rtnetlink: Introduce struct rtnl_nets and helpers.

rtnl_newlink() needs to hold 3 per-netns RTNL: 2 for a new device
and 1 for its peer.

We will add rtnl_nets_lock() later, which performs the nested locking
based on struct rtnl_nets, which has an array of struct net pointers.

rtnl_nets_add() adds a net pointer to the array and sorts it so that
rtnl_nets_lock() can simply acquire per-netns RTNL from array[0] to [2].

Before calling rtnl_nets_add(), get_net() must be called for the net,
and rtnl_nets_destroy() will call put_net() for each.

Let's apply the helpers to rtnl_newlink().

When CONFIG_DEBUG_NET_SMALL_RTNL is disabled, we do not call
rtnl_net_lock() thus do not care about the array order, so
rtnl_net_cmp_locks() returns -1 so that the loop in rtnl_nets_add()
can be optimised to NOP.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Nikolay Aleksandrov <razor@blackwall.org>
Link: https://patch.msgid.link/20241108004823.29419-5-kuniyu@amazon.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

1 files changed, 67 insertions, 3 deletions

diff --git a/net/core/rtnetlink.c b/net/core/rtnetlink.c
index e8357a3b9c7e..960d9d2c6aec 100644
--- a/net/core/rtnetlink.c
+++ b/net/core/rtnetlink.c
@@ -258,8 +258,67 @@ bool lockdep_rtnl_net_is_held(struct net *net)
 	return lockdep_rtnl_is_held() && lockdep_is_held(&net->rtnl_mutex);
 }
 EXPORT_SYMBOL(lockdep_rtnl_net_is_held);
+#else
+static int rtnl_net_cmp_locks(const struct net *net_a, const struct net *net_b)
+{
+	/* No need to swap */
+	return -1;
+}
 #endif
 
+struct rtnl_nets {
+	/* ->newlink() needs to freeze 3 netns at most;
+	 * 2 for the new device, 1 for its peer.
+	 */
+	struct net *net[3];
+	unsigned char len;
+};
+
+static void rtnl_nets_init(struct rtnl_nets *rtnl_nets)
+{
+	memset(rtnl_nets, 0, sizeof(*rtnl_nets));
+}
+
+static void rtnl_nets_destroy(struct rtnl_nets *rtnl_nets)
+{
+	int i;
+
+	for (i = 0; i < rtnl_nets->len; i++) {
+		put_net(rtnl_nets->net[i]);
+		rtnl_nets->net[i] = NULL;
+	}
+
+	rtnl_nets->len = 0;
+}
+
+/**
+ * rtnl_nets_add - Add netns to be locked before ->newlink().
+ *
+ * @rtnl_nets: rtnl_nets pointer passed to ->get_peer_net().
+ * @net: netns pointer with an extra refcnt held.
+ *
+ * The extra refcnt is released in rtnl_nets_destroy().
+ */
+static void rtnl_nets_add(struct rtnl_nets *rtnl_nets, struct net *net)
+{
+	int i;
+
+	DEBUG_NET_WARN_ON_ONCE(rtnl_nets->len == ARRAY_SIZE(rtnl_nets->net));
+
+	for (i = 0; i < rtnl_nets->len; i++) {
+		switch (rtnl_net_cmp_locks(rtnl_nets->net[i], net)) {
+		case 0:
+			put_net(net);
+			return;
+		case 1:
+			swap(rtnl_nets->net[i], net);
+		}
+	}
+
+	rtnl_nets->net[i] = net;
+	rtnl_nets->len++;
+}
+
 static struct rtnl_link __rcu *__rcu *rtnl_msg_handlers[RTNL_FAMILY_MAX + 1];
 
 static inline int rtm_msgindex(int msgtype)
@@ -3767,6 +3826,7 @@ static int rtnl_newlink(struct sk_buff *skb, struct nlmsghdr *nlh,
 	struct net *tgt_net, *link_net = NULL;
 	struct rtnl_link_ops *ops = NULL;
 	struct rtnl_newlink_tbs *tbs;
+	struct rtnl_nets rtnl_nets;
 	int ops_srcu_index;
 	int ret;
 
@@ -3810,6 +3870,8 @@ static int rtnl_newlink(struct sk_buff *skb, struct nlmsghdr *nlh,
 #endif
 	}
 
+	rtnl_nets_init(&rtnl_nets);
+
 	if (ops) {
 		if (ops->maxtype > RTNL_MAX_TYPE) {
 			ret = -EINVAL;
@@ -3839,6 +3901,8 @@ static int rtnl_newlink(struct sk_buff *skb, struct nlmsghdr *nlh,
 		goto put_ops;
 	}
 
+	rtnl_nets_add(&rtnl_nets, tgt_net);
+
 	if (tb[IFLA_LINK_NETNSID]) {
 		int id = nla_get_s32(tb[IFLA_LINK_NETNSID]);
 
@@ -3849,6 +3913,8 @@ static int rtnl_newlink(struct sk_buff *skb, struct nlmsghdr *nlh,
 			goto put_net;
 		}
 
+		rtnl_nets_add(&rtnl_nets, link_net);
+
 		if (!netlink_ns_capable(skb, link_net->user_ns, CAP_NET_ADMIN)) {
 			ret = -EPERM;
 			goto put_net;
@@ -3858,9 +3924,7 @@ static int rtnl_newlink(struct sk_buff *skb, struct nlmsghdr *nlh,
 	ret = __rtnl_newlink(skb, nlh, ops, tgt_net, link_net, tbs, data, extack);
 
 put_net:
-	if (link_net)
-		put_net(link_net);
-	put_net(tgt_net);
+	rtnl_nets_destroy(&rtnl_nets);
 put_ops:
 	if (ops)
 		rtnl_link_ops_put(ops, ops_srcu_index);