/*
   *		INETPEER - A storage for permanent information about peers
   *
   *  This source is covered by the GNU GPL, the same as all kernel sources.
   *
   *  Authors:	Andrey V. Savochkin <saw@msu.ru>
   */
  
  #include <linux/module.h>
  #include <linux/types.h>
  #include <linux/slab.h>
  #include <linux/interrupt.h>
  #include <linux/spinlock.h>
  #include <linux/random.h>
  #include <linux/timer.h>
  #include <linux/time.h>
  #include <linux/kernel.h>
  #include <linux/mm.h>
  #include <linux/net.h>
  #include <linux/workqueue.h>
  #include <net/ip.h>
  #include <net/inetpeer.h>
  #include <net/secure_seq.h>
  
  /*
   *  Theory of operations.
   *  We keep one entry for each peer IP address.  The nodes contains long-living
   *  information about the peer which doesn't depend on routes.
   *
   *  Nodes are removed only when reference counter goes to 0.
   *  When it's happened the node may be removed when a sufficient amount of
   *  time has been passed since its last use.  The less-recently-used entry can
   *  also be removed if the pool is overloaded i.e. if the total amount of
   *  entries is greater-or-equal than the threshold.
   *
   *  Node pool is organised as an AVL tree.
   *  Such an implementation has been chosen not just for fun.  It's a way to
   *  prevent easy and efficient DoS attacks by creating hash collisions.  A huge
   *  amount of long living nodes in a single hash slot would significantly delay
   *  lookups performed with disabled BHs.
   *
   *  Serialisation issues.
   *  1.  Nodes may appear in the tree only with the pool lock held.
   *  2.  Nodes may disappear from the tree only with the pool lock held
   *      AND reference count being 0.
   *  3.  Global variable peer_total is modified under the pool lock.
   *  4.  struct inet_peer fields modification:
   *		avl_left, avl_right, avl_parent, avl_height: pool lock
   *		refcnt: atomically against modifications on other CPU;
   *		   usually under some other lock to prevent node disappearing
   *		daddr: unchangeable
   */
  
  static struct kmem_cache *peer_cachep __read_mostly;
  
  static LIST_HEAD(gc_list);
  static const int gc_delay = 60 * HZ;
  static struct delayed_work gc_work;
  static DEFINE_SPINLOCK(gc_lock);
  
  #define node_height(x) x->avl_height
  
  #define peer_avl_empty ((struct inet_peer *)&peer_fake_node)
  #define peer_avl_empty_rcu ((struct inet_peer __rcu __force *)&peer_fake_node)
  static const struct inet_peer peer_fake_node = {
  	.avl_left	= peer_avl_empty_rcu,
  	.avl_right	= peer_avl_empty_rcu,
  	.avl_height	= 0
  };
  
  void inet_peer_base_init(struct inet_peer_base *bp)
  {
  	bp->root = peer_avl_empty_rcu;
  	seqlock_init(&bp->lock);
  	bp->total = 0;
  }
  EXPORT_SYMBOL_GPL(inet_peer_base_init);
  
  #define PEER_MAXDEPTH 40 /* sufficient for about 2^27 nodes */
  
  /* Exported for sysctl_net_ipv4.  */
  int inet_peer_threshold __read_mostly = 65536 + 128;	/* start to throw entries more
  					 * aggressively at this stage */
  int inet_peer_minttl __read_mostly = 120 * HZ;	/* TTL under high load: 120 sec */
  int inet_peer_maxttl __read_mostly = 10 * 60 * HZ;	/* usual time to live: 10 min */
  
  static void inetpeer_gc_worker(struct work_struct *work)
  {
  	struct inet_peer *p, *n, *c;
  	struct list_head list;
  
  	spin_lock_bh(&gc_lock);
  	list_replace_init(&gc_list, &list);
  	spin_unlock_bh(&gc_lock);
  
  	if (list_empty(&list))
  		return;
  
  	list_for_each_entry_safe(p, n, &list, gc_list) {
  
  		if (need_resched())
  			cond_resched();
  
  		c = rcu_dereference_protected(p->avl_left, 1);
  		if (c != peer_avl_empty) {
  			list_add_tail(&c->gc_list, &list);
  			p->avl_left = peer_avl_empty_rcu;
  		}
  
  		c = rcu_dereference_protected(p->avl_right, 1);
  		if (c != peer_avl_empty) {
  			list_add_tail(&c->gc_list, &list);
  			p->avl_right = peer_avl_empty_rcu;
  		}
  
  		n = list_entry(p->gc_list.next, struct inet_peer, gc_list);
  
  		if (!atomic_read(&p->refcnt)) {
  			list_del(&p->gc_list);
  			kmem_cache_free(peer_cachep, p);
  		}
  	}
  
  	if (list_empty(&list))
  		return;
  
  	spin_lock_bh(&gc_lock);
  	list_splice(&list, &gc_list);
  	spin_unlock_bh(&gc_lock);
  
  	schedule_delayed_work(&gc_work, gc_delay);
  }
  
  /* Called from ip_output.c:ip_init  */
  void __init inet_initpeers(void)
  {
  	struct sysinfo si;
  
  	/* Use the straight interface to information about memory. */
  	si_meminfo(&si);
  	/* The values below were suggested by Alexey Kuznetsov
  	 * <kuznet@ms2.inr.ac.ru>.  I don't have any opinion about the values
  	 * myself.  --SAW
  	 */
  	if (si.totalram <= (32768*1024)/PAGE_SIZE)
  		inet_peer_threshold >>= 1; /* max pool size about 1MB on IA32 */
  	if (si.totalram <= (16384*1024)/PAGE_SIZE)
  		inet_peer_threshold >>= 1; /* about 512KB */
  	if (si.totalram <= (8192*1024)/PAGE_SIZE)
  		inet_peer_threshold >>= 2; /* about 128KB */
  
  	peer_cachep = kmem_cache_create("inet_peer_cache",
  			sizeof(struct inet_peer),
  			0, SLAB_HWCACHE_ALIGN | SLAB_PANIC,
  			NULL);
  
  	INIT_DEFERRABLE_WORK(&gc_work, inetpeer_gc_worker);
  }
  
  #define rcu_deref_locked(X, BASE)				\
  	rcu_dereference_protected(X, lockdep_is_held(&(BASE)->lock.lock))
  
  /*
   * Called with local BH disabled and the pool lock held.
   */
  #define lookup(_daddr, _stack, _base)				\
  ({								\
  	struct inet_peer *u;					\
  	struct inet_peer __rcu **v;				\
  								\
  	stackptr = _stack;					\
  	*stackptr++ = &_base->root;				\
  	for (u = rcu_deref_locked(_base->root, _base);		\
  	     u != peer_avl_empty;) {				\
  		int cmp = inetpeer_addr_cmp(_daddr, &u->daddr);	\
  		if (cmp == 0)					\
  			break;					\
  		if (cmp == -1)					\
  			v = &u->avl_left;			\
  		else						\
  			v = &u->avl_right;			\
  		*stackptr++ = v;				\
  		u = rcu_deref_locked(*v, _base);		\
  	}							\
  	u;							\
  })
  
  /*
   * Called with rcu_read_lock()
   * Because we hold no lock against a writer, its quite possible we fall
   * in an endless loop.
   * But every pointer we follow is guaranteed to be valid thanks to RCU.
   * We exit from this function if number of links exceeds PEER_MAXDEPTH
   */
  static struct inet_peer *lookup_rcu(const struct inetpeer_addr *daddr,
  				    struct inet_peer_base *base)
  {
  	struct inet_peer *u = rcu_dereference(base->root);
  	int count = 0;
  
  	while (u != peer_avl_empty) {
  		int cmp = inetpeer_addr_cmp(daddr, &u->daddr);
  		if (cmp == 0) {
  			/* Before taking a reference, check if this entry was
  			 * deleted (refcnt=-1)
  			 */
  			if (!atomic_add_unless(&u->refcnt, 1, -1))
  				u = NULL;
  			return u;
  		}
  		if (cmp == -1)
  			u = rcu_dereference(u->avl_left);
  		else
  			u = rcu_dereference(u->avl_right);
  		if (unlikely(++count == PEER_MAXDEPTH))
  			break;
  	}
  	return NULL;
  }
  
  /* Called with local BH disabled and the pool lock held. */
  #define lookup_rightempty(start, base)				\
  ({								\
  	struct inet_peer *u;					\
  	struct inet_peer __rcu **v;				\
  	*stackptr++ = &start->avl_left;				\
  	v = &start->avl_left;					\
  	for (u = rcu_deref_locked(*v, base);			\
  	     u->avl_right != peer_avl_empty_rcu;) {		\
  		v = &u->avl_right;				\
  		*stackptr++ = v;				\
  		u = rcu_deref_locked(*v, base);			\
  	}							\
  	u;							\
  })
  
  /* Called with local BH disabled and the pool lock held.
   * Variable names are the proof of operation correctness.
   * Look into mm/map_avl.c for more detail description of the ideas.
   */
  static void peer_avl_rebalance(struct inet_peer __rcu **stack[],
  			       struct inet_peer __rcu ***stackend,
  			       struct inet_peer_base *base)
  {
  	struct inet_peer __rcu **nodep;
  	struct inet_peer *node, *l, *r;
  	int lh, rh;
  
  	while (stackend > stack) {
  		nodep = *--stackend;
  		node = rcu_deref_locked(*nodep, base);
  		l = rcu_deref_locked(node->avl_left, base);
  		r = rcu_deref_locked(node->avl_right, base);
  		lh = node_height(l);
  		rh = node_height(r);
  		if (lh > rh + 1) { /* l: RH+2 */
  			struct inet_peer *ll, *lr, *lrl, *lrr;
  			int lrh;
  			ll = rcu_deref_locked(l->avl_left, base);
  			lr = rcu_deref_locked(l->avl_right, base);
  			lrh = node_height(lr);
  			if (lrh <= node_height(ll)) {	/* ll: RH+1 */
  				RCU_INIT_POINTER(node->avl_left, lr);	/* lr: RH or RH+1 */
  				RCU_INIT_POINTER(node->avl_right, r);	/* r: RH */
  				node->avl_height = lrh + 1; /* RH+1 or RH+2 */
  				RCU_INIT_POINTER(l->avl_left, ll);       /* ll: RH+1 */
  				RCU_INIT_POINTER(l->avl_right, node);	/* node: RH+1 or RH+2 */
  				l->avl_height = node->avl_height + 1;
  				RCU_INIT_POINTER(*nodep, l);
  			} else { /* ll: RH, lr: RH+1 */
  				lrl = rcu_deref_locked(lr->avl_left, base);/* lrl: RH or RH-1 */
  				lrr = rcu_deref_locked(lr->avl_right, base);/* lrr: RH or RH-1 */
  				RCU_INIT_POINTER(node->avl_left, lrr);	/* lrr: RH or RH-1 */
  				RCU_INIT_POINTER(node->avl_right, r);	/* r: RH */
  				node->avl_height = rh + 1; /* node: RH+1 */
  				RCU_INIT_POINTER(l->avl_left, ll);	/* ll: RH */
  				RCU_INIT_POINTER(l->avl_right, lrl);	/* lrl: RH or RH-1 */
  				l->avl_height = rh + 1;	/* l: RH+1 */
  				RCU_INIT_POINTER(lr->avl_left, l);	/* l: RH+1 */
  				RCU_INIT_POINTER(lr->avl_right, node);	/* node: RH+1 */
  				lr->avl_height = rh + 2;
  				RCU_INIT_POINTER(*nodep, lr);
  			}
  		} else if (rh > lh + 1) { /* r: LH+2 */
  			struct inet_peer *rr, *rl, *rlr, *rll;
  			int rlh;
  			rr = rcu_deref_locked(r->avl_right, base);
  			rl = rcu_deref_locked(r->avl_left, base);
  			rlh = node_height(rl);
  			if (rlh <= node_height(rr)) {	/* rr: LH+1 */
  				RCU_INIT_POINTER(node->avl_right, rl);	/* rl: LH or LH+1 */
  				RCU_INIT_POINTER(node->avl_left, l);	/* l: LH */
  				node->avl_height = rlh + 1; /* LH+1 or LH+2 */
  				RCU_INIT_POINTER(r->avl_right, rr);	/* rr: LH+1 */
  				RCU_INIT_POINTER(r->avl_left, node);	/* node: LH+1 or LH+2 */
  				r->avl_height = node->avl_height + 1;
  				RCU_INIT_POINTER(*nodep, r);
  			} else { /* rr: RH, rl: RH+1 */
  				rlr = rcu_deref_locked(rl->avl_right, base);/* rlr: LH or LH-1 */
  				rll = rcu_deref_locked(rl->avl_left, base);/* rll: LH or LH-1 */
  				RCU_INIT_POINTER(node->avl_right, rll);	/* rll: LH or LH-1 */
  				RCU_INIT_POINTER(node->avl_left, l);	/* l: LH */
  				node->avl_height = lh + 1; /* node: LH+1 */
  				RCU_INIT_POINTER(r->avl_right, rr);	/* rr: LH */
  				RCU_INIT_POINTER(r->avl_left, rlr);	/* rlr: LH or LH-1 */
  				r->avl_height = lh + 1;	/* r: LH+1 */
  				RCU_INIT_POINTER(rl->avl_right, r);	/* r: LH+1 */
  				RCU_INIT_POINTER(rl->avl_left, node);	/* node: LH+1 */
  				rl->avl_height = lh + 2;
  				RCU_INIT_POINTER(*nodep, rl);
  			}
  		} else {
  			node->avl_height = (lh > rh ? lh : rh) + 1;
  		}
  	}
  }
  
  /* Called with local BH disabled and the pool lock held. */
  #define link_to_pool(n, base)					\
  do {								\
  	n->avl_height = 1;					\
  	n->avl_left = peer_avl_empty_rcu;			\
  	n->avl_right = peer_avl_empty_rcu;			\
  	/* lockless readers can catch us now */			\
  	rcu_assign_pointer(**--stackptr, n);			\
  	peer_avl_rebalance(stack, stackptr, base);		\
  } while (0)
  
  static void inetpeer_free_rcu(struct rcu_head *head)
  {
  	kmem_cache_free(peer_cachep, container_of(head, struct inet_peer, rcu));
  }
  
  static void unlink_from_pool(struct inet_peer *p, struct inet_peer_base *base,
  			     struct inet_peer __rcu **stack[PEER_MAXDEPTH])
  {
  	struct inet_peer __rcu ***stackptr, ***delp;
  
  	if (lookup(&p->daddr, stack, base) != p)
  		BUG();
  	delp = stackptr - 1; /* *delp[0] == p */
  	if (p->avl_left == peer_avl_empty_rcu) {
  		*delp[0] = p->avl_right;
  		--stackptr;
  	} else {
  		/* look for a node to insert instead of p */
  		struct inet_peer *t;
  		t = lookup_rightempty(p, base);
  		BUG_ON(rcu_deref_locked(*stackptr[-1], base) != t);
  		**--stackptr = t->avl_left;
  		/* t is removed, t->daddr > x->daddr for any
  		 * x in p->avl_left subtree.
  		 * Put t in the old place of p. */
  		RCU_INIT_POINTER(*delp[0], t);
  		t->avl_left = p->avl_left;
  		t->avl_right = p->avl_right;
  		t->avl_height = p->avl_height;
  		BUG_ON(delp[1] != &p->avl_left);
  		delp[1] = &t->avl_left; /* was &p->avl_left */
  	}
  	peer_avl_rebalance(stack, stackptr, base);
  	base->total--;
  	call_rcu(&p->rcu, inetpeer_free_rcu);
  }
  
  /* perform garbage collect on all items stacked during a lookup */
  static int inet_peer_gc(struct inet_peer_base *base,
  			struct inet_peer __rcu **stack[PEER_MAXDEPTH],
  			struct inet_peer __rcu ***stackptr)
  {
  	struct inet_peer *p, *gchead = NULL;
  	__u32 delta, ttl;
  	int cnt = 0;
  
  	if (base->total >= inet_peer_threshold)
  		ttl = 0; /* be aggressive */
  	else
  		ttl = inet_peer_maxttl
  				- (inet_peer_maxttl - inet_peer_minttl) / HZ *
  					base->total / inet_peer_threshold * HZ;
  	stackptr--; /* last stack slot is peer_avl_empty */
  	while (stackptr > stack) {
  		stackptr--;
  		p = rcu_deref_locked(**stackptr, base);
  		if (atomic_read(&p->refcnt) == 0) {
  			smp_rmb();
  			delta = (__u32)jiffies - p->dtime;
  			if (delta >= ttl &&
  			    atomic_cmpxchg(&p->refcnt, 0, -1) == 0) {
  				p->gc_next = gchead;
  				gchead = p;
  			}
  		}
  	}
  	while ((p = gchead) != NULL) {
  		gchead = p->gc_next;
  		cnt++;
  		unlink_from_pool(p, base, stack);
  	}
  	return cnt;
  }
  
  struct inet_peer *inet_getpeer(struct inet_peer_base *base,
  			       const struct inetpeer_addr *daddr,
  			       int create)
  {
  	struct inet_peer __rcu **stack[PEER_MAXDEPTH], ***stackptr;
  	struct inet_peer *p;
  	unsigned int sequence;
  	int invalidated, gccnt = 0;
  
  	/* Attempt a lockless lookup first.
  	 * Because of a concurrent writer, we might not find an existing entry.
  	 */
  	rcu_read_lock();
  	sequence = read_seqbegin(&base->lock);
  	p = lookup_rcu(daddr, base);
  	invalidated = read_seqretry(&base->lock, sequence);
  	rcu_read_unlock();
  
  	if (p)
  		return p;
  
  	/* If no writer did a change during our lookup, we can return early. */
  	if (!create && !invalidated)
  		return NULL;
  
  	/* retry an exact lookup, taking the lock before.
  	 * At least, nodes should be hot in our cache.
  	 */
  	write_seqlock_bh(&base->lock);
  relookup:
  	p = lookup(daddr, stack, base);
  	if (p != peer_avl_empty) {
  		atomic_inc(&p->refcnt);
  		write_sequnlock_bh(&base->lock);
  		return p;
  	}
  	if (!gccnt) {
  		gccnt = inet_peer_gc(base, stack, stackptr);
  		if (gccnt && create)
  			goto relookup;
  	}
  	p = create ? kmem_cache_alloc(peer_cachep, GFP_ATOMIC) : NULL;
  	if (p) {
  		p->daddr = *daddr;
  		atomic_set(&p->refcnt, 1);
  		atomic_set(&p->rid, 0);
  		p->metrics[RTAX_LOCK-1] = INETPEER_METRICS_NEW;
  		p->rate_tokens = 0;
  		/* 60*HZ is arbitrary, but chosen enough high so that the first
  		 * calculation of tokens is at its maximum.
  		 */
  		p->rate_last = jiffies - 60*HZ;
  		INIT_LIST_HEAD(&p->gc_list);
  
  		/* Link the node. */
  		link_to_pool(p, base);
  		base->total++;
  	}
  	write_sequnlock_bh(&base->lock);
  
  	return p;
  }
  EXPORT_SYMBOL_GPL(inet_getpeer);
  
  void inet_putpeer(struct inet_peer *p)
  {
  	p->dtime = (__u32)jiffies;
  	smp_mb__before_atomic();
  	atomic_dec(&p->refcnt);
  }
  EXPORT_SYMBOL_GPL(inet_putpeer);
  
  /*
   *	Check transmit rate limitation for given message.
   *	The rate information is held in the inet_peer entries now.
   *	This function is generic and could be used for other purposes
   *	too. It uses a Token bucket filter as suggested by Alexey Kuznetsov.
   *
   *	Note that the same inet_peer fields are modified by functions in
   *	route.c too, but these work for packet destinations while xrlim_allow
   *	works for icmp destinations. This means the rate limiting information
   *	for one "ip object" is shared - and these ICMPs are twice limited:
   *	by source and by destination.
   *
   *	RFC 1812: 4.3.2.8 SHOULD be able to limit error message rate
   *			  SHOULD allow setting of rate limits
   *
   * 	Shared between ICMPv4 and ICMPv6.
   */
  #define XRLIM_BURST_FACTOR 6
  bool inet_peer_xrlim_allow(struct inet_peer *peer, int timeout)
  {
  	unsigned long now, token;
  	bool rc = false;
  
  	if (!peer)
  		return true;
  
  	token = peer->rate_tokens;
  	now = jiffies;
  	token += now - peer->rate_last;
  	peer->rate_last = now;
  	if (token > XRLIM_BURST_FACTOR * timeout)
  		token = XRLIM_BURST_FACTOR * timeout;
  	if (token >= timeout) {
  		token -= timeout;
  		rc = true;
  	}
  	peer->rate_tokens = token;
  	return rc;
  }
  EXPORT_SYMBOL(inet_peer_xrlim_allow);
  
  static void inetpeer_inval_rcu(struct rcu_head *head)
  {
  	struct inet_peer *p = container_of(head, struct inet_peer, gc_rcu);
  
  	spin_lock_bh(&gc_lock);
  	list_add_tail(&p->gc_list, &gc_list);
  	spin_unlock_bh(&gc_lock);
  
  	schedule_delayed_work(&gc_work, gc_delay);
  }
  
  void inetpeer_invalidate_tree(struct inet_peer_base *base)
  {
  	struct inet_peer *root;
  
  	write_seqlock_bh(&base->lock);
  
  	root = rcu_deref_locked(base->root, base);
  	if (root != peer_avl_empty) {
  		base->root = peer_avl_empty_rcu;
  		base->total = 0;
  		call_rcu(&root->gc_rcu, inetpeer_inval_rcu);
  	}
  
  	write_sequnlock_bh(&base->lock);
  }
  EXPORT_SYMBOL(inetpeer_invalidate_tree);