is defined by and returned to userspace via the update_elem
 * and lookup functions.
 *
 * For instance, let's start with a trie that was created with a prefix length
 * of 32, so it can be used for IPv4 addresses, and one single element that
 * matches 192.168.0.0/16. The data array would hence contain
 * [0xc0, 0xa8, 0x00, 0x00] in big-endian notation. This documentation will
 * stick to IP-address notation for readability though.
 *
 * As the trie is empty initially, the new node (1) will be places as root
 * node, denoted as (R) in the example below. As there are no other node, both
 * child pointers are %NULL.
 *
 *              +----------------+
 *              |       (1)  (R) |
 *              | 192.168.0.0/16 |
 *              |    value: 1    |
 *              |   [0]    [1]   |
 *              +----------------+
 *
 * Next, let's add a new node (2) matching 192.168.0.0/24. As there is already
 * a node with the same data and a smaller prefix (ie, a less specific one),
 * node (2) will become a child of (1). In child index depends on the next bit
 * that is outside of what (1) matches, and that bit is 0, so (2) will be
 * child[0] of (1):
 *
 *              +----------------+
 *              |       (1)  (R) |
 *              | 192.168.0.0/16 |
 *              |    value: 1    |
 *              |   [0]    [1]   |
 *              +----------------+
 *                   |
 *    +----------------+
 *    |       (2)      |
 *    | 192.168.0.0/24 |
 *    |    value: 2    |
 *    |   [0]    [1]   |
 *    +----------------+
 *
 * The child[1] slot of (1) could be filled with another node which has bit #17
 * (the next bit after the ones that (1) matches on) set to 1. For instance,
 * 192.168.128.0/24:
 *
 *              +----------------+
 *              |       (1)  (R) |
 *              | 192.168.0.0/16 |
 *              |    value: 1    |
 *              |   [0]    [1]   |
 *              +----------------+
 *                   |      |
 *    +----------------+  +------------------+
 *    |       (2)      |  |        (3)       |
 *    | 192.168.0.0/24 |  | 192.168.128.0/24 |
 *    |    value: 2    |  |     value: 3     |
 *    |   [0]    [1]   |  |    [0]    [1]    |
 *    +----------------+  +------------------+
 *
 * Let's add another node (4) to the game for 192.168.1.0/24. In order to place
 * it, node (1) is looked at first, and because (4) of the semantics laid out
 * above (bit #17 is 0), it would normally be attached to (1) as child[0].
 * However, that slot is already allocated, so a new node is needed in between.
 * That node does not have a value attached to it and it will never be
 * returned to users as result of a lookup. It is only there to differentiate
 * the traversal further. It will get a prefix as wide as necessary to
 * distinguish its two children:
 *
 *                      +----------------+
 *                      |       (1)  (R) |
 *                      | 192.168.0.0/16 |
 *                      |    value: 1    |
 *                      |   [0]    [1]   |
 *                      +----------------+
 *                           |      |
 *            +----------------+  +------------------+
 *            |       (4)  (I) |  |        (3)       |
 *            | 192.168.0.0/23 |  | 192.168.128.0/24 |
 *            |    value: ---  |  |     value: 3     |
 *            |   [0]    [1]   |  |    [0]    [1]    |
 *            +----------------+  +------------------+
 *                 |      |
 *  +----------------+  +----------------+
 *  |       (2)      |  |       (5)      |
 *  | 192.168.0.0/24 |  | 192.168.1.0/24 |
 *  |    value: 2    |  |     value: 5   |
 *  |   [0]    [1]   |  |   [0]    [1]   |
 *  +----------------+  +----------------+
 *
 * 192.168.1.1/32 would be a child of (5) etc.
 *
 * An intermediate node will be turned into a 'real' node on demand. In the
 * example above, (4) would be re-used if 192.168.0.0/23 is added to the trie.
 *
 * A fully populated trie would have a height of 32 nodes, as the trie was
 * created with a prefix length of 32.
 *
 * The lookup starts at the root node. If the current node matches and if there
 * is a child that can be used to become more specific, the trie is traversed
 * downwards. The last node in the traversal that is a non-intermediate one is
 * returned.
 */

static inline int extract_bit(const u8 *data, size_t index)
{
	return !!(data[index / 8] & (1 << (7 - (index % 8))));
}

/**
 * longest_prefix_match() - determine the longest prefix
 * @trie:	The trie to get internal sizes from
 * @node:	The node to operate on
 * @key:	The key to compare to @node
 *
 * Determine the longest prefix of @node that matches the bits in @key.
 */
static size_t longest_prefix_match(const struct lpm_trie *trie,
				   const struct lpm_trie_node *node,
				   const struct bpf_lpm_trie_key *key)
{
	size_t prefixlen = 0;
	size_t i;

	for (i = 0; i < trie->data_size; i++) {
		size_t b;

		b = 8 - fls(node->data[i] ^ key->data[i]);
		prefixlen += b;

		if (prefixlen >= node->prefixlen || prefixlen >= key->prefixlen)
			return min(node->prefixlen, key->prefixlen);

		if (b < 8)
			break;
	}

	return prefixlen;
}

/* Called from syscall or from eBPF program */
static void *trie_lookup_elem(struct bpf_map *map, void *_key)
{
	struct lpm_trie *trie = container_of(map, struct lpm_trie, map);
	struct lpm_trie_node *node, *found = NULL;
	struct bpf_lpm_trie_key *key = _key;

	/* Start walking the trie from the root node ... */

	for (node = rcu_dereference(trie->root); node;) {
		unsigned int next_bit;
		size_t matchlen;

		/* Determine the longest prefix of @node that matches @key.
		 * If it's the maximum possible prefix for this trie, we have
		 * an exact match and can return it directly.
		 */
		matchlen = longest_prefix_match(trie, node, key);
		if (matchlen == trie->max_prefixlen) {
			found = node;
			break;
		}

		/* If the number of bits that match is smaller than the prefix
		 * length of @node, bail out and return the node we have seen
		 * last in the traversal (ie, the parent).
		 */
		if (matchlen < node->prefixlen)
			break;

		/* Consider this node as return candidate unless it is an
		 * artificially added intermediate one.
		 */
		if (!(node->flags & LPM_TREE_NODE_FLAG_IM))
			found = node;

		/* If the node match is fully satisfied, let's see if we can
		 * become more specific. Determine the next bit in the key and
		 * traverse down.
		 */
		next_bit = extract_bit(key->data, node->prefixlen);
		node = rcu_dereference(node->child[next_bit]);
	}

	if (!found)
		return NULL;

	return found->data + trie->data_size;
}

static struct lpm_trie_node *lpm_trie_node_alloc(const struct lpm_trie *trie,
						 const void *value)
{
	struct lpm_trie_node *node;
	size_t size = sizeof(struct lpm_trie_node) + trie->data_size;

	if (value)
		size += trie->map.value_size;

	node = kmalloc(size, GFP_ATOMIC | __GFP_NOWARN);
	if (!node)
		return NULL;

	node->flags = 0;

	if (value)
		memcpy(node->data + trie->data_size, value,
		       trie->map.value_size);

	return node;
}

/* Called from syscall or from eBPF program */
static int trie_update_elem(struct bpf_map *map,
			    void *_key, void *value, u64 flags)
{
	struct lpm_trie *trie = container_of(map, struct lpm_trie, map);
	struct lpm_trie_node *node, *im_node = NULL, *new_node = NULL;
	struct lpm_trie_node __rcu **slot;
	struct bpf_lpm_trie_key *key = _key;
	unsigned long irq_flags;
	unsigned int next_bit;
	size_t matchlen = 0;
	int ret = 0;

	if (unlikely(flags > BPF_EXIST))
		return -EINVAL;

	if (key->prefixlen > trie->max_prefixlen)
		return -EINVAL;

	raw_spin_lock_irqsave(&trie->lock, irq_flags);

	/* Allocate and fill a new node */

	if (trie->n_entries == trie->map.max_entries) {
		ret = -ENOSPC;
		goto out;
	}

	new_node = lpm_trie_node_alloc(trie, value);
	if (!new_node) {
		ret = -ENOMEM;
		goto out;
	}

	trie->n_entries++;

	new_node->prefixlen = key->prefixlen;
	RCU_INIT_POINTER(new_node->child[0], NULL);
	RCU_INIT_POINTER(new_node->child[1], NULL);
	memcpy(new_node->data, key->data, trie->data_size);

	/* Now find a slot to attach the new node. To do that, walk the tree
	 * from the root and match as many bits as possible for each node until
	 * we either find an empty slot or a slot that needs to be replaced by
	 * an intermediate node.
	 */
	slot = &trie->root;

	while ((node = rcu_dereference_protected(*slot,
					lockdep_is_held(&trie->lock)))) {
		matchlen = longest_prefix_match(trie, node, key);

		if (node->prefixlen != matchlen ||
		    node->prefixlen == key->prefixlen ||
		    node->prefixlen == trie->max_prefixlen)
			break;

		next_bit = extract_bit(key->data, node->prefixlen);
		slot = &node->child[next_bit];
	}

	/* If the slot is empty (a free child pointer or an empty root),
	 * simply assign the @new_node to that slot and be done.
	 */
	if (!node) {
		rcu_assign_pointer(*slot, new_node);
		goto out;
	}

	/* If the slot we picked already exists, replace it with @new_node
	 * which already has the correct data array set.
	 */
	if (node->prefixlen == matchlen) {
		new_node->child[0] = node->child[0];
		new_node->child[1] = node->child[1];

		if (!(node->flags & LPM_TREE_NODE_FLAG_IM))
			trie->n_entries--;

		rcu_assign_pointer(*slot, new_node);
		kfree_rcu(node, rcu);

		goto out;
	}

	/* If the new node matches the prefix completely, it must be inserted
	 * as an ancestor. Simply insert it between @node and *@slot.
	 */
	if (matchlen == key->prefixlen) {
		next_bit = extract_bit(node->data, matchlen);
		rcu_assign_pointer(new_node->child[next_bit], node);
		rcu_assign_pointer(*slot, new_node);
		goto out;
	}

	im_node = lpm_trie_node_alloc(trie, NULL);
	if (!im_node) {
		ret = -ENOMEM;
		goto out;
	}

	im_node->prefixlen = matchlen;
	im_node->flags |= LPM_TREE_NODE_FLAG_IM;
	memcpy(im_node->data, node->data, trie->data_size);

	/* Now determine which child to install in which slot */
	if (extract_bit(key->data, matchlen)) {
		rcu_assign_pointer(im_node->child[0], node);
		rcu_assign_pointer(im_node->child[1], new_node);
	} else {
		rcu_assign_pointer(im_node->child[0], new_node);
		rcu_assign_pointer(im_node->child[1], node);
	}

	/* Finally, assign the intermediate node to the determined spot */
	rcu_assign_pointer(*slot, im_node);

out:
	if (ret) {
		if (new_node)
			trie->n_entries--;

		kfree(new_node);
		kfree(im_node);
	}

	raw_spin_unlock_irqrestore(&trie->lock, irq_flags);

	return ret;
}

static int trie_delete_elem(struct bpf_map *map, void *key)
{
	/* TODO */
	return -ENOSYS;
}

#define LPM_DATA_SIZE_MAX	256
#define LPM_DATA_SIZE_MIN	1

#define LPM_VAL_SIZE_MAX	(KMALLOC_MAX_SIZE - LPM_DATA_SIZE_MAX - \
				 sizeof(struct lpm_trie_node))
#define LPM_VAL_SIZE_MIN	1

#define LPM_KEY_SIZE(X)		(sizeof(struct bpf_lpm_trie_key) + (X))
#define LPM_KEY_SIZE_MAX	LPM_KEY_SIZE(LPM_DATA_SIZE_MAX)
#define LPM_KEY_SIZE_MIN	LPM_KEY_SIZE(LPM_DATA_SIZE_MIN)

static struct bpf_map *trie_alloc(union bpf_attr *attr)
{
	struct lpm_trie *trie;
	u64 cost = sizeof(*trie), cost_per_node;
	int ret;

	if (!capable(CAP_SYS_ADMIN))
		return ERR_PTR(-EPERM);

	/* check sanity of attributes */
	if (attr->max_entries == 0 ||
	    attr->map_flags != BPF_F_NO_PREALLOC ||
	    attr->key_size < LPM_KEY_SIZE_MIN ||
	    attr->key_size > LPM_KEY_SIZE_MAX ||
	    attr->value_size < LPM_VAL_SIZE_MIN ||
	    attr->value_size > LPM_VAL_SIZE_MAX)
		return ERR_PTR(-EINVAL);

	trie = kzalloc(sizeof(*trie), GFP_USER | __GFP_NOWARN);
	if (!trie)
		return ERR_PTR(-ENOMEM);

	/* copy mandatory map attributes */
	trie->map.map_type = attr->map_type;
	trie->map.key_size = attr->key_size;
	trie->map.value_size = attr->value_size;
	trie->map.max_entries = attr->max_entries;
	trie->map.map_flags = attr->map_flags;
	trie->data_size = attr->key_size -
			  offsetof(struct bpf_lpm_trie_key, data);
	trie->max_prefixlen = trie->data_size * 8;

	cost_per_node = sizeof(struct lpm_trie_node) +
			attr->value_size + trie->data_size;
	cost += (u64) attr->max_entries * cost_per_node;
	if (cost >= U32_MAX - PAGE_SIZE) {
		ret = -E2BIG;
		goto out_err;
	}

	trie->map.pages = round_up(cost, PAGE_SIZE) >> PAGE_SHIFT;

	ret = bpf_map_precharge_memlock(trie->map.pages);
	if (ret)
		goto out_err;

	raw_spin_lock_init(&trie->lock);

	return &trie->map;
out_err:
	kfree(trie);
	return ERR_PTR(ret);
}

static void trie_free(struct bpf_map *map)
{
	struct lpm_trie *trie = container_of(map, struct lpm_trie, map);
	struct lpm_trie_node __rcu **slot;
	struct lpm_trie_node *node;

	raw_spin_lock(&trie->lock);

	/* Always start at the root and walk down to a node that has no
	 * children. Then free that node, nullify its reference in the parent
	 * and start over.
	 */

	for (;;) {
		slot = &trie->root;

		for (;;) {
			node = rcu_dereference_protected(*slot,
					lockdep_is_held(&trie->lock));
			if (!node)
				goto unlock;

			if (rcu_access_pointer(node->child[0])) {
				slot = &node->child[0];
				continue;
			}

			if (rcu_access_pointer(node->child[1])) {
				slot = &node->child[1];
				continue;
			}

			kfree(node);
			RCU_INIT_POINTER(*slot, NULL);
			break;
		}
	}

unlock:
	raw_spin_unlock(&trie->lock);
}

static int trie_get_next_key(struct bpf_map *map, void *key, void *next_key)
{
	return -ENOTSUPP;
}

const struct bpf_map_ops trie_map_ops = {
	.map_alloc = trie_alloc,
	.map_free = trie_free,
	.map_get_next_key = trie_get_next_key,
	.map_lookup_elem = trie_lookup_elem,
	.map_update_elem = trie_update_elem,
	.map_delete_elem = trie_delete_elem,
};