ipv6: anycast: Don't hold RTNL for IPV6_JOIN_ANYCAST.

JIRA: https://issues.redhat.com/browse/RHEL-115325

commit eb1ac9f
Author: Kuniyuki Iwashima <kuniyu@google.com>
Date:   Wed Jul 2 16:01:31 2025 -0700

    ipv6: anycast: Don't hold RTNL for IPV6_JOIN_ANYCAST.

    inet6_sk(sk)->ipv6_ac_list is protected by lock_sock().

    In ipv6_sock_ac_join(), only __dev_get_by_index(), __dev_get_by_flags(),
    and __in6_dev_get() require RTNL.

    __dev_get_by_flags() is only used by ipv6_sock_ac_join() and can be
    converted to RCU version.

    Let's replace RCU version helper and drop RTNL from IPV6_JOIN_ANYCAST.

    setsockopt_needs_rtnl() will be removed in the next patch.

    Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
    Reviewed-by: Eric Dumazet <edumazet@google.com>
    Link: https://patch.msgid.link/20250702230210.3115355-15-kuni1840@gmail.com
    Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Signed-off-by: CKI Backport Bot <cki-ci-bot+cki-gitlab-backport-bot@redhat.com>

Author: CKI Backport Bot

include/linux/netdevice.h

@@ -3160,8 +3160,8 @@ int dev_get_iflink(const struct net_device *dev);
 int dev_fill_metadata_dst(struct net_device *dev, struct sk_buff *skb);
 int dev_fill_forward_path(const struct net_device *dev, const u8 *daddr,
 			  struct net_device_path_stack *stack);
-struct net_device *__dev_get_by_flags(struct net *net, unsigned short flags,
-				      unsigned short mask);
+struct net_device *dev_get_by_flags_rcu(struct net *net, unsigned short flags,
+					unsigned short mask);
 struct net_device *dev_get_by_name(struct net *net, const char *name);
 struct net_device *dev_get_by_name_rcu(struct net *net, const char *name);
 struct net_device *__dev_get_by_name(struct net *net, const char *name);

net/core/dev.c

@@ -1102,33 +1102,31 @@ struct net_device *dev_getfirstbyhwtype(struct net *net, unsigned short type)
 EXPORT_SYMBOL(dev_getfirstbyhwtype);
 
 /**
- *	__dev_get_by_flags - find any device with given flags
- *	@net: the applicable net namespace
- *	@if_flags: IFF_* values
- *	@mask: bitmask of bits in if_flags to check
+ * dev_get_by_flags_rcu - find any device with given flags
+ * @net: the applicable net namespace
+ * @if_flags: IFF_* values
+ * @mask: bitmask of bits in if_flags to check
  *
- *	Search for any interface with the given flags. Returns NULL if a device
- *	is not found or a pointer to the device. Must be called inside
- *	rtnl_lock(), and result refcount is unchanged.
+ * Search for any interface with the given flags.
+ *
+ * Context: rcu_read_lock() must be held.
+ * Returns: NULL if a device is not found or a pointer to the device.
  */
-
-struct net_device *__dev_get_by_flags(struct net *net, unsigned short if_flags,
-				      unsigned short mask)
+struct net_device *dev_get_by_flags_rcu(struct net *net, unsigned short if_flags,
+					unsigned short mask)
 {
-	struct net_device *dev, *ret;
-
-	ASSERT_RTNL();
+	struct net_device *dev;
 
-	ret = NULL;
-	for_each_netdev(net, dev) {
-		if (((dev->flags ^ if_flags) & mask) == 0) {
-			ret = dev;
-			break;
+	for_each_netdev_rcu(net, dev) {
+		if (((READ_ONCE(dev->flags) ^ if_flags) & mask) == 0) {
+			dev_hold(dev);
+			return dev;
 		}
 	}
-	return ret;
+
+	return NULL;
 }
-EXPORT_SYMBOL(__dev_get_by_flags);
+EXPORT_IPV6_MOD(dev_get_by_flags_rcu);
 
 /**
  *	dev_valid_name - check if name is okay for network device

net/ipv6/anycast.c

@@ -73,15 +73,13 @@ int ipv6_sock_ac_join(struct sock *sk, int ifindex, const struct in6_addr *addr)
 	struct inet6_dev *idev;
 	int err = 0, ishost;
 
-	ASSERT_RTNL();
-
 	if (!ns_capable(net->user_ns, CAP_NET_ADMIN))
 		return -EPERM;
 	if (ipv6_addr_is_multicast(addr))
 		return -EINVAL;
 
 	if (ifindex)
-		dev = __dev_get_by_index(net, ifindex);
+		dev = dev_get_by_index(net, ifindex);
 
 	if (ipv6_chk_addr_and_flags(net, addr, dev, true, 0, IFA_F_TENTATIVE)) {
 		err = -EINVAL;
@@ -102,26 +100,30 @@ int ipv6_sock_ac_join(struct sock *sk, int ifindex, const struct in6_addr *addr)
 	if (ifindex == 0) {
 		struct rt6_info *rt;
 
+		rcu_read_lock();
 		rt = rt6_lookup(net, addr, NULL, 0, NULL, 0);
 		if (rt) {
-			dev = rt->dst.dev;
+			dev = dst_dev(&rt->dst);
+			dev_hold(dev);
 			ip6_rt_put(rt);
 		} else if (ishost) {
+			rcu_read_unlock();
 			err = -EADDRNOTAVAIL;
 			goto error;
 		} else {
 			/* router, no matching interface: just pick one */
-			dev = __dev_get_by_flags(net, IFF_UP,
-						 IFF_UP | IFF_LOOPBACK);
+			dev = dev_get_by_flags_rcu(net, IFF_UP,
+						   IFF_UP | IFF_LOOPBACK);
 		}
+		rcu_read_unlock();
 	}
 
 	if (!dev) {
 		err = -ENODEV;
 		goto error;
 	}
 
-	idev = __in6_dev_get(dev);
+	idev = in6_dev_get(dev);
 	if (!idev) {
 		if (ifindex)
 			err = -ENODEV;
@@ -144,7 +146,7 @@ int ipv6_sock_ac_join(struct sock *sk, int ifindex, const struct in6_addr *addr)
 		if (ishost)
 			err = -EADDRNOTAVAIL;
 		if (err)
-			goto error;
+			goto error_idev;
 	}
 
 	err = __ipv6_dev_ac_inc(idev, addr);
@@ -154,7 +156,11 @@ int ipv6_sock_ac_join(struct sock *sk, int ifindex, const struct in6_addr *addr)
 		pac = NULL;
 	}
 
+error_idev:
+	in6_dev_put(idev);
 error:
+	dev_put(dev);
+
 	if (pac)
 		sock_kfree_s(sk, pac, sizeof(*pac));
 	return err;

net/ipv6/ipv6_sockglue.c

@@ -119,10 +119,6 @@ struct ipv6_txoptions *ipv6_update_options(struct sock *sk,
 
 static bool setsockopt_needs_rtnl(int optname)
 {
-	switch (optname) {
-	case IPV6_JOIN_ANYCAST:
-		return true;
-	}
 	return false;
 }