rtnetlink: use xarray iterator to implement rtnl_dump_ifinfo()

Adopt net->dev_by_index as I did in commit 0e0939c
("net-procfs: use xarray iterator to implement /proc/net/dev")

This makes sure an existing device is always visible in the dump,
regardless of concurrent insertions/deletions.

v2: added suggestions from Jakub Kicinski and Ido Schimmel,
    thanks for the help !

Link: https://lore.kernel.org/all/20240209142441.6c56435b@kernel.org/
Link: https://lore.kernel.org/all/ZckR-XOsULLI9EHc@shredder/
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://lore.kernel.org/r/20240211214404.1882191-3-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/core/rtnetlink.c

@@ -2188,25 +2188,22 @@ static int rtnl_valid_dump_ifinfo_req(const struct nlmsghdr *nlh,
 
 static int rtnl_dump_ifinfo(struct sk_buff *skb, struct netlink_callback *cb)
 {
+	const struct rtnl_link_ops *kind_ops = NULL;
 	struct netlink_ext_ack *extack = cb->extack;
 	const struct nlmsghdr *nlh = cb->nlh;
 	struct net *net = sock_net(skb->sk);
-	struct net *tgt_net = net;
-	int h, s_h;
-	int idx = 0, s_idx;
-	struct net_device *dev;
-	struct hlist_head *head;
+	unsigned int flags = NLM_F_MULTI;
 	struct nlattr *tb[IFLA_MAX+1];
+	struct {
+		unsigned long ifindex;
+	} *ctx = (void *)cb->ctx;
+	struct net *tgt_net = net;
 	u32 ext_filter_mask = 0;
-	const struct rtnl_link_ops *kind_ops = NULL;
-	unsigned int flags = NLM_F_MULTI;
+	struct net_device *dev;
 	int master_idx = 0;
 	int netnsid = -1;
 	int err, i;
 
-	s_h = cb->args[0];
-	s_idx = cb->args[1];
-
 	err = rtnl_valid_dump_ifinfo_req(nlh, cb->strict_check, tb, extack);
 	if (err < 0) {
 		if (cb->strict_check)
@@ -2250,36 +2247,21 @@ static int rtnl_dump_ifinfo(struct sk_buff *skb, struct netlink_callback *cb)
 		flags |= NLM_F_DUMP_FILTERED;
 
 walk_entries:
-	for (h = s_h; h < NETDEV_HASHENTRIES; h++, s_idx = 0) {
-		idx = 0;
-		head = &tgt_net->dev_index_head[h];
-		hlist_for_each_entry(dev, head, index_hlist) {
-			if (link_dump_filtered(dev, master_idx, kind_ops))
-				goto cont;
-			if (idx < s_idx)
-				goto cont;
-			err = rtnl_fill_ifinfo(skb, dev, net,
-					       RTM_NEWLINK,
-					       NETLINK_CB(cb->skb).portid,
-					       nlh->nlmsg_seq, 0, flags,
-					       ext_filter_mask, 0, NULL, 0,
-					       netnsid, GFP_KERNEL);
-
-			if (err < 0) {
-				if (likely(skb->len))
-					goto out;
-
-				goto out_err;
-			}
-cont:
-			idx++;
+	err = 0;
+	for_each_netdev_dump(tgt_net, dev, ctx->ifindex) {
+		if (link_dump_filtered(dev, master_idx, kind_ops))
+			continue;
+		err = rtnl_fill_ifinfo(skb, dev, net, RTM_NEWLINK,
+				       NETLINK_CB(cb->skb).portid,
+				       nlh->nlmsg_seq, 0, flags,
+				       ext_filter_mask, 0, NULL, 0,
+				       netnsid, GFP_KERNEL);
+		if (err < 0) {
+			if (likely(skb->len))
+				err = skb->len;
+			break;
 		}
 	}
-out:
-	err = skb->len;
-out_err:
-	cb->args[1] = idx;
-	cb->args[0] = h;
 	cb->seq = tgt_net->dev_base_seq;
 	nl_dump_check_consistent(cb, nlmsg_hdr(skb));
 	if (netnsid >= 0)