[NETFILTER]: ctnetlink: avoid unneccessary event message generation

Avoid unneccessary event message generation by checking for netlink
listeners before building a message.
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/netfilter/nfnetlink.h

@@ -164,6 +164,7 @@ extern void nfattr_parse(struct nfattr *tb[], int maxattr,
  	__res;								\
 })
 
+extern int nfnetlink_has_listeners(unsigned int group);
 extern int nfnetlink_send(struct sk_buff *skb, u32 pid, unsigned group, 
 			  int echo);
 extern int nfnetlink_unicast(struct sk_buff *skb, u_int32_t pid, int flags);

net/ipv4/netfilter/ip_conntrack_netlink.c

@@ -328,7 +328,8 @@ static int ctnetlink_conntrack_event(struct notifier_block *this,
 	} else 
 		return NOTIFY_DONE;
 
-  /* FIXME: Check if there are any listeners before, don't hurt performance */
+	if (!nfnetlink_has_listeners(group))
+		return NOTIFY_DONE;
 
 	skb = alloc_skb(NLMSG_GOODSIZE, GFP_ATOMIC);
 	if (!skb)

net/netfilter/nf_conntrack_netlink.c

@@ -339,7 +339,8 @@ static int ctnetlink_conntrack_event(struct notifier_block *this,
 	} else
 		return NOTIFY_DONE;
 
-  /* FIXME: Check if there are any listeners before, don't hurt performance */
+	if (!nfnetlink_has_listeners(group))
+		return NOTIFY_DONE;
 
 	skb = alloc_skb(NLMSG_GOODSIZE, GFP_ATOMIC);
 	if (!skb)

net/netfilter/nfnetlink.c

@@ -191,6 +191,12 @@ nfnetlink_check_attributes(struct nfnetlink_subsystem *subsys,
         return 0;
 }
 
+int nfnetlink_has_listeners(unsigned int group)
+{
+	return netlink_has_listeners(nfnl, group);
+}
+EXPORT_SYMBOL_GPL(nfnetlink_has_listeners);
+
 int nfnetlink_send(struct sk_buff *skb, u32 pid, unsigned group, int echo)
 {
 	gfp_t allocation = in_interrupt() ? GFP_ATOMIC : GFP_KERNEL;