rtnetlink: add RTNL_FLAG_DOIT_UNLOCKED

Allow callers to tell rtnetlink core that its doit callback
should be invoked without holding rtnl mutex.
Signed-off-by: Florian Westphal <fw@strlen.de>
Reviewed-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/rtnetlink.h

@@ -8,6 +8,10 @@ typedef int (*rtnl_doit_func)(struct sk_buff *, struct nlmsghdr *,
 			      struct netlink_ext_ack *);
 typedef int (*rtnl_dumpit_func)(struct sk_buff *, struct netlink_callback *);
 
+enum rtnl_link_flags {
+	RTNL_FLAG_DOIT_UNLOCKED = 1,
+};
+
 int __rtnl_register(int protocol, int msgtype,
 		    rtnl_doit_func, rtnl_dumpit_func, unsigned int flags);
 void rtnl_register(int protocol, int msgtype,

net/core/rtnetlink.c

@@ -62,6 +62,7 @@
 struct rtnl_link {
 	rtnl_doit_func		doit;
 	rtnl_dumpit_func	dumpit;
+	unsigned int		flags;
 };
 
 static DEFINE_MUTEX(rtnl_mutex);
@@ -184,6 +185,7 @@ int __rtnl_register(int protocol, int msgtype,
 		tab[msgindex].doit = doit;
 	if (dumpit)
 		tab[msgindex].dumpit = dumpit;
+	tab[msgindex].flags |= flags;
 
 	return 0;
 }
@@ -233,6 +235,7 @@ int rtnl_unregister(int protocol, int msgtype)
 
 	handlers[msgindex].doit = NULL;
 	handlers[msgindex].dumpit = NULL;
+	handlers[msgindex].flags = 0;
 	rtnl_unlock();
 
 	return 0;
@@ -4143,6 +4146,7 @@ static int rtnetlink_rcv_msg(struct sk_buff *skb, struct nlmsghdr *nlh,
 	struct rtnl_link *handlers;
 	int err = -EOPNOTSUPP;
 	rtnl_doit_func doit;
+	unsigned int flags;
 	int kind;
 	int family;
 	int type;
@@ -4209,6 +4213,17 @@ static int rtnetlink_rcv_msg(struct sk_buff *skb, struct nlmsghdr *nlh,
 		return err;
 	}
 
+	flags = READ_ONCE(handlers[type].flags);
+	if (flags & RTNL_FLAG_DOIT_UNLOCKED) {
+		refcount_inc(&rtnl_msg_handlers_ref[family]);
+		doit = READ_ONCE(handlers[type].doit);
+		rcu_read_unlock();
+		if (doit)
+			err = doit(skb, nlh, extack);
+		refcount_dec(&rtnl_msg_handlers_ref[family]);
+		return err;
+	}
+
 	rcu_read_unlock();
 
 	rtnl_lock();