/*
   * Plugable TCP congestion control support and newReno
   * congestion control.
   * Based on ideas from I/O scheduler support and Web100.
   *
   * Copyright (C) 2005 Stephen Hemminger <shemminger@osdl.org>
   */
  
  #define pr_fmt(fmt) "TCP: " fmt
  
  #include <linux/module.h>
  #include <linux/mm.h>
  #include <linux/types.h>
  #include <linux/list.h>
  #include <linux/gfp.h>
  #include <net/tcp.h>
  
  static DEFINE_SPINLOCK(tcp_cong_list_lock);
  static LIST_HEAD(tcp_cong_list);
  
  /* Simple linear search, don't expect many entries! */
  static struct tcp_congestion_ops *tcp_ca_find(const char *name)
  {
  	struct tcp_congestion_ops *e;
  
  	list_for_each_entry_rcu(e, &tcp_cong_list, list) {
  		if (strcmp(e->name, name) == 0)
  			return e;
  	}
  
  	return NULL;
  }
  
  /*
   * Attach new congestion control algorithm to the list
   * of available options.
   */
  int tcp_register_congestion_control(struct tcp_congestion_ops *ca)
  {
  	int ret = 0;
  
  	/* all algorithms must implement ssthresh and cong_avoid ops */
  	if (!ca->ssthresh || !ca->cong_avoid) {
  		pr_err("%s does not implement required ops
  ", ca->name);
  		return -EINVAL;
  	}
  
  	spin_lock(&tcp_cong_list_lock);
  	if (tcp_ca_find(ca->name)) {
  		pr_notice("%s already registered
  ", ca->name);
  		ret = -EEXIST;
  	} else {
  		list_add_tail_rcu(&ca->list, &tcp_cong_list);
  		pr_info("%s registered
  ", ca->name);
  	}
  	spin_unlock(&tcp_cong_list_lock);
  
  	return ret;
  }
  EXPORT_SYMBOL_GPL(tcp_register_congestion_control);
  
  /*
   * Remove congestion control algorithm, called from
   * the module's remove function.  Module ref counts are used
   * to ensure that this can't be done till all sockets using
   * that method are closed.
   */
  void tcp_unregister_congestion_control(struct tcp_congestion_ops *ca)
  {
  	spin_lock(&tcp_cong_list_lock);
  	list_del_rcu(&ca->list);
  	spin_unlock(&tcp_cong_list_lock);
  }
  EXPORT_SYMBOL_GPL(tcp_unregister_congestion_control);
  
  /* Assign choice of congestion control. */
  void tcp_init_congestion_control(struct sock *sk)
  {
  	struct inet_connection_sock *icsk = inet_csk(sk);
  	struct tcp_congestion_ops *ca;
  
  	/* if no choice made yet assign the current value set as default */
  	if (icsk->icsk_ca_ops == &tcp_init_congestion_ops) {
  		rcu_read_lock();
  		list_for_each_entry_rcu(ca, &tcp_cong_list, list) {
  			if (try_module_get(ca->owner)) {
  				icsk->icsk_ca_ops = ca;
  				break;
  			}
  
  			/* fallback to next available */
  		}
  		rcu_read_unlock();
  	}
  
  	if (icsk->icsk_ca_ops->init)
  		icsk->icsk_ca_ops->init(sk);
  }
  
  /* Manage refcounts on socket close. */
  void tcp_cleanup_congestion_control(struct sock *sk)
  {
  	struct inet_connection_sock *icsk = inet_csk(sk);
  
  	if (icsk->icsk_ca_ops->release)
  		icsk->icsk_ca_ops->release(sk);
  	module_put(icsk->icsk_ca_ops->owner);
  }
  
  /* Used by sysctl to change default congestion control */
  int tcp_set_default_congestion_control(const char *name)
  {
  	struct tcp_congestion_ops *ca;
  	int ret = -ENOENT;
  
  	spin_lock(&tcp_cong_list_lock);
  	ca = tcp_ca_find(name);
  #ifdef CONFIG_MODULES
  	if (!ca && capable(CAP_NET_ADMIN)) {
  		spin_unlock(&tcp_cong_list_lock);
  
  		request_module("tcp_%s", name);
  		spin_lock(&tcp_cong_list_lock);
  		ca = tcp_ca_find(name);
  	}
  #endif
  
  	if (ca) {
  		ca->flags |= TCP_CONG_NON_RESTRICTED;	/* default is always allowed */
  		list_move(&ca->list, &tcp_cong_list);
  		ret = 0;
  	}
  	spin_unlock(&tcp_cong_list_lock);
  
  	return ret;
  }
  
  /* Set default value from kernel configuration at bootup */
  static int __init tcp_congestion_default(void)
  {
  	return tcp_set_default_congestion_control(CONFIG_DEFAULT_TCP_CONG);
  }
  late_initcall(tcp_congestion_default);
  
  
  /* Build string with list of available congestion control values */
  void tcp_get_available_congestion_control(char *buf, size_t maxlen)
  {
  	struct tcp_congestion_ops *ca;
  	size_t offs = 0;
  
  	rcu_read_lock();
  	list_for_each_entry_rcu(ca, &tcp_cong_list, list) {
  		offs += snprintf(buf + offs, maxlen - offs,
  				 "%s%s",
  				 offs == 0 ? "" : " ", ca->name);
  
  	}
  	rcu_read_unlock();
  }
  
  /* Get current default congestion control */
  void tcp_get_default_congestion_control(char *name)
  {
  	struct tcp_congestion_ops *ca;
  	/* We will always have reno... */
  	BUG_ON(list_empty(&tcp_cong_list));
  
  	rcu_read_lock();
  	ca = list_entry(tcp_cong_list.next, struct tcp_congestion_ops, list);
  	strncpy(name, ca->name, TCP_CA_NAME_MAX);
  	rcu_read_unlock();
  }
  
  /* Built list of non-restricted congestion control values */
  void tcp_get_allowed_congestion_control(char *buf, size_t maxlen)
  {
  	struct tcp_congestion_ops *ca;
  	size_t offs = 0;
  
  	*buf = '\0';
  	rcu_read_lock();
  	list_for_each_entry_rcu(ca, &tcp_cong_list, list) {
  		if (!(ca->flags & TCP_CONG_NON_RESTRICTED))
  			continue;
  		offs += snprintf(buf + offs, maxlen - offs,
  				 "%s%s",
  				 offs == 0 ? "" : " ", ca->name);
  
  	}
  	rcu_read_unlock();
  }
  
  /* Change list of non-restricted congestion control */
  int tcp_set_allowed_congestion_control(char *val)
  {
  	struct tcp_congestion_ops *ca;
  	char *saved_clone, *clone, *name;
  	int ret = 0;
  
  	saved_clone = clone = kstrdup(val, GFP_USER);
  	if (!clone)
  		return -ENOMEM;
  
  	spin_lock(&tcp_cong_list_lock);
  	/* pass 1 check for bad entries */
  	while ((name = strsep(&clone, " ")) && *name) {
  		ca = tcp_ca_find(name);
  		if (!ca) {
  			ret = -ENOENT;
  			goto out;
  		}
  	}
  
  	/* pass 2 clear old values */
  	list_for_each_entry_rcu(ca, &tcp_cong_list, list)
  		ca->flags &= ~TCP_CONG_NON_RESTRICTED;
  
  	/* pass 3 mark as allowed */
  	while ((name = strsep(&val, " ")) && *name) {
  		ca = tcp_ca_find(name);
  		WARN_ON(!ca);
  		if (ca)
  			ca->flags |= TCP_CONG_NON_RESTRICTED;
  	}
  out:
  	spin_unlock(&tcp_cong_list_lock);
  	kfree(saved_clone);
  
  	return ret;
  }
  
  
  /* Change congestion control for socket */
  int tcp_set_congestion_control(struct sock *sk, const char *name)
  {
  	struct inet_connection_sock *icsk = inet_csk(sk);
  	struct tcp_congestion_ops *ca;
  	int err = 0;
  
  	rcu_read_lock();
  	ca = tcp_ca_find(name);
  
  	/* no change asking for existing value */
  	if (ca == icsk->icsk_ca_ops)
  		goto out;
  
  #ifdef CONFIG_MODULES
  	/* not found attempt to autoload module */
  	if (!ca && capable(CAP_NET_ADMIN)) {
  		rcu_read_unlock();
  		request_module("tcp_%s", name);
  		rcu_read_lock();
  		ca = tcp_ca_find(name);
  	}
  #endif
  	if (!ca)
  		err = -ENOENT;
  
  	else if (!((ca->flags & TCP_CONG_NON_RESTRICTED) ||
  		   ns_capable(sock_net(sk)->user_ns, CAP_NET_ADMIN)))
  		err = -EPERM;
  
  	else if (!try_module_get(ca->owner))
  		err = -EBUSY;
  
  	else {
  		tcp_cleanup_congestion_control(sk);
  		icsk->icsk_ca_ops = ca;
  
  		if (sk->sk_state != TCP_CLOSE && icsk->icsk_ca_ops->init)
  			icsk->icsk_ca_ops->init(sk);
  	}
   out:
  	rcu_read_unlock();
  	return err;
  }
  
  /* RFC2861 Check whether we are limited by application or congestion window
   * This is the inverse of cwnd check in tcp_tso_should_defer
   */
  bool tcp_is_cwnd_limited(const struct sock *sk, u32 in_flight)
  {
  	const struct tcp_sock *tp = tcp_sk(sk);
  	u32 left;
  
  	if (in_flight >= tp->snd_cwnd)
  		return true;
  
  	left = tp->snd_cwnd - in_flight;
  	if (sk_can_gso(sk) &&
  	    left * sysctl_tcp_tso_win_divisor < tp->snd_cwnd &&
  	    left < tp->xmit_size_goal_segs)
  		return true;
  	return left <= tcp_max_tso_deferred_mss(tp);
  }
  EXPORT_SYMBOL_GPL(tcp_is_cwnd_limited);
  
  /* Slow start is used when congestion window is no greater than the slow start
   * threshold. We base on RFC2581 and also handle stretch ACKs properly.
   * We do not implement RFC3465 Appropriate Byte Counting (ABC) per se but
   * something better;) a packet is only considered (s)acked in its entirety to
   * defend the ACK attacks described in the RFC. Slow start processes a stretch
   * ACK of degree N as if N acks of degree 1 are received back to back except
   * ABC caps N to 2. Slow start exits when cwnd grows over ssthresh and
   * returns the leftover acks to adjust cwnd in congestion avoidance mode.
   */
  int tcp_slow_start(struct tcp_sock *tp, u32 acked)
  {
  	u32 cwnd = tp->snd_cwnd + acked;
  
  	if (cwnd > tp->snd_ssthresh)
  		cwnd = tp->snd_ssthresh + 1;
  	acked -= cwnd - tp->snd_cwnd;
  	tp->snd_cwnd = min(cwnd, tp->snd_cwnd_clamp);
  	return acked;
  }
  EXPORT_SYMBOL_GPL(tcp_slow_start);
  
  /* In theory this is tp->snd_cwnd += 1 / tp->snd_cwnd (or alternative w) */
  void tcp_cong_avoid_ai(struct tcp_sock *tp, u32 w)
  {
  	if (tp->snd_cwnd_cnt >= w) {
  		if (tp->snd_cwnd < tp->snd_cwnd_clamp)
  			tp->snd_cwnd++;
  		tp->snd_cwnd_cnt = 0;
  	} else {
  		tp->snd_cwnd_cnt++;
  	}
  }
  EXPORT_SYMBOL_GPL(tcp_cong_avoid_ai);
  
  /*
   * TCP Reno congestion control
   * This is special case used for fallback as well.
   */
  /* This is Jacobson's slow start and congestion avoidance.
   * SIGCOMM '88, p. 328.
   */
  void tcp_reno_cong_avoid(struct sock *sk, u32 ack, u32 acked, u32 in_flight)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  
  	if (!tcp_is_cwnd_limited(sk, in_flight))
  		return;
  
  	/* In "safe" area, increase. */
  	if (tp->snd_cwnd <= tp->snd_ssthresh)
  		tcp_slow_start(tp, acked);
  	/* In dangerous area, increase slowly. */
  	else
  		tcp_cong_avoid_ai(tp, tp->snd_cwnd);
  }
  EXPORT_SYMBOL_GPL(tcp_reno_cong_avoid);
  
  /* Slow start threshold is half the congestion window (min 2) */
  u32 tcp_reno_ssthresh(struct sock *sk)
  {
  	const struct tcp_sock *tp = tcp_sk(sk);
  	return max(tp->snd_cwnd >> 1U, 2U);
  }
  EXPORT_SYMBOL_GPL(tcp_reno_ssthresh);
  
  /* Lower bound on congestion window with halving. */
  u32 tcp_reno_min_cwnd(const struct sock *sk)
  {
  	const struct tcp_sock *tp = tcp_sk(sk);
  	return tp->snd_ssthresh/2;
  }
  EXPORT_SYMBOL_GPL(tcp_reno_min_cwnd);
  
  struct tcp_congestion_ops tcp_reno = {
  	.flags		= TCP_CONG_NON_RESTRICTED,
  	.name		= "reno",
  	.owner		= THIS_MODULE,
  	.ssthresh	= tcp_reno_ssthresh,
  	.cong_avoid	= tcp_reno_cong_avoid,
  	.min_cwnd	= tcp_reno_min_cwnd,
  };
  
  /* Initial congestion control used (until SYN)
   * really reno under another name so we can tell difference
   * during tcp_set_default_congestion_control
   */
  struct tcp_congestion_ops tcp_init_congestion_ops  = {
  	.name		= "",
  	.owner		= THIS_MODULE,
  	.ssthresh	= tcp_reno_ssthresh,
  	.cong_avoid	= tcp_reno_cong_avoid,
  	.min_cwnd	= tcp_reno_min_cwnd,
  };
  EXPORT_SYMBOL_GPL(tcp_init_congestion_ops);