netdevsim: add dummy device notifiers

In order to exercise and verify notifiers' locking assumptions,
register dummy notifiers (via register_netdevice_notifier_dev_net).
Share notifier event handler that enforces the assumptions with
lock_debug.c (rename and export rtnl_net_debug_event as
netdev_debug_event). Add ops lock asserts to netdev_debug_event.

Signed-off-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://patch.msgid.link/20250401163452.622454-6-sdf@fomichev.me
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/netdevsim/netdev.c

@@ -939,6 +939,7 @@ static int nsim_init_netdevsim(struct netdevsim *ns)
 	ns->netdev->netdev_ops = &nsim_netdev_ops;
 	ns->netdev->stat_ops = &nsim_stat_ops;
 	ns->netdev->queue_mgmt_ops = &nsim_queue_mgmt_ops;
+	netdev_lockdep_set_classes(ns->netdev);
 
 	err = nsim_udp_tunnels_info_create(ns->nsim_dev, ns->netdev);
 	if (err)
@@ -960,6 +961,14 @@ static int nsim_init_netdevsim(struct netdevsim *ns)
 	if (err)
 		goto err_ipsec_teardown;
 	rtnl_unlock();
+
+	if (IS_ENABLED(CONFIG_DEBUG_NET)) {
+		ns->nb.notifier_call = netdev_debug_event;
+		if (register_netdevice_notifier_dev_net(ns->netdev, &ns->nb,
+							&ns->nn))
+			ns->nb.notifier_call = NULL;
+	}
+
 	return 0;
 
 err_ipsec_teardown:
@@ -1043,6 +1052,10 @@ void nsim_destroy(struct netdevsim *ns)
 	debugfs_remove(ns->qr_dfs);
 	debugfs_remove(ns->pp_dfs);
 
+	if (ns->nb.notifier_call)
+		unregister_netdevice_notifier_dev_net(ns->netdev, &ns->nb,
+						      &ns->nn);
+
 	rtnl_lock();
 	peer = rtnl_dereference(ns->peer);
 	if (peer)

drivers/net/netdevsim/netdevsim.h

@@ -144,6 +144,9 @@ struct netdevsim {
 
 	struct nsim_ethtool ethtool;
 	struct netdevsim __rcu *peer;
+
+	struct notifier_block nb;
+	struct netdev_net_notifier nn;
 };
 
 struct netdevsim *

include/net/netdev_lock.h

@@ -98,4 +98,7 @@ static inline int netdev_lock_cmp_fn(const struct lockdep_map *a,
 				  &qdisc_xmit_lock_key);	\
 }
 
+int netdev_debug_event(struct notifier_block *nb, unsigned long event,
+		       void *ptr);
+
 #endif

net/core/lock_debug.c

@@ -6,22 +6,25 @@
 #include <linux/notifier.h>
 #include <linux/rtnetlink.h>
 #include <net/net_namespace.h>
+#include <net/netdev_lock.h>
 #include <net/netns/generic.h>
 
-static int rtnl_net_debug_event(struct notifier_block *nb,
-				unsigned long event, void *ptr)
+int netdev_debug_event(struct notifier_block *nb, unsigned long event,
+		       void *ptr)
 {
 	struct net_device *dev = netdev_notifier_info_to_dev(ptr);
 	struct net *net = dev_net(dev);
 	enum netdev_cmd cmd = event;
 
 	/* Keep enum and don't add default to trigger -Werror=switch */
 	switch (cmd) {
+	case NETDEV_REGISTER:
 	case NETDEV_UP:
+		netdev_ops_assert_locked(dev);
+		fallthrough;
 	case NETDEV_DOWN:
 	case NETDEV_REBOOT:
 	case NETDEV_CHANGE:
-	case NETDEV_REGISTER:
 	case NETDEV_UNREGISTER:
 	case NETDEV_CHANGEMTU:
 	case NETDEV_CHANGEADDR:
@@ -66,6 +69,7 @@ static int rtnl_net_debug_event(struct notifier_block *nb,
 
 	return NOTIFY_DONE;
 }
+EXPORT_SYMBOL_NS_GPL(netdev_debug_event, "NETDEV_INTERNAL");
 
 static int rtnl_net_debug_net_id;
 
@@ -74,7 +78,7 @@ static int __net_init rtnl_net_debug_net_init(struct net *net)
 	struct notifier_block *nb;
 
 	nb = net_generic(net, rtnl_net_debug_net_id);
-	nb->notifier_call = rtnl_net_debug_event;
+	nb->notifier_call = netdev_debug_event;
 
 	return register_netdevice_notifier_net(net, nb);
 }
@@ -95,7 +99,7 @@ static struct pernet_operations rtnl_net_debug_net_ops __net_initdata = {
 };
 
 static struct notifier_block rtnl_net_debug_block = {
-	.notifier_call = rtnl_net_debug_event,
+	.notifier_call = netdev_debug_event,
 };
 
 static int __init rtnl_net_debug_init(void)