sock_diag: allow concurrent operation in sock_diag_rcv_msg()

TCPDIAG_GETSOCK and DCCPDIAG_GETSOCK diag are serialized
on sock_diag_table_mutex.

This is to make sure inet_diag module is not unloaded
while diag was ongoing.

It is time to get rid of this mutex and use RCU protection,
allowing full parallelism.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Guillaume Nault <gnault@redhat.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

include/linux/sock_diag.h

@@ -23,8 +23,13 @@ struct sock_diag_handler {
 int sock_diag_register(const struct sock_diag_handler *h);
 void sock_diag_unregister(const struct sock_diag_handler *h);
 
-void sock_diag_register_inet_compat(int (*fn)(struct sk_buff *skb, struct nlmsghdr *nlh));
-void sock_diag_unregister_inet_compat(int (*fn)(struct sk_buff *skb, struct nlmsghdr *nlh));
+struct sock_diag_inet_compat {
+	struct module *owner;
+	int (*fn)(struct sk_buff *skb, struct nlmsghdr *nlh);
+};
+
+void sock_diag_register_inet_compat(const struct sock_diag_inet_compat *ptr);
+void sock_diag_unregister_inet_compat(const struct sock_diag_inet_compat *ptr);
 
 u64 __sock_gen_cookie(struct sock *sk);
 

net/core/sock_diag.c

@@ -17,8 +17,9 @@
 #include <linux/sock_diag.h>
 
 static const struct sock_diag_handler __rcu *sock_diag_handlers[AF_MAX];
-static int (*inet_rcv_compat)(struct sk_buff *skb, struct nlmsghdr *nlh);
-static DEFINE_MUTEX(sock_diag_table_mutex);
+
+static struct sock_diag_inet_compat __rcu *inet_rcv_compat;
+
 static struct workqueue_struct *broadcast_wq;
 
 DEFINE_COOKIE(sock_cookie);
@@ -184,19 +185,20 @@ void sock_diag_broadcast_destroy(struct sock *sk)
 	queue_work(broadcast_wq, &bsk->work);
 }
 
-void sock_diag_register_inet_compat(int (*fn)(struct sk_buff *skb, struct nlmsghdr *nlh))
+void sock_diag_register_inet_compat(const struct sock_diag_inet_compat *ptr)
 {
-	mutex_lock(&sock_diag_table_mutex);
-	inet_rcv_compat = fn;
-	mutex_unlock(&sock_diag_table_mutex);
+	xchg((__force const struct sock_diag_inet_compat **)&inet_rcv_compat,
+	     ptr);
 }
 EXPORT_SYMBOL_GPL(sock_diag_register_inet_compat);
 
-void sock_diag_unregister_inet_compat(int (*fn)(struct sk_buff *skb, struct nlmsghdr *nlh))
+void sock_diag_unregister_inet_compat(const struct sock_diag_inet_compat *ptr)
 {
-	mutex_lock(&sock_diag_table_mutex);
-	inet_rcv_compat = NULL;
-	mutex_unlock(&sock_diag_table_mutex);
+	const struct sock_diag_inet_compat *old;
+
+	old = xchg((__force const struct sock_diag_inet_compat **)&inet_rcv_compat,
+		   NULL);
+	WARN_ON_ONCE(old != ptr);
 }
 EXPORT_SYMBOL_GPL(sock_diag_unregister_inet_compat);
 
@@ -259,20 +261,27 @@ static int __sock_diag_cmd(struct sk_buff *skb, struct nlmsghdr *nlh)
 static int sock_diag_rcv_msg(struct sk_buff *skb, struct nlmsghdr *nlh,
 			     struct netlink_ext_ack *extack)
 {
+	const struct sock_diag_inet_compat *ptr;
 	int ret;
 
 	switch (nlh->nlmsg_type) {
 	case TCPDIAG_GETSOCK:
 	case DCCPDIAG_GETSOCK:
-		if (inet_rcv_compat == NULL)
+
+		if (!rcu_access_pointer(inet_rcv_compat))
 			sock_load_diag_module(AF_INET, 0);
 
-		mutex_lock(&sock_diag_table_mutex);
-		if (inet_rcv_compat != NULL)
-			ret = inet_rcv_compat(skb, nlh);
-		else
+		rcu_read_lock();
+		ptr = rcu_dereference(inet_rcv_compat);
+		if (ptr && !try_module_get(ptr->owner))
+			ptr = NULL;
+		rcu_read_unlock();
+
 		ret = -EOPNOTSUPP;
-		mutex_unlock(&sock_diag_table_mutex);
+		if (ptr) {
+			ret = ptr->fn(skb, nlh);
+			module_put(ptr->owner);
+		}
 
 		return ret;
 	case SOCK_DIAG_BY_FAMILY:

net/ipv4/inet_diag.c

@@ -1527,6 +1527,11 @@ void inet_diag_unregister(const struct inet_diag_handler *h)
 }
 EXPORT_SYMBOL_GPL(inet_diag_unregister);
 
+static const struct sock_diag_inet_compat inet_diag_compat = {
+	.owner	= THIS_MODULE,
+	.fn	= inet_diag_rcv_msg_compat,
+};
+
 static int __init inet_diag_init(void)
 {
 	const int inet_diag_table_size = (IPPROTO_MAX *
@@ -1545,7 +1550,7 @@ static int __init inet_diag_init(void)
 	if (err)
 		goto out_free_inet;
 
-	sock_diag_register_inet_compat(inet_diag_rcv_msg_compat);
+	sock_diag_register_inet_compat(&inet_diag_compat);
 out:
 	return err;
 
@@ -1560,7 +1565,7 @@ static void __exit inet_diag_exit(void)
 {
 	sock_diag_unregister(&inet6_diag_handler);
 	sock_diag_unregister(&inet_diag_handler);
-	sock_diag_unregister_inet_compat(inet_diag_rcv_msg_compat);
+	sock_diag_unregister_inet_compat(&inet_diag_compat);
 	kfree(inet_diag_table);
 }