ipv6: mcast: Don't hold RTNL for IPV6_DROP_MEMBERSHIP and MCAST_LEAVE_GROUP.

In __ipv6_sock_mc_drop(), per-socket mld data is protected by lock_sock(),
and only __dev_get_by_index() and __in6_dev_get() require RTNL.

Let's use dev_get_by_index() and in6_dev_get() and drop RTNL for
IPV6_ADD_MEMBERSHIP and MCAST_JOIN_GROUP.

Note that __ipv6_sock_mc_drop() is factorised to reuse in the next patch.

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20250702230210.3115355-8-kuni1840@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: q2ven

net/ipv6/ipv6_sockglue.c

@@ -121,10 +121,8 @@ static bool setsockopt_needs_rtnl(int optname)
 {
 	switch (optname) {
 	case IPV6_ADDRFORM:
-	case IPV6_DROP_MEMBERSHIP:
 	case IPV6_JOIN_ANYCAST:
 	case IPV6_LEAVE_ANYCAST:
-	case MCAST_LEAVE_GROUP:
 	case MCAST_JOIN_SOURCE_GROUP:
 	case MCAST_LEAVE_SOURCE_GROUP:
 	case MCAST_BLOCK_SOURCE:

net/ipv6/mcast.c

@@ -253,14 +253,36 @@ int ipv6_sock_mc_join_ssm(struct sock *sk, int ifindex,
 /*
  *	socket leave on multicast group
  */
+static void __ipv6_sock_mc_drop(struct sock *sk, struct ipv6_mc_socklist *mc_lst)
+{
+	struct net *net = sock_net(sk);
+	struct net_device *dev;
+
+	dev = dev_get_by_index(net, mc_lst->ifindex);
+	if (dev) {
+		struct inet6_dev *idev = in6_dev_get(dev);
+
+		ip6_mc_leave_src(sk, mc_lst, idev);
+
+		if (idev) {
+			__ipv6_dev_mc_dec(idev, &mc_lst->addr);
+			in6_dev_put(idev);
+		}
+
+		dev_put(dev);
+	} else {
+		ip6_mc_leave_src(sk, mc_lst, NULL);
+	}
+
+	atomic_sub(sizeof(*mc_lst), &sk->sk_omem_alloc);
+	kfree_rcu(mc_lst, rcu);
+}
+
 int ipv6_sock_mc_drop(struct sock *sk, int ifindex, const struct in6_addr *addr)
 {
 	struct ipv6_pinfo *np = inet6_sk(sk);
-	struct ipv6_mc_socklist *mc_lst;
 	struct ipv6_mc_socklist __rcu **lnk;
-	struct net *net = sock_net(sk);
-
-	ASSERT_RTNL();
+	struct ipv6_mc_socklist *mc_lst;
 
 	if (!ipv6_addr_is_multicast(addr))
 		return -EINVAL;
@@ -270,23 +292,8 @@ int ipv6_sock_mc_drop(struct sock *sk, int ifindex, const struct in6_addr *addr)
 	      lnk = &mc_lst->next) {
 		if ((ifindex == 0 || mc_lst->ifindex == ifindex) &&
 		    ipv6_addr_equal(&mc_lst->addr, addr)) {
-			struct net_device *dev;
-
 			*lnk = mc_lst->next;
-
-			dev = __dev_get_by_index(net, mc_lst->ifindex);
-			if (dev) {
-				struct inet6_dev *idev = __in6_dev_get(dev);
-
-				ip6_mc_leave_src(sk, mc_lst, idev);
-				if (idev)
-					__ipv6_dev_mc_dec(idev, &mc_lst->addr);
-			} else {
-				ip6_mc_leave_src(sk, mc_lst, NULL);
-			}
-
-			atomic_sub(sizeof(*mc_lst), &sk->sk_omem_alloc);
-			kfree_rcu(mc_lst, rcu);
+			__ipv6_sock_mc_drop(sk, mc_lst);
 			return 0;
 		}
 	}