static int tc_ctl_chain(struct sk_buff *skb, struct nlmsghdr *n,
			struct netlink_ext_ack *extack)
{
	struct net *net = sock_net(skb->sk);
	struct nlattr *tca[TCA_MAX + 1];
	struct tcmsg *t;
	u32 parent;
	u32 chain_index;
	struct Qdisc *q = NULL;
	struct tcf_chain *chain = NULL;
	struct tcf_block *block;
	unsigned long cl;
	int err;

	if (n->nlmsg_type != RTM_GETCHAIN &&
	    !netlink_ns_capable(skb, net->user_ns, CAP_NET_ADMIN))
		return -EPERM;

replay:
	err = nlmsg_parse_deprecated(n, sizeof(*t), tca, TCA_MAX,
				     rtm_tca_policy, extack);
	if (err < 0)
		return err;

	t = nlmsg_data(n);
	parent = t->tcm_parent;
	cl = 0;

	block = tcf_block_find(net, &q, &parent, &cl,
			       t->tcm_ifindex, t->tcm_block_index, extack);
	if (IS_ERR(block))
		return PTR_ERR(block);

	chain_index = tca[TCA_CHAIN] ? nla_get_u32(tca[TCA_CHAIN]) : 0;
	if (chain_index > TC_ACT_EXT_VAL_MASK) {
		NL_SET_ERR_MSG(extack, "Specified chain index exceeds upper limit");
		err = -EINVAL;
		goto errout_block;
	}

	mutex_lock(&block->lock);
	chain = tcf_chain_lookup(block, chain_index);
	if (n->nlmsg_type == RTM_NEWCHAIN) {
		if (chain) {
			if (tcf_chain_held_by_acts_only(chain)) {
				/* The chain exists only because there is
				 * some action referencing it.
				 */
				tcf_chain_hold(chain);
			} else {
				NL_SET_ERR_MSG(extack, "Filter chain already exists");
				err = -EEXIST;
				goto errout_block_locked;
			}
		} else {
			if (!(n->nlmsg_flags & NLM_F_CREATE)) {
				NL_SET_ERR_MSG(extack, "Need both RTM_NEWCHAIN and NLM_F_CREATE to create a new chain");
				err = -ENOENT;
				goto errout_block_locked;
			}
			chain = tcf_chain_create(block, chain_index);
			if (!chain) {
				NL_SET_ERR_MSG(extack, "Failed to create filter chain");
				err = -ENOMEM;
				goto errout_block_locked;
			}
		}
	} else {
		if (!chain || tcf_chain_held_by_acts_only(chain)) {
			NL_SET_ERR_MSG(extack, "Cannot find specified filter chain");
			err = -EINVAL;
			goto errout_block_locked;
		}
		tcf_chain_hold(chain);
	}

	if (n->nlmsg_type == RTM_NEWCHAIN) {
		/* Modifying chain requires holding parent block lock. In case
		 * the chain was successfully added, take a reference to the
		 * chain. This ensures that an empty chain does not disappear at
		 * the end of this function.
		 */
		tcf_chain_hold(chain);
		chain->explicitly_created = true;
	}
	mutex_unlock(&block->lock);

	switch (n->nlmsg_type) {
	case RTM_NEWCHAIN:
		err = tc_chain_tmplt_add(chain, net, tca, extack);
		if (err) {
			tcf_chain_put_explicitly_created(chain);
			goto errout;
		}

		tc_chain_notify(chain, NULL, 0, NLM_F_CREATE | NLM_F_EXCL,
				RTM_NEWCHAIN, false);
		break;
	case RTM_DELCHAIN:
		tfilter_notify_chain(net, skb, block, q, parent, n,
				     chain, RTM_DELTFILTER);
		/* Flush the chain first as the user requested chain removal. */
		tcf_chain_flush(chain, true);
		/* In case the chain was successfully deleted, put a reference
		 * to the chain previously taken during addition.
		 */
		tcf_chain_put_explicitly_created(chain);
		break;
	case RTM_GETCHAIN:
		err = tc_chain_notify(chain, skb, n->nlmsg_seq,
				      n->nlmsg_flags, n->nlmsg_type, true);
		if (err < 0)
			NL_SET_ERR_MSG(extack, "Failed to send chain notify message");
		break;
	default:
		err = -EOPNOTSUPP;
		NL_SET_ERR_MSG(extack, "Unsupported message type");
		goto errout;
	}

errout:
	tcf_chain_put(chain);
errout_block:
	tcf_block_release(q, block, true);
	if (err == -EAGAIN)
		/* Replay the request. */
		goto replay;
	return err;

errout_block_locked:
	mutex_unlock(&block->lock);
	goto errout_block;
}