static int tc_dump_chain(struct sk_buff *skb, struct netlink_callback *cb)
{
	struct net *net = sock_net(skb->sk);
	struct nlattr *tca[TCA_MAX + 1];
	struct Qdisc *q = NULL;
	struct tcf_block *block;
	struct tcmsg *tcm = nlmsg_data(cb->nlh);
	struct tcf_chain *chain;
	long index_start;
	long index;
	int err;

	if (nlmsg_len(cb->nlh) < sizeof(*tcm))
		return skb->len;

	err = nlmsg_parse_deprecated(cb->nlh, sizeof(*tcm), tca, TCA_MAX,
				     rtm_tca_policy, cb->extack);
	if (err)
		return err;

	if (tcm->tcm_ifindex == TCM_IFINDEX_MAGIC_BLOCK) {
		block = tcf_block_refcnt_get(net, tcm->tcm_block_index);
		if (!block)
			goto out;
	} else {
		const struct Qdisc_class_ops *cops;
		struct net_device *dev;
		unsigned long cl = 0;

		dev = __dev_get_by_index(net, tcm->tcm_ifindex);
		if (!dev)
			return skb->len;

		if (!tcm->tcm_parent)
			q = dev->qdisc;
		else
			q = qdisc_lookup(dev, TC_H_MAJ(tcm->tcm_parent));

		if (!q)
			goto out;
		cops = q->ops->cl_ops;
		if (!cops)
			goto out;
		if (!cops->tcf_block)
			goto out;
		if (TC_H_MIN(tcm->tcm_parent)) {
			cl = cops->find(q, tcm->tcm_parent);
			if (cl == 0)
				goto out;
		}
		block = cops->tcf_block(q, cl, NULL);
		if (!block)
			goto out;
		if (tcf_block_shared(block))
			q = NULL;
	}

	index_start = cb->args[0];
	index = 0;

	mutex_lock(&block->lock);
	list_for_each_entry(chain, &block->chain_list, list) {
		if ((tca[TCA_CHAIN] &&
		     nla_get_u32(tca[TCA_CHAIN]) != chain->index))
			continue;
		if (index < index_start) {
			index++;
			continue;
		}
		if (tcf_chain_held_by_acts_only(chain))
			continue;
		err = tc_chain_fill_node(chain->tmplt_ops, chain->tmplt_priv,
					 chain->index, net, skb, block,
					 NETLINK_CB(cb->skb).portid,
					 cb->nlh->nlmsg_seq, NLM_F_MULTI,
					 RTM_NEWCHAIN);
		if (err <= 0)
			break;
		index++;
	}
	mutex_unlock(&block->lock);

	if (tcm->tcm_ifindex == TCM_IFINDEX_MAGIC_BLOCK)
		tcf_block_refcnt_put(block, true);
	cb->args[0] = index;

out:
	/* If we did no progress, the error (EMSGSIZE) is real */
	if (skb->len == 0 && err)
		return err;
	return skb->len;
}