sock_diag: allow concurrent operations

sock_diag_broadcast_destroy_work() and __sock_diag_cmd()
are currently using sock_diag_table_mutex to protect
against concurrent sock_diag_handlers[] changes.

This makes inet_diag dump serialized, thus less scalable
than legacy /proc files.

It is time to switch to full RCU protection.
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>

net/core/sock_diag.c

@@ -16,7 +16,7 @@
 #include <linux/inet_diag.h>
 #include <linux/sock_diag.h>
 
-static const struct sock_diag_handler *sock_diag_handlers[AF_MAX];
+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 workqueue_struct *broadcast_wq;
@@ -122,6 +122,24 @@ static size_t sock_diag_nlmsg_size(void)
 	       + nla_total_size_64bit(sizeof(struct tcp_info))); /* INET_DIAG_INFO */
 }
 
+static const struct sock_diag_handler *sock_diag_lock_handler(int family)
+{
+	const struct sock_diag_handler *handler;
+
+	rcu_read_lock();
+	handler = rcu_dereference(sock_diag_handlers[family]);
+	if (handler && !try_module_get(handler->owner))
+		handler = NULL;
+	rcu_read_unlock();
+
+	return handler;
+}
+
+static void sock_diag_unlock_handler(const struct sock_diag_handler *handler)
+{
+	module_put(handler->owner);
+}
+
 static void sock_diag_broadcast_destroy_work(struct work_struct *work)
 {
 	struct broadcast_sk *bsk =
@@ -138,12 +156,12 @@ static void sock_diag_broadcast_destroy_work(struct work_struct *work)
 	if (!skb)
 		goto out;
 
-	mutex_lock(&sock_diag_table_mutex);
-	hndl = sock_diag_handlers[sk->sk_family];
-	if (hndl && hndl->get_info)
+	hndl = sock_diag_lock_handler(sk->sk_family);
+	if (hndl) {
+		if (hndl->get_info)
 			err = hndl->get_info(skb, sk);
-	mutex_unlock(&sock_diag_table_mutex);
-
+		sock_diag_unlock_handler(hndl);
+	}
 	if (!err)
 		nlmsg_multicast(sock_net(sk)->diag_nlsk, skb, 0, group,
 				GFP_KERNEL);
@@ -184,33 +202,26 @@ EXPORT_SYMBOL_GPL(sock_diag_unregister_inet_compat);
 
 int sock_diag_register(const struct sock_diag_handler *hndl)
 {
-	int err = 0;
+	int family = hndl->family;
 
-	if (hndl->family >= AF_MAX)
+	if (family >= AF_MAX)
 		return -EINVAL;
 
-	mutex_lock(&sock_diag_table_mutex);
-	if (sock_diag_handlers[hndl->family])
-		err = -EBUSY;
-	else
-		WRITE_ONCE(sock_diag_handlers[hndl->family], hndl);
-	mutex_unlock(&sock_diag_table_mutex);
-
-	return err;
+	return !cmpxchg((const struct sock_diag_handler **)
+				&sock_diag_handlers[family],
+			NULL, hndl) ? 0 : -EBUSY;
 }
 EXPORT_SYMBOL_GPL(sock_diag_register);
 
-void sock_diag_unregister(const struct sock_diag_handler *hnld)
+void sock_diag_unregister(const struct sock_diag_handler *hndl)
 {
-	int family = hnld->family;
+	int family = hndl->family;
 
 	if (family >= AF_MAX)
 		return;
 
-	mutex_lock(&sock_diag_table_mutex);
-	BUG_ON(sock_diag_handlers[family] != hnld);
-	WRITE_ONCE(sock_diag_handlers[family], NULL);
-	mutex_unlock(&sock_diag_table_mutex);
+	xchg((const struct sock_diag_handler **)&sock_diag_handlers[family],
+	     NULL);
 }
 EXPORT_SYMBOL_GPL(sock_diag_unregister);
 
@@ -227,20 +238,20 @@ static int __sock_diag_cmd(struct sk_buff *skb, struct nlmsghdr *nlh)
 		return -EINVAL;
 	req->sdiag_family = array_index_nospec(req->sdiag_family, AF_MAX);
 
-	if (READ_ONCE(sock_diag_handlers[req->sdiag_family]) == NULL)
+	if (!rcu_access_pointer(sock_diag_handlers[req->sdiag_family]))
 		sock_load_diag_module(req->sdiag_family, 0);
 
-	mutex_lock(&sock_diag_table_mutex);
-	hndl = sock_diag_handlers[req->sdiag_family];
+	hndl = sock_diag_lock_handler(req->sdiag_family);
 	if (hndl == NULL)
-		err = -ENOENT;
-	else if (nlh->nlmsg_type == SOCK_DIAG_BY_FAMILY)
+		return -ENOENT;
+
+	if (nlh->nlmsg_type == SOCK_DIAG_BY_FAMILY)
 		err = hndl->dump(skb, nlh);
 	else if (nlh->nlmsg_type == SOCK_DESTROY && hndl->destroy)
 		err = hndl->destroy(skb, nlh);
 	else
 		err = -EOPNOTSUPP;
-	mutex_unlock(&sock_diag_table_mutex);
+	sock_diag_unlock_handler(hndl);
 
 	return err;
 }
@@ -286,12 +297,12 @@ static int sock_diag_bind(struct net *net, int group)
 	switch (group) {
 	case SKNLGRP_INET_TCP_DESTROY:
 	case SKNLGRP_INET_UDP_DESTROY:
-		if (!READ_ONCE(sock_diag_handlers[AF_INET]))
+		if (!rcu_access_pointer(sock_diag_handlers[AF_INET]))
 			sock_load_diag_module(AF_INET, 0);
 		break;
 	case SKNLGRP_INET6_TCP_DESTROY:
 	case SKNLGRP_INET6_UDP_DESTROY:
-		if (!READ_ONCE(sock_diag_handlers[AF_INET6]))
+		if (!rcu_access_pointer(sock_diag_handlers[AF_INET6]))
 			sock_load_diag_module(AF_INET6, 0);
 		break;
 	}