[NETLINK]: Add netlink_has_listeners for avoiding unneccessary event message generation

Keep a bitmask of multicast groups with subscribed listeners to let
netlink users check for listeners before generating multicast
messages.

Queries don't perform any locking, which may result in false
positives, it is guaranteed however that any new subscriptions are
visible before bind() or setsockopt() return.
Signed-off-by: Patrick McHardy <kaber@trash.net>
ACKed-by: Jamal Hadi Salim<hadi@cyberus.ca>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/netlink.h

@@ -151,6 +151,7 @@ struct netlink_skb_parms
 
 extern struct sock *netlink_kernel_create(int unit, unsigned int groups, void (*input)(struct sock *sk, int len), struct module *module);
 extern void netlink_ack(struct sk_buff *in_skb, struct nlmsghdr *nlh, int err);
+extern int netlink_has_listeners(struct sock *sk, unsigned int group);
 extern int netlink_unicast(struct sock *ssk, struct sk_buff *skb, __u32 pid, int nonblock);
 extern int netlink_broadcast(struct sock *ssk, struct sk_buff *skb, __u32 pid,
 			     __u32 group, gfp_t allocation);

net/netlink/af_netlink.c

@@ -106,6 +106,7 @@ struct nl_pid_hash {
 struct netlink_table {
 	struct nl_pid_hash hash;
 	struct hlist_head mc_list;
+	unsigned long *listeners;
 	unsigned int nl_nonroot;
 	unsigned int groups;
 	struct module *module;
@@ -296,6 +297,24 @@ static inline int nl_pid_hash_dilute(struct nl_pid_hash *hash, int len)
 
 static const struct proto_ops netlink_ops;
 
+static void
+netlink_update_listeners(struct sock *sk)
+{
+	struct netlink_table *tbl = &nl_table[sk->sk_protocol];
+	struct hlist_node *node;
+	unsigned long mask;
+	unsigned int i;
+
+	for (i = 0; i < NLGRPSZ(tbl->groups)/sizeof(unsigned long); i++) {
+		mask = 0;
+		sk_for_each_bound(sk, node, &tbl->mc_list)
+			mask |= nlk_sk(sk)->groups[i];
+		tbl->listeners[i] = mask;
+	}
+	/* this function is only called with the netlink table "grabbed", which
+	 * makes sure updates are visible before bind or setsockopt return. */
+}
+
 static int netlink_insert(struct sock *sk, u32 pid)
 {
 	struct nl_pid_hash *hash = &nl_table[sk->sk_protocol].hash;
@@ -456,12 +475,14 @@ static int netlink_release(struct socket *sock)
 	if (nlk->module)
 		module_put(nlk->module);
 
-	if (nlk->flags & NETLINK_KERNEL_SOCKET) {
 	netlink_table_grab();
+	if (nlk->flags & NETLINK_KERNEL_SOCKET) {
+		kfree(nl_table[sk->sk_protocol].listeners);
 		nl_table[sk->sk_protocol].module = NULL;
 		nl_table[sk->sk_protocol].registered = 0;
+	} else if (nlk->subscriptions)
+		netlink_update_listeners(sk);
 	netlink_table_ungrab();
-	}
 
 	kfree(nlk->groups);
 	nlk->groups = NULL;
@@ -589,6 +610,7 @@ static int netlink_bind(struct socket *sock, struct sockaddr *addr, int addr_len
 	                                 hweight32(nladdr->nl_groups) -
 	                                 hweight32(nlk->groups[0]));
 	nlk->groups[0] = (nlk->groups[0] & ~0xffffffffUL) | nladdr->nl_groups; 
+	netlink_update_listeners(sk);
 	netlink_table_ungrab();
 
 	return 0;
@@ -807,6 +829,17 @@ int netlink_unicast(struct sock *ssk, struct sk_buff *skb, u32 pid, int nonblock
 	return netlink_sendskb(sk, skb, ssk->sk_protocol);
 }
 
+int netlink_has_listeners(struct sock *sk, unsigned int group)
+{
+	int res = 0;
+
+	BUG_ON(!(nlk_sk(sk)->flags & NETLINK_KERNEL_SOCKET));
+	if (group - 1 < nl_table[sk->sk_protocol].groups)
+		res = test_bit(group - 1, nl_table[sk->sk_protocol].listeners);
+	return res;
+}
+EXPORT_SYMBOL_GPL(netlink_has_listeners);
+
 static __inline__ int netlink_broadcast_deliver(struct sock *sk, struct sk_buff *skb)
 {
 	struct netlink_sock *nlk = nlk_sk(sk);
@@ -1011,6 +1044,7 @@ static int netlink_setsockopt(struct socket *sock, int level, int optname,
 		else
 			__clear_bit(val - 1, nlk->groups);
 		netlink_update_subscriptions(sk, subscriptions);
+		netlink_update_listeners(sk);
 		netlink_table_ungrab();
 		err = 0;
 		break;
@@ -1237,6 +1271,7 @@ netlink_kernel_create(int unit, unsigned int groups,
 	struct socket *sock;
 	struct sock *sk;
 	struct netlink_sock *nlk;
+	unsigned long *listeners = NULL;
 
 	if (!nl_table)
 		return NULL;
@@ -1250,6 +1285,13 @@ netlink_kernel_create(int unit, unsigned int groups,
 	if (__netlink_create(sock, unit) < 0)
 		goto out_sock_release;
 
+	if (groups < 32)
+		groups = 32;
+
+	listeners = kzalloc(NLGRPSZ(groups), GFP_KERNEL);
+	if (!listeners)
+		goto out_sock_release;
+
 	sk = sock->sk;
 	sk->sk_data_ready = netlink_data_ready;
 	if (input)
@@ -1262,7 +1304,8 @@ netlink_kernel_create(int unit, unsigned int groups,
 	nlk->flags |= NETLINK_KERNEL_SOCKET;
 
 	netlink_table_grab();
-	nl_table[unit].groups = groups < 32 ? 32 : groups;
+	nl_table[unit].groups = groups;
+	nl_table[unit].listeners = listeners;
 	nl_table[unit].module = module;
 	nl_table[unit].registered = 1;
 	netlink_table_ungrab();
@@ -1270,6 +1313,7 @@ netlink_kernel_create(int unit, unsigned int groups,
 	return sk;
 
 out_sock_release:
+	kfree(listeners);
 	sock_release(sock);
 	return NULL;
 }