ipv6: remove RTNL protection from inet6_dump_fib()

No longer hold RTNL while calling inet6_dump_fib().

Also change return value for a completed dump,
so that NLMSG_DONE can be appended to current skb,
saving one recvmsg() system call.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/20240329183053.644630-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: edumazet

net/ipv6/ip6_fib.c

@@ -623,23 +623,22 @@ static int inet6_dump_fib(struct sk_buff *skb, struct netlink_callback *cb)
 	struct rt6_rtnl_dump_arg arg = {
 		.filter.dump_exceptions = true,
 		.filter.dump_routes = true,
-		.filter.rtnl_held = true,
+		.filter.rtnl_held = false,
 	};
 	const struct nlmsghdr *nlh = cb->nlh;
 	struct net *net = sock_net(skb->sk);
-	unsigned int h, s_h;
 	unsigned int e = 0, s_e;
+	struct hlist_head *head;
 	struct fib6_walker *w;
 	struct fib6_table *tb;
-	struct hlist_head *head;
-	int res = 0;
+	unsigned int h, s_h;
+	int err = 0;
 
+	rcu_read_lock();
 	if (cb->strict_check) {
-		int err;
-
 		err = ip_valid_fib_dump_req(net, nlh, &arg.filter, cb);
 		if (err < 0)
-			return err;
+			goto unlock;
 	} else if (nlmsg_len(nlh) >= sizeof(struct rtmsg)) {
 		struct rtmsg *rtm = nlmsg_data(nlh);
 
@@ -660,8 +659,10 @@ static int inet6_dump_fib(struct sk_buff *skb, struct netlink_callback *cb)
 		 * 2. allocate and initialize walker.
 		 */
 		w = kzalloc(sizeof(*w), GFP_ATOMIC);
-		if (!w)
-			return -ENOMEM;
+		if (!w) {
+			err = -ENOMEM;
+			goto unlock;
+		}
 		w->func = fib6_dump_node;
 		cb->args[2] = (long)w;
 	}
@@ -675,46 +676,46 @@ static int inet6_dump_fib(struct sk_buff *skb, struct netlink_callback *cb)
 		tb = fib6_get_table(net, arg.filter.table_id);
 		if (!tb) {
 			if (rtnl_msg_family(cb->nlh) != PF_INET6)
-				goto out;
+				goto unlock;
 
 			NL_SET_ERR_MSG_MOD(cb->extack, "FIB table does not exist");
-			return -ENOENT;
+			err = -ENOENT;
+			goto unlock;
 		}
 
 		if (!cb->args[0]) {
-			res = fib6_dump_table(tb, skb, cb);
-			if (!res)
+			err = fib6_dump_table(tb, skb, cb);
+			if (!err)
 				cb->args[0] = 1;
 		}
-		goto out;
+		goto unlock;
 	}
 
 	s_h = cb->args[0];
 	s_e = cb->args[1];
 
-	rcu_read_lock();
 	for (h = s_h; h < FIB6_TABLE_HASHSZ; h++, s_e = 0) {
 		e = 0;
 		head = &net->ipv6.fib_table_hash[h];
 		hlist_for_each_entry_rcu(tb, head, tb6_hlist) {
 			if (e < s_e)
 				goto next;
-			res = fib6_dump_table(tb, skb, cb);
-			if (res != 0)
-				goto out_unlock;
+			err = fib6_dump_table(tb, skb, cb);
+			if (err != 0)
+				goto out;
 next:
 			e++;
 		}
 	}
-out_unlock:
-	rcu_read_unlock();
+out:
 	cb->args[1] = e;
 	cb->args[0] = h;
-out:
-	res = res < 0 ? res : skb->len;
-	if (res <= 0)
+
+unlock:
+	rcu_read_unlock();
+	if (err <= 0)
 		fib6_dump_end(cb);
-	return res;
+	return err;
 }
 
 void fib6_metric_set(struct fib6_info *f6i, int metric, u32 val)
@@ -2506,7 +2507,7 @@ int __init fib6_init(void)
 		goto out_kmem_cache_create;
 
 	ret = rtnl_register_module(THIS_MODULE, PF_INET6, RTM_GETROUTE, NULL,
-				   inet6_dump_fib, 0);
+				   inet6_dump_fib, RTNL_FLAG_DUMP_UNLOCKED);
 	if (ret)
 		goto out_unregister_subsys;