ipv6: use xa_array iterator to implement inet6_netconf_dump_devconf()

1) inet6_netconf_dump_devconf() can run under RCU protection
   instead of RTNL.

2) properly return 0 at the end of a dump, avoiding an
   an extra recvmsg() system call.

3) Do not use inet6_base_seq() anymore, for_each_netdev_dump()
   has nice properties. Restarting a GETDEVCONF dump if a device has
   been added/removed or if net->ipv6.dev_addr_genid has changed is moot.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/addrconf.c

@@ -727,17 +727,18 @@ static u32 inet6_base_seq(const struct net *net)
 	return res;
 }
 
-
 static int inet6_netconf_dump_devconf(struct sk_buff *skb,
 				      struct netlink_callback *cb)
 {
 	const struct nlmsghdr *nlh = cb->nlh;
 	struct net *net = sock_net(skb->sk);
-	int h, s_h;
-	int idx, s_idx;
+	struct {
+		unsigned long ifindex;
+		unsigned int all_default;
+	} *ctx = (void *)cb->ctx;
 	struct net_device *dev;
 	struct inet6_dev *idev;
-	struct hlist_head *head;
+	int err = 0;
 
 	if (cb->strict_check) {
 		struct netlink_ext_ack *extack = cb->extack;
@@ -754,64 +755,48 @@ static int inet6_netconf_dump_devconf(struct sk_buff *skb,
 		}
 	}
 
-	s_h = cb->args[0];
-	s_idx = idx = cb->args[1];
-
-	for (h = s_h; h < NETDEV_HASHENTRIES; h++, s_idx = 0) {
-		idx = 0;
-		head = &net->dev_index_head[h];
-		rcu_read_lock();
-		cb->seq = inet6_base_seq(net);
-		hlist_for_each_entry_rcu(dev, head, index_hlist) {
-			if (idx < s_idx)
-				goto cont;
-			idev = __in6_dev_get(dev);
-			if (!idev)
-				goto cont;
-
-			if (inet6_netconf_fill_devconf(skb, dev->ifindex,
-						       &idev->cnf,
-						       NETLINK_CB(cb->skb).portid,
-						       nlh->nlmsg_seq,
-						       RTM_NEWNETCONF,
-						       NLM_F_MULTI,
-						       NETCONFA_ALL) < 0) {
-				rcu_read_unlock();
-				goto done;
-			}
-			nl_dump_check_consistent(cb, nlmsg_hdr(skb));
-cont:
-			idx++;
-		}
-		rcu_read_unlock();
+	rcu_read_lock();
+	for_each_netdev_dump(net, dev, ctx->ifindex) {
+		idev = __in6_dev_get(dev);
+		if (!idev)
+			continue;
+		err = inet6_netconf_fill_devconf(skb, dev->ifindex,
+					         &idev->cnf,
+						 NETLINK_CB(cb->skb).portid,
+						 nlh->nlmsg_seq,
+						 RTM_NEWNETCONF,
+						 NLM_F_MULTI,
+						 NETCONFA_ALL);
+		if (err < 0)
+			goto done;
 	}
-	if (h == NETDEV_HASHENTRIES) {
-		if (inet6_netconf_fill_devconf(skb, NETCONFA_IFINDEX_ALL,
-					       net->ipv6.devconf_all,
-					       NETLINK_CB(cb->skb).portid,
-					       nlh->nlmsg_seq,
-					       RTM_NEWNETCONF, NLM_F_MULTI,
-					       NETCONFA_ALL) < 0)
+	if (ctx->all_default == 0) {
+		err = inet6_netconf_fill_devconf(skb, NETCONFA_IFINDEX_ALL,
+						 net->ipv6.devconf_all,
+						 NETLINK_CB(cb->skb).portid,
+						 nlh->nlmsg_seq,
+						 RTM_NEWNETCONF, NLM_F_MULTI,
+						 NETCONFA_ALL);
+		if (err < 0)
 			goto done;
-		else
-			h++;
-	}
-	if (h == NETDEV_HASHENTRIES + 1) {
-		if (inet6_netconf_fill_devconf(skb, NETCONFA_IFINDEX_DEFAULT,
-					       net->ipv6.devconf_dflt,
-					       NETLINK_CB(cb->skb).portid,
-					       nlh->nlmsg_seq,
-					       RTM_NEWNETCONF, NLM_F_MULTI,
-					       NETCONFA_ALL) < 0)
+		ctx->all_default++;
+	}
+	if (ctx->all_default == 1) {
+		err = inet6_netconf_fill_devconf(skb, NETCONFA_IFINDEX_DEFAULT,
+						 net->ipv6.devconf_dflt,
+						 NETLINK_CB(cb->skb).portid,
+						 nlh->nlmsg_seq,
+						 RTM_NEWNETCONF, NLM_F_MULTI,
+						 NETCONFA_ALL);
+		if (err < 0)
 			goto done;
-		else
-			h++;
+		ctx->all_default++;
 	}
 done:
-	cb->args[0] = h;
-	cb->args[1] = idx;
-
-	return skb->len;
+	if (err < 0 && likely(skb->len))
+		err = skb->len;
+	rcu_read_unlock();
+	return err;
 }
 
 #ifdef CONFIG_SYSCTL
@@ -7503,7 +7488,8 @@ int __init addrconf_init(void)
 	err = rtnl_register_module(THIS_MODULE, PF_INET6, RTM_GETNETCONF,
 				   inet6_netconf_get_devconf,
 				   inet6_netconf_dump_devconf,
-				   RTNL_FLAG_DOIT_UNLOCKED);
+				   RTNL_FLAG_DOIT_UNLOCKED |
+				   RTNL_FLAG_DUMP_UNLOCKED);
 	if (err < 0)
 		goto errout;
 	err = ipv6_addr_label_rtnl_register();