netfilter: nft_hash: bug fixes and resizing

The hash set type is very broken and was never meant to be merged in this
state. Missing RCU synchronization on element removal, leaking chain
refcounts when used as a verdict map, races during lookups, a fixed table
size are probably just some of the problems. Luckily it is currently
never chosen by the kernel when the rbtree type is also available.

Rewrite it to be usable.

The new implementation supports automatic hash table resizing using RCU,
based on Paul McKenney's and Josh Triplett's algorithm "Optimized Resizing
For RCU-Protected Hash Tables" described in [1].

Resizing doesn't require a second list head in the elements, it works by
chosing a hash function that remaps elements to a predictable set of buckets,
only resizing by integral factors and

- during expansion: linking new buckets to the old bucket that contains
  elements for any of the new buckets, thereby creating imprecise chains,
  then incrementally seperating the elements until the new buckets only
  contain elements that hash directly to them.

- during shrinking: linking the hash chains of all old buckets that hash
  to the same new bucket to form a single chain.

Expansion requires at most the number of elements in the longest hash chain
grace periods, shrinking requires a single grace period.

Due to the requirement of having hash chains/elements linked to multiple
buckets during resizing, homemade single linked lists are used instead of
the existing list helpers, that don't support this in a clean fashion.
As a side effect, the amount of memory required per element is reduced by
one pointer.

Expansion is triggered when the load factors exceeds 75%, shrinking when
the load factor goes below 30%. Both operations are allowed to fail and
will be retried on the next insertion or removal if their respective
conditions still hold.

[1] http://dl.acm.org/citation.cfm?id=2002181.2002192

Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Reviewed-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nft_hash.c

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2008-2009 Patrick McHardy <kaber@trash.net>
+ * Copyright (c) 2008-2014 Patrick McHardy <kaber@trash.net>
  *
  * This program is free software; you can redistribute it and/or modify
  * it under the terms of the GNU General Public License version 2 as
@@ -18,17 +18,29 @@
 #include <linux/netfilter/nf_tables.h>
 #include <net/netfilter/nf_tables.h>
 
+#define NFT_HASH_MIN_SIZE	4
+
 struct nft_hash {
-	struct hlist_head	*hash;
-	unsigned int		hsize;
+	struct nft_hash_table __rcu	*tbl;
+};
+
+struct nft_hash_table {
+	unsigned int			size;
+	unsigned int			elements;
+	struct nft_hash_elem __rcu	*buckets[];
 };
 
 struct nft_hash_elem {
-	struct hlist_node	hnode;
-	struct nft_data		key;
-	struct nft_data		data[];
+	struct nft_hash_elem __rcu	*next;
+	struct nft_data			key;
+	struct nft_data			data[];
 };
 
+#define nft_hash_for_each_entry(i, head) \
+	for (i = nft_dereference(head); i != NULL; i = nft_dereference(i->next))
+#define nft_hash_for_each_entry_rcu(i, head) \
+	for (i = rcu_dereference(head); i != NULL; i = rcu_dereference(i->next))
+
 static u32 nft_hash_rnd __read_mostly;
 static bool nft_hash_rnd_initted __read_mostly;
 
@@ -38,7 +50,7 @@ static unsigned int nft_hash_data(const struct nft_data *data,
 	unsigned int h;
 
 	h = jhash(data->data, len, nft_hash_rnd);
-	return ((u64)h * hsize) >> 32;
+	return h & (hsize - 1);
 }
 
 static bool nft_hash_lookup(const struct nft_set *set,
@@ -46,11 +58,12 @@ static bool nft_hash_lookup(const struct nft_set *set,
 			    struct nft_data *data)
 {
 	const struct nft_hash *priv = nft_set_priv(set);
+	const struct nft_hash_table *tbl = rcu_dereference(priv->tbl);
 	const struct nft_hash_elem *he;
 	unsigned int h;
 
-	h = nft_hash_data(key, priv->hsize, set->klen);
-	hlist_for_each_entry(he, &priv->hash[h], hnode) {
+	h = nft_hash_data(key, tbl->size, set->klen);
+	nft_hash_for_each_entry_rcu(he, tbl->buckets[h]) {
 		if (nft_data_cmp(&he->key, key, set->klen))
 			continue;
 		if (set->flags & NFT_SET_MAP)
@@ -60,19 +73,148 @@ static bool nft_hash_lookup(const struct nft_set *set,
 	return false;
 }
 
-static void nft_hash_elem_destroy(const struct nft_set *set,
-				  struct nft_hash_elem *he)
+static void nft_hash_tbl_free(const struct nft_hash_table *tbl)
 {
-	nft_data_uninit(&he->key, NFT_DATA_VALUE);
-	if (set->flags & NFT_SET_MAP)
-		nft_data_uninit(he->data, set->dtype);
-	kfree(he);
+	if (is_vmalloc_addr(tbl))
+		vfree(tbl);
+	else
+		kfree(tbl);
+}
+
+static struct nft_hash_table *nft_hash_tbl_alloc(unsigned int nbuckets)
+{
+	struct nft_hash_table *tbl;
+	size_t size;
+
+	size = sizeof(*tbl) + nbuckets * sizeof(tbl->buckets[0]);
+	tbl = kzalloc(size, GFP_KERNEL | __GFP_REPEAT | __GFP_NOWARN);
+	if (tbl == NULL)
+		tbl = vzalloc(size);
+	if (tbl == NULL)
+		return NULL;
+	tbl->size = nbuckets;
+
+	return tbl;
+}
+
+static void nft_hash_chain_unzip(const struct nft_set *set,
+				 const struct nft_hash_table *ntbl,
+				 struct nft_hash_table *tbl, unsigned int n)
+{
+	struct nft_hash_elem *he, *last, *next;
+	unsigned int h;
+
+	he = nft_dereference(tbl->buckets[n]);
+	if (he == NULL)
+		return;
+	h = nft_hash_data(&he->key, ntbl->size, set->klen);
+
+	/* Find last element of first chain hashing to bucket h */
+	last = he;
+	nft_hash_for_each_entry(he, he->next) {
+		if (nft_hash_data(&he->key, ntbl->size, set->klen) != h)
+			break;
+		last = he;
+	}
+
+	/* Unlink first chain from the old table */
+	RCU_INIT_POINTER(tbl->buckets[n], last->next);
+
+	/* If end of chain reached, done */
+	if (he == NULL)
+		return;
+
+	/* Find first element of second chain hashing to bucket h */
+	next = NULL;
+	nft_hash_for_each_entry(he, he->next) {
+		if (nft_hash_data(&he->key, ntbl->size, set->klen) != h)
+			continue;
+		next = he;
+		break;
+	}
+
+	/* Link the two chains */
+	RCU_INIT_POINTER(last->next, next);
+}
+
+static int nft_hash_tbl_expand(const struct nft_set *set, struct nft_hash *priv)
+{
+	struct nft_hash_table *tbl = nft_dereference(priv->tbl), *ntbl;
+	struct nft_hash_elem *he;
+	unsigned int i, h;
+	bool complete;
+
+	ntbl = nft_hash_tbl_alloc(tbl->size * 2);
+	if (ntbl == NULL)
+		return -ENOMEM;
+
+	/* Link new table's buckets to first element in the old table
+	 * hashing to the new bucket.
+	 */
+	for (i = 0; i < ntbl->size; i++) {
+		h = i < tbl->size ? i : i - tbl->size;
+		nft_hash_for_each_entry(he, tbl->buckets[h]) {
+			if (nft_hash_data(&he->key, ntbl->size, set->klen) != i)
+				continue;
+			RCU_INIT_POINTER(ntbl->buckets[i], he);
+			break;
+		}
+	}
+	ntbl->elements = tbl->elements;
+
+	/* Publish new table */
+	rcu_assign_pointer(priv->tbl, ntbl);
+
+	/* Unzip interleaved hash chains */
+	do {
+		/* Wait for readers to use new table/unzipped chains */
+		synchronize_rcu();
+
+		complete = true;
+		for (i = 0; i < tbl->size; i++) {
+			nft_hash_chain_unzip(set, ntbl, tbl, i);
+			if (tbl->buckets[i] != NULL)
+				complete = false;
+		}
+	} while (!complete);
+
+	nft_hash_tbl_free(tbl);
+	return 0;
+}
+
+static int nft_hash_tbl_shrink(const struct nft_set *set, struct nft_hash *priv)
+{
+	struct nft_hash_table *tbl = nft_dereference(priv->tbl), *ntbl;
+	struct nft_hash_elem __rcu **pprev;
+	unsigned int i;
+
+	ntbl = nft_hash_tbl_alloc(tbl->size / 2);
+	if (ntbl == NULL)
+		return -ENOMEM;
+
+	for (i = 0; i < ntbl->size; i++) {
+		ntbl->buckets[i] = tbl->buckets[i];
+
+		for (pprev = &ntbl->buckets[i]; *pprev != NULL;
+		     pprev = &nft_dereference(*pprev)->next)
+			;
+		RCU_INIT_POINTER(*pprev, tbl->buckets[i + ntbl->size]);
+	}
+	ntbl->elements = tbl->elements;
+
+	/* Publish new table */
+	rcu_assign_pointer(priv->tbl, ntbl);
+	synchronize_rcu();
+
+	nft_hash_tbl_free(tbl);
+	return 0;
 }
 
 static int nft_hash_insert(const struct nft_set *set,
 			   const struct nft_set_elem *elem)
 {
 	struct nft_hash *priv = nft_set_priv(set);
+	struct nft_hash_table *tbl = nft_dereference(priv->tbl);
 	struct nft_hash_elem *he;
 	unsigned int size, h;
 
@@ -91,33 +233,66 @@ static int nft_hash_insert(const struct nft_set *set,
 	if (set->flags & NFT_SET_MAP)
 		nft_data_copy(he->data, &elem->data);
 
-	h = nft_hash_data(&he->key, priv->hsize, set->klen);
-	hlist_add_head_rcu(&he->hnode, &priv->hash[h]);
+	h = nft_hash_data(&he->key, tbl->size, set->klen);
+	RCU_INIT_POINTER(he->next, tbl->buckets[h]);
+	rcu_assign_pointer(tbl->buckets[h], he);
+	tbl->elements++;
+
+	/* Expand table when exceeding 75% load */
+	if (tbl->elements > tbl->size / 4 * 3)
+		nft_hash_tbl_expand(set, priv);
+
 	return 0;
 }
 
+static void nft_hash_elem_destroy(const struct nft_set *set,
+				  struct nft_hash_elem *he)
+{
+	nft_data_uninit(&he->key, NFT_DATA_VALUE);
+	if (set->flags & NFT_SET_MAP)
+		nft_data_uninit(he->data, set->dtype);
+	kfree(he);
+}
+
 static void nft_hash_remove(const struct nft_set *set,
 			    const struct nft_set_elem *elem)
 {
-	struct nft_hash_elem *he = elem->cookie;
+	struct nft_hash *priv = nft_set_priv(set);
+	struct nft_hash_table *tbl = nft_dereference(priv->tbl);
+	struct nft_hash_elem *he, __rcu **pprev;
 
-	hlist_del_rcu(&he->hnode);
+	pprev = elem->cookie;
+	he = nft_dereference((*pprev));
+
+	RCU_INIT_POINTER(*pprev, he->next);
+	synchronize_rcu();
 	kfree(he);
+	tbl->elements--;
+
+	/* Shrink table beneath 30% load */
+	if (tbl->elements < tbl->size * 3 / 10 &&
+	    tbl->size > NFT_HASH_MIN_SIZE)
+		nft_hash_tbl_shrink(set, priv);
 }
 
 static int nft_hash_get(const struct nft_set *set, struct nft_set_elem *elem)
 {
 	const struct nft_hash *priv = nft_set_priv(set);
+	const struct nft_hash_table *tbl = nft_dereference(priv->tbl);
+	struct nft_hash_elem __rcu * const *pprev;
 	struct nft_hash_elem *he;
 	unsigned int h;
 
-	h = nft_hash_data(&elem->key, priv->hsize, set->klen);
-	hlist_for_each_entry(he, &priv->hash[h], hnode) {
-		if (nft_data_cmp(&he->key, &elem->key, set->klen))
+	h = nft_hash_data(&elem->key, tbl->size, set->klen);
+	pprev = &tbl->buckets[h];
+	nft_hash_for_each_entry(he, tbl->buckets[h]) {
+		if (nft_data_cmp(&he->key, &elem->key, set->klen)) {
+			pprev = &he->next;
 			continue;
+		}
 
-		elem->cookie = he;
-		elem->flags  = 0;
+		elem->cookie = (void *)pprev;
+		elem->flags = 0;
 		if (set->flags & NFT_SET_MAP)
 			nft_data_copy(&elem->data, he->data);
 		return 0;
@@ -129,12 +304,13 @@ static void nft_hash_walk(const struct nft_ctx *ctx, const struct nft_set *set,
 			  struct nft_set_iter *iter)
 {
 	const struct nft_hash *priv = nft_set_priv(set);
+	const struct nft_hash_table *tbl = nft_dereference(priv->tbl);
 	const struct nft_hash_elem *he;
 	struct nft_set_elem elem;
 	unsigned int i;
 
-	for (i = 0; i < priv->hsize; i++) {
-		hlist_for_each_entry(he, &priv->hash[i], hnode) {
+	for (i = 0; i < tbl->size; i++) {
+		nft_hash_for_each_entry(he, tbl->buckets[i]) {
 			if (iter->count < iter->skip)
 				goto cont;
 
@@ -161,43 +337,35 @@ static int nft_hash_init(const struct nft_set *set,
 			 const struct nlattr * const tb[])
 {
 	struct nft_hash *priv = nft_set_priv(set);
-	unsigned int cnt, i;
+	struct nft_hash_table *tbl;
 
 	if (unlikely(!nft_hash_rnd_initted)) {
 		get_random_bytes(&nft_hash_rnd, 4);
 		nft_hash_rnd_initted = true;
 	}
 
-	/* Aim for a load factor of 0.75 */
-	// FIXME: temporarily broken until we have set descriptions
-	cnt = 100;
-	cnt = cnt * 4 / 3;
-
-	priv->hash = kcalloc(cnt, sizeof(struct hlist_head), GFP_KERNEL);
-	if (priv->hash == NULL)
+	tbl = nft_hash_tbl_alloc(NFT_HASH_MIN_SIZE);
+	if (tbl == NULL)
 		return -ENOMEM;
-	priv->hsize = cnt;
-
-	for (i = 0; i < cnt; i++)
-		INIT_HLIST_HEAD(&priv->hash[i]);
-
+	RCU_INIT_POINTER(priv->tbl, tbl);
 	return 0;
 }
 
 static void nft_hash_destroy(const struct nft_set *set)
 {
 	const struct nft_hash *priv = nft_set_priv(set);
-	const struct hlist_node *next;
-	struct nft_hash_elem *elem;
+	const struct nft_hash_table *tbl = nft_dereference(priv->tbl);
+	struct nft_hash_elem *he, *next;
 	unsigned int i;
 
-	for (i = 0; i < priv->hsize; i++) {
-		hlist_for_each_entry_safe(elem, next, &priv->hash[i], hnode) {
-			hlist_del(&elem->hnode);
-			nft_hash_elem_destroy(set, elem);
+	for (i = 0; i < tbl->size; i++) {
+		for (he = nft_dereference(tbl->buckets[i]); he != NULL;
+		     he = next) {
+			next = nft_dereference(he->next);
+			nft_hash_elem_destroy(set, he);
 		}
 	}
-	kfree(priv->hash);
+	kfree(tbl);
 }
 
 static struct nft_set_ops nft_hash_ops __read_mostly = {