// SPDX-License-Identifier: GPL-2.0-only
/*
 * Copyright (c) 2006 Patrick McHardy <kaber@trash.net>
 * Copyright © CC Computer Consultants GmbH, 2007 - 2008
 *
 * This is a replacement of the old ipt_recent module, which carried the
 * following copyright notice:
 *
 * Author: Stephen Frost <sfrost@snowman.net>
 * Copyright 2002-2003, Stephen Frost, 2.5.x port by laforge@netfilter.org
 */
#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
#include <linux/init.h>
#include <linux/ip.h>
#include <linux/ipv6.h>
#include <linux/module.h>
#include <linux/moduleparam.h>
#include <linux/proc_fs.h>
#include <linux/seq_file.h>
#include <linux/string.h>
#include <linux/ctype.h>
#include <linux/list.h>
#include <linux/random.h>
#include <linux/jhash.h>
#include <linux/bitops.h>
#include <linux/skbuff.h>
#include <linux/inet.h>
#include <linux/slab.h>
#include <linux/vmalloc.h>
#include <net/net_namespace.h>
#include <net/netns/generic.h>

#include <linux/netfilter/x_tables.h>
#include <linux/netfilter/xt_recent.h>

MODULE_AUTHOR("Patrick McHardy <kaber@trash.net>");
MODULE_AUTHOR("Jan Engelhardt <jengelh@medozas.de>");
MODULE_DESCRIPTION("Xtables: \"recently-seen\" host matching");
MODULE_LICENSE("GPL");
MODULE_ALIAS("ipt_recent");
MODULE_ALIAS("ip6t_recent");

static unsigned int ip_list_tot __read_mostly = 100;
static unsigned int ip_list_hash_size __read_mostly;
static unsigned int ip_list_perms __read_mostly = 0644;
static unsigned int ip_list_uid __read_mostly;
static unsigned int ip_list_gid __read_mostly;
module_param(ip_list_tot, uint, 0400);
module_param(ip_list_hash_size, uint, 0400);
module_param(ip_list_perms, uint, 0400);
module_param(ip_list_uid, uint, 0644);
module_param(ip_list_gid, uint, 0644);
MODULE_PARM_DESC(ip_list_tot, "number of IPs to remember per list");
MODULE_PARM_DESC(ip_list_hash_size, "size of hash table used to look up IPs");
MODULE_PARM_DESC(ip_list_perms, "permissions on /proc/net/xt_recent/* files");
MODULE_PARM_DESC(ip_list_uid, "default owner of /proc/net/xt_recent/* files");
MODULE_PARM_DESC(ip_list_gid, "default owning group of /proc/net/xt_recent/* files");

/* retained for backwards compatibility */
static unsigned int ip_pkt_list_tot __read_mostly;
module_param(ip_pkt_list_tot, uint, 0400);
MODULE_PARM_DESC(ip_pkt_list_tot, "number of packets per IP address to remember (max. 255)");

#define XT_RECENT_MAX_NSTAMPS	256

struct recent_entry {
	struct list_head	list;
	struct list_head	lru_list;
	union nf_inet_addr	addr;
	u_int16_t		family;
	u_int8_t		ttl;
	u_int8_t		index;
	u_int16_t		nstamps;
	unsigned long		stamps[];
};

struct recent_table {
	struct list_head	list;
	char			name[XT_RECENT_NAME_LEN];
	union nf_inet_addr	mask;
	unsigned int		refcnt;
	unsigned int		entries;
	u8			nstamps_max_mask;
	struct list_head	lru_list;
	struct list_head	iphash[];
};

struct recent_net {
	struct list_head	tables;
#ifdef CONFIG_PROC_FS
	struct proc_dir_entry	*xt_recent;
#endif
};

static unsigned int recent_net_id __read_mostly;

static inline struct recent_net *recent_pernet(struct net *net)
{
	return net_generic(net, recent_net_id);
}

static DEFINE_SPINLOCK(recent_lock);
static DEFINE_MUTEX(recent_mutex);

#ifdef CONFIG_PROC_FS
static const struct proc_ops recent_mt_proc_ops;
#endif

static u_int32_t hash_rnd __read_mostly;

static inline unsigned int recent_entry_hash4(const union nf_inet_addr *addr)
{
	return jhash_1word((__force u32)addr->ip, hash_rnd) &
	       (ip_list_hash_size - 1);
}

static inline unsigned int recent_entry_hash6(const union nf_inet_addr *addr)
{
	return jhash2((u32 *)addr->ip6, ARRAY_SIZE(addr->ip6), hash_rnd) &
	       (ip_list_hash_size - 1);
}

static struct recent_entry *
recent_entry_lookup(const struct recent_table *table,
		    const union nf_inet_addr *addrp, u_int16_t family,
		    u_int8_t ttl)
{
	struct recent_entry *e;
	unsigned int h;

	if (family == NFPROTO_IPV4)
		h = recent_entry_hash4(addrp);
	else
		h = recent_entry_hash6(addrp);

	list_for_each_entry(e, &table->iphash[h], list)
		if (e->family == family &&
		    memcmp(&e->addr, addrp, sizeof(e->addr)) == 0 &&
		    (ttl == e->ttl || ttl == 0 || e->ttl == 0))
			return e;
	return NULL;
}

static void recent_entry_remove(struct recent_table *t, struct recent_entry *e)
{
	list_del(&e->list);
	list_del(&e->lru_list);
	kfree(e);
	t->entries--;
}

/*
 * Drop entries with timestamps older then 'time'.
 */
static void recent_entry_reap(struct recent_table *t, unsigned long time,
			      struct recent_entry *working, bool update)
{
	struct recent_entry *e;

	/*
	 * The head of the LRU list is always the oldest entry.
	 */
	e = list_entry(t->lru_list.next, struct recent_entry, lru_list);

	/*
	 * Do not reap the entry which are going to be updated.
	 */
	if (e == working && update)
		return;

	/*
	 * The last time stamp is the most recent.
	 */
	if (time_after(time, e->stamps[e->index-1]))
		recent_entry_remove(t, e);
}

static struct recent_entry *
recent_entry_init(struct recent_table *t, const union nf_inet_addr *addr,
		  u_int16_t family, u_int8_t ttl)
{
	struct recent_entry *e;
	unsigned int nstamps_max = t->nstamps_max_mask;

	if (t->entries >= ip_list_tot) {
		e = list_entry(t->lru_list.next, struct recent_entry, lru_list);
		recent_entry_remove(t, e);
	}

	nstamps_max += 1;
	e = kmalloc(struct_size(e, stamps, nstamps_max), GFP_ATOMIC);
	if (e == NULL)
		return NULL;
	memcpy(&e->addr, addr, sizeof(e->addr));
	e->ttl       = ttl;
	e->stamps[0] = jiffies;
	e->nstamps   = 1;
	e->index     = 1;
	e->family    = family;
	if (family == NFPROTO_IPV4)
		list_add_tail(&e->list, &t->iphash[recent_entry_hash4(addr)]);
	else
		list_add_tail(&e->list, &t->iphash[recent_entry_hash6(addr)]);
	list_add_tail(&e->lru_list, &t->lru_list);
	t->entries++;
	return e;
}

static void recent_entry_update(struct recent_table *t, struct recent_entry *e)
{
	e->index &= t->nstamps_max_mask;
	e->stamps[e->index++] = jiffies;
	if (e->index > e->nstamps)
		e->nstamps = e->index;
	list_move_tail(&e->lru_list, &t->lru_list);
}

static struct recent_table *recent_table_lookup(struct recent_net *recent_net,
						const char *name)
{
	struct recent_table *t;

	list_for_each_entry(t, &recent_net->tables, list)
		if (!strcmp(t->name, name))
			return t;
	return NULL;
}

static void recent_table_flush(struct recent_table *t)
{
	struct recent_entry *e, *next;
	unsigned int i;

	for (i = 0; i < ip_list_hash_size; i++)
		list_for_each_entry_safe(e, next, &t->iphash[i], list)
			recent_entry_remove(t, e);
}

static bool
recent_mt(const struct sk_buff *skb, struct xt_action_param *par)
{
	struct net *net = xt_net(par);
	struct recent_net *recent_net = recent_pernet(net);
	const struct xt_recent_mtinfo_v1 *info = par->matchinfo;
	struct recent_table *t;
	struct recent_entry *e;
	union nf_inet_addr addr = {}, addr_mask;
	u_int8_t ttl;
	bool ret = info->invert;

	if (xt_family(par) == NFPROTO_IPV4) {
		const struct iphdr *iph = ip_hdr(skb);

		if (info->side == XT_RECENT_DEST)
			addr.ip = iph->daddr;
		else
			addr.ip = iph->saddr;

		ttl = iph->ttl;
	} else {
		const struct ipv6hdr *iph = ipv6_hdr(skb);

		if (info->side == XT_RECENT_DEST)
			memcpy(&addr.in6, &iph->daddr, sizeof(addr.in6));
		else
			memcpy(&addr.in6, &iph->saddr, sizeof(addr.in6));

		ttl = iph->hop_limit;
	}

	/* use TTL as seen before forwarding */
	if (xt_out(par) != NULL &&
	    (!skb->sk || !net_eq(net, sock_net(skb->sk))))
		ttl++;

	spin_lock_bh(&recent_lock);
	t = recent_table_lookup(recent_net, info->name);

	nf_inet_addr_mask(&addr, &addr_mask, &t->mask);

	e = recent_entry_lookup(t, &addr_mask, xt_family(par),
				(info->check_set & XT_RECENT_TTL) ? ttl : 0);
	if (e == NULL) {
		if (!(info->check_set & XT_RECENT_SET))
			goto out;
		e = recent_entry_init(t, &addr_mask, xt_family(par), ttl);
		if (e == NULL)
			par->hotdrop = true;
		ret = !ret;
		goto out;
	}

	if (info->check_set & XT_RECENT_SET)
		ret = !ret;
	else if (info->check_set & XT_RECENT_REMOVE) {
		recent_entry_remove(t, e);
		ret = !ret;
	} else if (info->check_set & (XT_RECENT_CHECK | XT_RECENT_UPDATE)) {
		unsigned long time = jiffies - info->seconds * HZ;
		unsigned int i, hits = 0;

		for (i = 0; i < e->nstamps; i++) {
			if (info->seconds && time_after(time, e->stamps[i]))
				continue;
			if (!info->hit_count || ++hits >= info->hit_count) {
				ret = !ret;
				break;
			}
		}

		/* info->seconds must be non-zero */
		if (info->check_set & XT_RECENT_REAP)
			recent_entry_reap(t, time, e,
				info->check_set & XT_RECENT_UPDATE && ret);
	}

	if (info->check_set & XT_RECENT_SET ||
	    (info->check_set & XT_RECENT_UPDATE && ret)) {
		recent_entry_update(t, e);
		e->ttl = ttl;
	}
out:
	spin_unlock_bh(&recent_lock);
	return ret;
}

static void recent_table_free(void *addr)
{
	kvfree(addr);
}

static int recent_mt_check(const struct xt_mtchk_param *par,
			   const struct xt_recent_mtinfo_v1 *info)
{
	struct recent_net *recent_net = recent_pernet(par->net);
	struct recent_table *t;
#ifdef CONFIG_PROC_FS
	struct proc_dir_entry *pde;
	kuid_t uid;
	kgid_t gid;
#endif
	unsigned int nstamp_mask;
	unsigned int i;
	int ret = -EINVAL;

	net_get_random_once(&hash_rnd, sizeof(hash_rnd));

	if (info->check_set & ~XT_RECENT_VALID_FLAGS) {
		pr_info_ratelimited("Unsupported userspace flags (%08x)\n",
				    info->check_set);
		return -EINVAL;
	}
	if (hweight8(info->check_set &
		     (XT_RECENT_SET | XT_RECENT_REMOVE |
		      XT_RECENT_CHECK | XT_RECENT_UPDATE)) != 1)
		return -EINVAL;
	if ((info->check_set & (XT_RECENT_SET | XT_RECENT_REMOVE)) &&
	    (info->seconds || info->hit_count ||
	    (info->check_set & XT_RECENT_MODIFIERS)))
		return -EINVAL;
	if ((info->check_set & XT_RECENT_REAP) && !info->seconds)
		return -EINVAL;
	if (info->hit_count >= XT_RECENT_MAX_NSTAMPS) {
		pr_info_ratelimited("hitcount (%u) is larger than allowed maximum (%u)\n",
				    info->hit_count, XT_RECENT_MAX_NSTAMPS - 1);
		return -EINVAL;
	}
	ret = xt_check_proc_name(info->name, sizeof(info->name));
	if (ret)
		return ret;

	if (ip_pkt_list_tot && info->hit_count < ip_pkt_list_tot)
		nstamp_mask = roundup_pow_of_two(ip_pkt_list_tot) - 1;
	else if (info->hit_count)
		nstamp_mask = roundup_pow_of_two(info->hit_count) - 1;
	else
		nstamp_mask = 32 - 1;

	mutex_lock(&recent_mutex);
	t = recent_table_lookup(recent_net, info->name);
	if (t != NULL) {
		if (nstamp_mask > t->nstamps_max_mask) {
			spin_lock_bh(&recent_lock);
			recent_table_flush(t);
			t->nstamps_max_mask = nstamp_mask;
			spin_unlock_bh(&recent_lock);
		}

		t->refcnt++;
		ret = 0;
		goto out;
	}

	t = kvzalloc(struct_size(t, iphash, ip_list_hash_size), GFP_KERNEL);
	if (t == NULL) {
		ret = -ENOMEM;
		goto out;
	}
	t->refcnt = 1;
	t->nstamps_max_mask = nstamp_mask;

	memcpy(&t->mask, &info->mask, sizeof(t->mask));
	strcpy(t->name, info->name);
	INIT_LIST_HEAD(&t->lru_list);
	for (i = 0; i < ip_list_hash_size; i++)
		INIT_LIST_HEAD(&t->iphash[i]);
#ifdef CONFIG_PROC_FS
	uid = make_kuid(&init_user_ns, ip_list_uid);
	gid = make_kgid(&init_user_ns, ip_list_gid);
	if (!uid_valid(uid) || !gid_valid(gid)) {
		recent_table_free(t);
		ret = -EINVAL;
		goto out;
	}
	pde = proc_create_data(t->name, ip_list_perms, recent_net->xt_recent,
			       &recent_mt_proc_ops, t);
	if (pde == NULL) {
		recent_table_free(t);
		ret = -ENOMEM;
		goto out;
	}
	proc_set_user(pde, uid, gid);
#endif
	spin_lock_bh(&recent_lock);
	list_add_tail(&t->list, &recent_net->tables);
	spin_unlock_bh(&recent_lock);
	ret = 0;
out:
	mutex_unlock(&recent_mutex);
	return ret;
}

static int recent_mt_check_v0(const struct xt_mtchk_param *par)
{
	const struct xt_recent_mtinfo_v0 *info_v0 = par->matchinfo;
	struct xt_recent_mtinfo_v1 info_v1;

	/* Copy revision 0 structure to revision 1 */
	memcpy(&info_v1, info_v0, sizeof(struct xt_recent_mtinfo));
	/* Set default mask to ensure backward compatible behaviour */
	memset(info_v1.mask.all, 0xFF, sizeof(info_v1.mask.all));

	return recent_mt_check(par, &info_v1);
}

static int recent_mt_check_v1(const struct xt_mtchk_param *par)
{
	return recent_mt_check(par, par->matchinfo);
}

static void recent_mt_destroy(const struct xt_mtdtor_param *par)
{
	struct recent_net *recent_net = recent_pernet(par->net);
	const struct xt_recent_mtinfo_v1 *info = par->matchinfo;
	struct recent_table *t;

	mutex_lock(&recent_mutex);
	t = recent_table_lookup(recent_net, info->name);
	if (--t->refcnt == 0) {
		spin_lock_bh(&recent_lock);
		list_del(&t->list);
		spin_unlock_bh(&recent_lock);
#ifdef CONFIG_PROC_FS
		if (recent_net->xt_recent != NULL)
			remove_proc_entry(t->name, recent_net->xt_recent);
#endif
		recent_table_flush(t);
		recent_table_free(t);
	}
	mutex_unlock(&recent_mutex);
}

#ifdef CONFIG_PROC_FS
struct recent_iter_state {
	const struct recent_table *table;
	unsigned int		bucket;
};

static void *recent_seq_start(struct seq_file *seq, loff_t *pos)
	__acquires(recent_lock)
{
	struct recent_iter_state *st = seq->private;
	const struct recent_table *t = st->table;
	struct recent_entry *e;
	loff_t p = *pos;

	spin_lock_bh(&recent_lock);

	for (st->bucket = 0; st->bucket < ip_list_hash_size; st->bucket++)
		list_for_each_entry(e, &t->iphash[st->bucket], list)
			if (p-- == 0)
				return e;
	return NULL;
}

static void *recent_seq_next(struct seq_file *seq, void *v, loff_t *pos)
{
	struct recent_iter_state *st = seq->private;
	const struct recent_table *t = st->table;
	const struct recent_entry *e = v;
	const struct list_head *head = e->list.next;

	(*pos)++;
	while (head == &t->iphash[st->bucket]) {
		if (++st->bucket >= ip_list_hash_size)
			return NULL;
		head = t->iphash[st->bucket].next;
	}
	return list_entry(head, struct recent_entry, list);
}

static void recent_seq_stop(struct seq_file *s, void *v)
	__releases(recent_lock)
{
	spin_unlock_bh(&recent_lock);
}

static int recent_seq_show(struct seq_file *seq, void *v)
{
	const struct recent_entry *e = v;
	struct recent_iter_state *st = seq->private;
	const struct recent_table *t = st->table;
	unsigned int i;

	i = (e->index - 1) & t->nstamps_max_mask;

	if (e->family == NFPROTO_IPV4)
		seq_printf(seq, "src=%pI4 ttl: %u last_seen: %lu oldest_pkt: %u",
			   &e->addr.ip, e->ttl, e->stamps[i], e->index);
	else
		seq_printf(seq, "src=%pI6 ttl: %u last_seen: %lu oldest_pkt: %u",
			   &e->addr.in6, e->ttl, e->stamps[i], e->index);
	for (i = 0; i < e->nstamps; i++)
		seq_printf(seq, "%s %lu", i ? "," : "", e->stamps[i]);
	seq_putc(seq, '\n');
	return 0;
}

static const struct seq_operations recent_seq_ops = {
	.start		= recent_seq_start,
	.next		= recent_seq_next,
	.stop		= recent_seq_stop,
	.show		= recent_seq_show,
};

static int recent_seq_open(struct inode *inode, struct file *file)
{
	struct recent_iter_state *st;

	st = __seq_open_private(file, &recent_seq_ops, sizeof(*st));
	if (st == NULL)
		return -ENOMEM;

	st->table    = PDE_DATA(inode);
	return 0;
}

static ssize_t
recent_mt_proc_write(struct file *file, const char __user *input,
		     size_t size, loff_t *loff)
{
	struct recent_table *t = PDE_DATA(file_inode(file));
	struct recent_entry *e;
	char buf[sizeof("+b335:1d35:1e55:dead:c0de:1715:5afe:c0de")];
	const char *c = buf;
	union nf_inet_addr addr = {};
	u_int16_t family;
	bool add, succ;

	if (size == 0)
		return 0;
	if (size > sizeof(buf))
		size = sizeof(buf);
	if (copy_from_user(buf, input, size) != 0)
		return -EFAULT;

	/* Strict protocol! */
	if (*loff != 0)
		return -ESPIPE;
	switch (*c) {
	case '/': /* flush table */
		spin_lock_bh(&recent_lock);
		recent_table_flush(t);
		spin_unlock_bh(&recent_lock);
		return size;
	case '-': /* remove address */
		add = false;
		break;
	case '+': /* add address */
		add = true;
		break;
	default:
		pr_info_ratelimited("Need \"+ip\", \"-ip\" or \"/\"\n");
		return -EINVAL;
	}

	++c;
	--size;
	if (strnchr(c, size, ':') != NULL) {
		family = NFPROTO_IPV6;
		succ   = in6_pton(c, size, (void *)&addr, '\n', NULL);
	} else {
		family = NFPROTO_IPV4;
		succ   = in4_pton(c, size, (void *)&addr, '\n', NULL);
	}

	if (!succ)
		return -EINVAL;

	spin_lock_bh(&recent_lock);
	e = recent_entry_lookup(t, &addr, family, 0);
	if (e == NULL) {
		if (add)
			recent_entry_init(t, &addr, family, 0);
	} else {
		if (add)
			recent_entry_update(t, e);
		else
			recent_entry_remove(t, e);
	}
	spin_unlock_bh(&recent_lock);
	/* Note we removed one above */
	*loff += size + 1;
	return size + 1;
}

static const struct proc_ops recent_mt_proc_ops = {
	.proc_open	= recent_seq_open,
	.proc_read	= seq_read,
	.proc_write	= recent_mt_proc_write,
	.proc_release	= seq_release_private,
	.proc_lseek	= seq_lseek,
};

static int __net_init recent_proc_net_init(struct net *net)
{
	struct recent_net *recent_net = recent_pernet(net);

	recent_net->xt_recent = proc_mkdir("xt_recent", net->proc_net);
	if (!recent_net->xt_recent)
		return -ENOMEM;
	return 0;
}

static void __net_exit recent_proc_net_exit(struct net *net)
{
	struct recent_net *recent_net = recent_pernet(net);
	struct recent_table *t;

	/* recent_net_exit() is called before recent_mt_destroy(). Make sure
	 * that the parent xt_recent proc entry is empty before trying to
	 * remove it.
	 */
	spin_lock_bh(&recent_lock);
	list_for_each_entry(t, &recent_net->tables, list)
	        remove_proc_entry(t->name, recent_net->xt_recent);

	recent_net->xt_recent = NULL;
	spin_unlock_bh(&recent_lock);

	remove_proc_entry("xt_recent", net->proc_net);
}
#else
static inline int recent_proc_net_init(struct net *net)
{
	return 0;
}

static inline void recent_proc_net_exit(struct net *net)
{
}
#endif /* CONFIG_PROC_FS */

static int __net_init recent_net_init(struct net *net)
{
	struct recent_net *recent_net = recent_pernet(net);

	INIT_LIST_HEAD(&recent_net->tables);
	return recent_proc_net_init(net);
}

static void __net_exit recent_net_exit(struct net *net)
{
	recent_proc_net_exit(net);
}

static struct pernet_operations recent_net_ops = {
	.init	= recent_net_init,
	.exit	= recent_net_exit,
	.id	= &recent_net_id,
	.size	= sizeof(struct recent_net),
};

static struct xt_match recent_mt_reg[] __read_mostly = {
	{
		.name       = "recent",
		.revision   = 0,
		.family     = NFPROTO_IPV4,
		.match      = recent_mt,
		.matchsize  = sizeof(struct xt_recent_mtinfo),
		.checkentry = recent_mt_check_v0,
		.destroy    = recent_mt_destroy,
		.me         = THIS_MODULE,
	},
	{
		.name       = "recent",
		.revision   = 0,
		.family     = NFPROTO_IPV6,
		.match      = recent_mt,
		.matchsize  = sizeof(struct xt_recent_mtinfo),
		.checkentry = recent_mt_check_v0,
		.destroy    = recent_mt_destroy,
		.me         = THIS_MODULE,
	},
	{
		.name       = "recent",
		.revision   = 1,
		.family     = NFPROTO_IPV4,
		.match      = recent_mt,
		.matchsize  = sizeof(struct xt_recent_mtinfo_v1),
		.checkentry = recent_mt_check_v1,
		.destroy    = recent_mt_destroy,
		.me         = THIS_MODULE,
	},
	{
		.name       = "recent",
		.revision   = 1,
		.family     = NFPROTO_IPV6,
		.match      = recent_mt,
		.matchsize  = sizeof(struct xt_recent_mtinfo_v1),
		.checkentry = recent_mt_check_v1,
		.destroy    = recent_mt_destroy,
		.me         = THIS_MODULE,
	}
};

static int __init recent_mt_init(void)
{
	int err;

	BUILD_BUG_ON_NOT_POWER_OF_2(XT_RECENT_MAX_NSTAMPS);

	if (!ip_list_tot || ip_pkt_list_tot >= XT_RECENT_MAX_NSTAMPS)
		return -EINVAL;
	ip_list_hash_size = 1 << fls(ip_list_tot);

	err = register_pernet_subsys(&recent_net_ops);
	if (err)
		return err;
	err = xt_register_matches(recent_mt_reg, ARRAY_SIZE(recent_mt_reg));
	if (err)
		unregister_pernet_subsys(&recent_net_ops);
	return err;
}

static void __exit recent_mt_exit(void)
{
	xt_unregister_matches(recent_mt_reg, ARRAY_SIZE(recent_mt_reg));
	unregister_pernet_subsys(&recent_net_ops);
}

module_init(recent_mt_init);
module_exit