netfilter: nf_tables: implement proper set selection

The current set selection simply choses the first set type that provides
the requested features, which always results in the rbtree being chosen
by virtue of being the first set in the list.

What we actually want to do is choose the implementation that can provide
the requested features and is optimal from either a performance or memory
perspective depending on the characteristics of the elements and the
preferences specified by the user.

The elements are not known when creating a set. Even if we would provide
them for anonymous (literal) sets, we'd still have standalone sets where
the elements are not known in advance. We therefore need an abstract
description of the data charcteristics.

The kernel already knows the size of the key, this patch starts by
introducing a nested set description which so far contains only the maximum
amount of elements. Based on this the set implementations are changed to
provide an estimate of the required amount of memory and the lookup
complexity class.

The set ops have a new callback ->estimate() that is invoked during set
selection. It receives a structure containing the attributes known to the
kernel and is supposed to populate a struct nft_set_estimate with the
complexity class and, in case the size is known, the complete amount of
memory required, or the amount of memory required per element otherwise.

Based on the policy specified by the user (performance/memory, defaulting
to performance) the kernel will then select the best suited implementation.

Even if the set implementation would allow to add more than the specified
maximum amount of elements, they are enforced since new implementations
might not be able to add more than maximum based on which they were
selected.
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

include/net/netfilter/nf_tables.h

@@ -145,6 +145,44 @@ struct nft_set_iter {
 			      const struct nft_set_elem *elem);
 };
 
+/**
+ *	struct nft_set_desc - description of set elements
+ *
+ *	@klen: key length
+ *	@dlen: data length
+ *	@size: number of set elements
+ */
+struct nft_set_desc {
+	unsigned int		klen;
+	unsigned int		dlen;
+	unsigned int		size;
+};
+
+/**
+ *	enum nft_set_class - performance class
+ *
+ *	@NFT_LOOKUP_O_1: constant, O(1)
+ *	@NFT_LOOKUP_O_LOG_N: logarithmic, O(log N)
+ *	@NFT_LOOKUP_O_N: linear, O(N)
+ */
+enum nft_set_class {
+	NFT_SET_CLASS_O_1,
+	NFT_SET_CLASS_O_LOG_N,
+	NFT_SET_CLASS_O_N,
+};
+
+/**
+ *	struct nft_set_estimate - estimation of memory and performance
+ *				  characteristics
+ *
+ *	@size: required memory
+ *	@class: lookup performance class
+ */
+struct nft_set_estimate {
+	unsigned int		size;
+	enum nft_set_class	class;
+};
+
 /**
  *	struct nft_set_ops - nf_tables set operations
  *
@@ -174,7 +212,11 @@ struct nft_set_ops {
 						struct nft_set_iter *iter);
 
 	unsigned int			(*privsize)(const struct nlattr * const nla[]);
+	bool				(*estimate)(const struct nft_set_desc *desc,
+						    u32 features,
+						    struct nft_set_estimate *est);
 	int				(*init)(const struct nft_set *set,
+						const struct nft_set_desc *desc,
 						const struct nlattr * const nla[]);
 	void				(*destroy)(const struct nft_set *set);
 
@@ -194,6 +236,8 @@ void nft_unregister_set(struct nft_set_ops *ops);
  * 	@name: name of the set
  * 	@ktype: key type (numeric type defined by userspace, not used in the kernel)
  * 	@dtype: data type (verdict or numeric type defined by userspace)
+ * 	@size: maximum set size
+ * 	@nelems: number of elements
  * 	@ops: set ops
  * 	@flags: set flags
  * 	@klen: key length
@@ -206,6 +250,8 @@ struct nft_set {
 	char				name[IFNAMSIZ];
 	u32				ktype;
 	u32				dtype;
+	u32				size;
+	u32				nelems;
 	/* runtime data below here */
 	const struct nft_set_ops	*ops ____cacheline_aligned;
 	u16				flags;

include/uapi/linux/netfilter/nf_tables.h

@@ -211,6 +211,29 @@ enum nft_set_flags {
 	NFT_SET_MAP			= 0x8,
 };
 
+/**
+ * enum nft_set_policies - set selection policy
+ *
+ * @NFT_SET_POL_PERFORMANCE: prefer high performance over low memory use
+ * @NFT_SET_POL_MEMORY: prefer low memory use over high performance
+ */
+enum nft_set_policies {
+	NFT_SET_POL_PERFORMANCE,
+	NFT_SET_POL_MEMORY,
+};
+
+/**
+ * enum nft_set_desc_attributes - set element description
+ *
+ * @NFTA_SET_DESC_SIZE: number of elements in set (NLA_U32)
+ */
+enum nft_set_desc_attributes {
+	NFTA_SET_DESC_UNSPEC,
+	NFTA_SET_DESC_SIZE,
+	__NFTA_SET_DESC_MAX
+};
+#define NFTA_SET_DESC_MAX	(__NFTA_SET_DESC_MAX - 1)
+
 /**
  * enum nft_set_attributes - nf_tables set netlink attributes
  *
@@ -221,6 +244,8 @@ enum nft_set_flags {
  * @NFTA_SET_KEY_LEN: key data length (NLA_U32)
  * @NFTA_SET_DATA_TYPE: mapping data type (NLA_U32)
  * @NFTA_SET_DATA_LEN: mapping data length (NLA_U32)
+ * @NFTA_SET_POLICY: selection policy (NLA_U32)
+ * @NFTA_SET_DESC: set description (NLA_NESTED)
  */
 enum nft_set_attributes {
 	NFTA_SET_UNSPEC,
@@ -231,6 +256,8 @@ enum nft_set_attributes {
 	NFTA_SET_KEY_LEN,
 	NFTA_SET_DATA_TYPE,
 	NFTA_SET_DATA_LEN,
+	NFTA_SET_POLICY,
+	NFTA_SET_DESC,
 	__NFTA_SET_MAX
 };
 #define NFTA_SET_MAX		(__NFTA_SET_MAX - 1)

net/netfilter/nf_tables_api.c

@@ -1912,9 +1912,18 @@ void nft_unregister_set(struct nft_set_ops *ops)
 }
 EXPORT_SYMBOL_GPL(nft_unregister_set);
 
-static const struct nft_set_ops *nft_select_set_ops(const struct nlattr * const nla[])
+/*
+ * Select a set implementation based on the data characteristics and the
+ * given policy. The total memory use might not be known if no size is
+ * given, in that case the amount of memory per element is used.
+ */
+static const struct nft_set_ops *
+nft_select_set_ops(const struct nlattr * const nla[],
+		   const struct nft_set_desc *desc,
+		   enum nft_set_policies policy)
 {
-	const struct nft_set_ops *ops;
+	const struct nft_set_ops *ops, *bops;
+	struct nft_set_estimate est, best;
 	u32 features;
 
 #ifdef CONFIG_MODULES
@@ -1932,15 +1941,45 @@ static const struct nft_set_ops *nft_select_set_ops(const struct nlattr * const
 		features &= NFT_SET_INTERVAL | NFT_SET_MAP;
 	}
 
-	// FIXME: implement selection properly
+	bops	   = NULL;
+	best.size  = ~0;
+	best.class = ~0;
+
 	list_for_each_entry(ops, &nf_tables_set_ops, list) {
 		if ((ops->features & features) != features)
 			continue;
+		if (!ops->estimate(desc, features, &est))
+			continue;
+
+		switch (policy) {
+		case NFT_SET_POL_PERFORMANCE:
+			if (est.class < best.class)
+				break;
+			if (est.class == best.class && est.size < best.size)
+				break;
+			continue;
+		case NFT_SET_POL_MEMORY:
+			if (est.size < best.size)
+				break;
+			if (est.size == best.size && est.class < best.class)
+				break;
+			continue;
+		default:
+			break;
+		}
+
 		if (!try_module_get(ops->owner))
 			continue;
-		return ops;
+		if (bops != NULL)
+			module_put(bops->owner);
+
+		bops = ops;
+		best = est;
 	}
 
+	if (bops != NULL)
+		return bops;
+
 	return ERR_PTR(-EOPNOTSUPP);
 }
 
@@ -1952,6 +1991,12 @@ static const struct nla_policy nft_set_policy[NFTA_SET_MAX + 1] = {
 	[NFTA_SET_KEY_LEN]		= { .type = NLA_U32 },
 	[NFTA_SET_DATA_TYPE]		= { .type = NLA_U32 },
 	[NFTA_SET_DATA_LEN]		= { .type = NLA_U32 },
+	[NFTA_SET_POLICY]		= { .type = NLA_U32 },
+	[NFTA_SET_DESC]			= { .type = NLA_NESTED },
+};
+
+static const struct nla_policy nft_set_desc_policy[NFTA_SET_DESC_MAX + 1] = {
+	[NFTA_SET_DESC_SIZE]		= { .type = NLA_U32 },
 };
 
 static int nft_ctx_init_from_setattr(struct nft_ctx *ctx,
@@ -2043,6 +2088,7 @@ static int nf_tables_fill_set(struct sk_buff *skb, const struct nft_ctx *ctx,
 {
 	struct nfgenmsg *nfmsg;
 	struct nlmsghdr *nlh;
+	struct nlattr *desc;
 	u32 portid = NETLINK_CB(ctx->skb).portid;
 	u32 seq = ctx->nlh->nlmsg_seq;
 
@@ -2076,6 +2122,14 @@ static int nf_tables_fill_set(struct sk_buff *skb, const struct nft_ctx *ctx,
 			goto nla_put_failure;
 	}
 
+	desc = nla_nest_start(skb, NFTA_SET_DESC);
+	if (desc == NULL)
+		goto nla_put_failure;
+	if (set->size &&
+	    nla_put_be32(skb, NFTA_SET_DESC_SIZE, htonl(set->size)))
+		goto nla_put_failure;
+	nla_nest_end(skb, desc);
+
 	return nlmsg_end(skb, nlh);
 
 nla_put_failure:
@@ -2304,6 +2358,23 @@ static int nf_tables_getset(struct sock *nlsk, struct sk_buff *skb,
 	return err;
 }
 
+static int nf_tables_set_desc_parse(const struct nft_ctx *ctx,
+				    struct nft_set_desc *desc,
+				    const struct nlattr *nla)
+{
+	struct nlattr *da[NFTA_SET_DESC_MAX + 1];
+	int err;
+
+	err = nla_parse_nested(da, NFTA_SET_DESC_MAX, nla, nft_set_desc_policy);
+	if (err < 0)
+		return err;
+
+	if (da[NFTA_SET_DESC_SIZE] != NULL)
+		desc->size = ntohl(nla_get_be32(da[NFTA_SET_DESC_SIZE]));
+
+	return 0;
+}
+
 static int nf_tables_newset(struct sock *nlsk, struct sk_buff *skb,
 			    const struct nlmsghdr *nlh,
 			    const struct nlattr * const nla[])
@@ -2318,7 +2389,8 @@ static int nf_tables_newset(struct sock *nlsk, struct sk_buff *skb,
 	char name[IFNAMSIZ];
 	unsigned int size;
 	bool create;
-	u32 ktype, klen, dlen, dtype, flags;
+	u32 ktype, dtype, flags, policy;
+	struct nft_set_desc desc;
 	int err;
 
 	if (nla[NFTA_SET_TABLE] == NULL ||
@@ -2326,6 +2398,8 @@ static int nf_tables_newset(struct sock *nlsk, struct sk_buff *skb,
 	    nla[NFTA_SET_KEY_LEN] == NULL)
 		return -EINVAL;
 
+	memset(&desc, 0, sizeof(desc));
+
 	ktype = NFT_DATA_VALUE;
 	if (nla[NFTA_SET_KEY_TYPE] != NULL) {
 		ktype = ntohl(nla_get_be32(nla[NFTA_SET_KEY_TYPE]));
@@ -2333,8 +2407,8 @@ static int nf_tables_newset(struct sock *nlsk, struct sk_buff *skb,
 			return -EINVAL;
 	}
 
-	klen = ntohl(nla_get_be32(nla[NFTA_SET_KEY_LEN]));
-	if (klen == 0 || klen > FIELD_SIZEOF(struct nft_data, data))
+	desc.klen = ntohl(nla_get_be32(nla[NFTA_SET_KEY_LEN]));
+	if (desc.klen == 0 || desc.klen > FIELD_SIZEOF(struct nft_data, data))
 		return -EINVAL;
 
 	flags = 0;
@@ -2346,7 +2420,6 @@ static int nf_tables_newset(struct sock *nlsk, struct sk_buff *skb,
 	}
 
 	dtype = 0;
-	dlen  = 0;
 	if (nla[NFTA_SET_DATA_TYPE] != NULL) {
 		if (!(flags & NFT_SET_MAP))
 			return -EINVAL;
@@ -2359,15 +2432,25 @@ static int nf_tables_newset(struct sock *nlsk, struct sk_buff *skb,
 		if (dtype != NFT_DATA_VERDICT) {
 			if (nla[NFTA_SET_DATA_LEN] == NULL)
 				return -EINVAL;
-			dlen = ntohl(nla_get_be32(nla[NFTA_SET_DATA_LEN]));
-			if (dlen == 0 ||
-			    dlen > FIELD_SIZEOF(struct nft_data, data))
+			desc.dlen = ntohl(nla_get_be32(nla[NFTA_SET_DATA_LEN]));
+			if (desc.dlen == 0 ||
+			    desc.dlen > FIELD_SIZEOF(struct nft_data, data))
 				return -EINVAL;
 		} else
-			dlen = sizeof(struct nft_data);
+			desc.dlen = sizeof(struct nft_data);
 	} else if (flags & NFT_SET_MAP)
 		return -EINVAL;
 
+	policy = NFT_SET_POL_PERFORMANCE;
+	if (nla[NFTA_SET_POLICY] != NULL)
+		policy = ntohl(nla_get_be32(nla[NFTA_SET_POLICY]));
+
+	if (nla[NFTA_SET_DESC] != NULL) {
+		err = nf_tables_set_desc_parse(&ctx, &desc, nla[NFTA_SET_DESC]);
+		if (err < 0)
+			return err;
+	}
+
 	create = nlh->nlmsg_flags & NLM_F_CREATE ? true : false;
 
 	afi = nf_tables_afinfo_lookup(net, nfmsg->nfgen_family, create);
@@ -2398,7 +2481,7 @@ static int nf_tables_newset(struct sock *nlsk, struct sk_buff *skb,
 	if (!(nlh->nlmsg_flags & NLM_F_CREATE))
 		return -ENOENT;
 
-	ops = nft_select_set_ops(nla);
+	ops = nft_select_set_ops(nla, &desc, policy);
 	if (IS_ERR(ops))
 		return PTR_ERR(ops);
 
@@ -2419,12 +2502,13 @@ static int nf_tables_newset(struct sock *nlsk, struct sk_buff *skb,
 	INIT_LIST_HEAD(&set->bindings);
 	set->ops   = ops;
 	set->ktype = ktype;
-	set->klen  = klen;
+	set->klen  = desc.klen;
 	set->dtype = dtype;
-	set->dlen  = dlen;
+	set->dlen  = desc.dlen;
 	set->flags = flags;
+	set->size  = desc.size;
 
-	err = ops->init(set, nla);
+	err = ops->init(set, &desc, nla);
 	if (err < 0)
 		goto err2;
 
@@ -2733,6 +2817,9 @@ static int nft_add_set_elem(const struct nft_ctx *ctx, struct nft_set *set,
 	enum nft_registers dreg;
 	int err;
 
+	if (set->size && set->nelems == set->size)
+		return -ENFILE;
+
 	err = nla_parse_nested(nla, NFTA_SET_ELEM_MAX, attr,
 			       nft_set_elem_policy);
 	if (err < 0)
@@ -2798,6 +2885,7 @@ static int nft_add_set_elem(const struct nft_ctx *ctx, struct nft_set *set,
 	err = set->ops->insert(set, &elem);
 	if (err < 0)
 		goto err3;
+	set->nelems++;
 
 	return 0;
 
@@ -2867,6 +2955,7 @@ static int nft_del_setelem(const struct nft_ctx *ctx, struct nft_set *set,
 		goto err2;
 
 	set->ops->remove(set, &elem);
+	set->nelems--;
 
 	nft_data_uninit(&elem.key, NFT_DATA_VALUE);
 	if (set->flags & NFT_SET_MAP)

net/netfilter/nft_hash.c

@@ -12,6 +12,7 @@
 #include <linux/init.h>
 #include <linux/module.h>
 #include <linux/list.h>
+#include <linux/log2.h>
 #include <linux/jhash.h>
 #include <linux/netlink.h>
 #include <linux/vmalloc.h>
@@ -19,7 +20,7 @@
 #include <linux/netfilter/nf_tables.h>
 #include <net/netfilter/nf_tables.h>
 
-#define NFT_HASH_MIN_SIZE	4
+#define NFT_HASH_MIN_SIZE	4UL
 
 struct nft_hash {
 	struct nft_hash_table __rcu	*tbl;
@@ -82,6 +83,11 @@ static void nft_hash_tbl_free(const struct nft_hash_table *tbl)
 		kfree(tbl);
 }
 
+static unsigned int nft_hash_tbl_size(unsigned int nelem)
+{
+	return max(roundup_pow_of_two(nelem * 4 / 3), NFT_HASH_MIN_SIZE);
+}
+
 static struct nft_hash_table *nft_hash_tbl_alloc(unsigned int nbuckets)
 {
 	struct nft_hash_table *tbl;
@@ -335,17 +341,23 @@ static unsigned int nft_hash_privsize(const struct nlattr * const nla[])
 }
 
 static int nft_hash_init(const struct nft_set *set,
+			 const struct nft_set_desc *desc,
 			 const struct nlattr * const tb[])
 {
 	struct nft_hash *priv = nft_set_priv(set);
 	struct nft_hash_table *tbl;
+	unsigned int size;
 
 	if (unlikely(!nft_hash_rnd_initted)) {
 		get_random_bytes(&nft_hash_rnd, 4);
 		nft_hash_rnd_initted = true;
 	}
 
-	tbl = nft_hash_tbl_alloc(NFT_HASH_MIN_SIZE);
+	size = NFT_HASH_MIN_SIZE;
+	if (desc->size)
+		size = nft_hash_tbl_size(desc->size);
+
+	tbl = nft_hash_tbl_alloc(size);
 	if (tbl == NULL)
 		return -ENOMEM;
 	RCU_INIT_POINTER(priv->tbl, tbl);
@@ -369,8 +381,37 @@ static void nft_hash_destroy(const struct nft_set *set)
 	kfree(tbl);
 }
 
+static bool nft_hash_estimate(const struct nft_set_desc *desc, u32 features,
+			      struct nft_set_estimate *est)
+{
+	unsigned int esize;
+
+	esize = sizeof(struct nft_hash_elem);
+	if (features & NFT_SET_MAP)
+		esize += FIELD_SIZEOF(struct nft_hash_elem, data[0]);
+
+	if (desc->size) {
+		est->size = sizeof(struct nft_hash) +
+			    nft_hash_tbl_size(desc->size) *
+			    sizeof(struct nft_hash_elem *) +
+			    desc->size * esize;
+	} else {
+		/* Resizing happens when the load drops below 30% or goes
+		 * above 75%. The average of 52.5% load (approximated by 50%)
+		 * is used for the size estimation of the hash buckets,
+		 * meaning we calculate two buckets per element.
+		 */
+		est->size = esize + 2 * sizeof(struct nft_hash_elem *);
+	}
+
+	est->class = NFT_SET_CLASS_O_1;
+
+	return true;
+}
+
 static struct nft_set_ops nft_hash_ops __read_mostly = {
 	.privsize       = nft_hash_privsize,
+	.estimate	= nft_hash_estimate,
 	.init		= nft_hash_init,
 	.destroy	= nft_hash_destroy,
 	.get		= nft_hash_get,

net/netfilter/nft_rbtree.c

@@ -201,6 +201,7 @@ static unsigned int nft_rbtree_privsize(const struct nlattr * const nla[])
 }
 
 static int nft_rbtree_init(const struct nft_set *set,
+			   const struct nft_set_desc *desc,
 			   const struct nlattr * const nla[])
 {
 	struct nft_rbtree *priv = nft_set_priv(set);
@@ -222,8 +223,28 @@ static void nft_rbtree_destroy(const struct nft_set *set)
 	}
 }
 
+static bool nft_rbtree_estimate(const struct nft_set_desc *desc, u32 features,
+				struct nft_set_estimate *est)
+{
+	unsigned int nsize;
+
+	nsize = sizeof(struct nft_rbtree_elem);
+	if (features & NFT_SET_MAP)
+		nsize += FIELD_SIZEOF(struct nft_rbtree_elem, data[0]);
+
+	if (desc->size)
+		est->size = sizeof(struct nft_rbtree) + desc->size * nsize;
+	else
+		est->size = nsize;
+
+	est->class = NFT_SET_CLASS_O_LOG_N;
+
+	return true;
+}
+
 static struct nft_set_ops nft_rbtree_ops __read_mostly = {
 	.privsize	= nft_rbtree_privsize,
+	.estimate	= nft_rbtree_estimate,
 	.init		= nft_rbtree_init,
 	.destroy	= nft_rbtree_destroy,
 	.insert		= nft_rbtree_insert,