/*
   * NET		Generic infrastructure for Network protocols.
   *
   * Authors:	Arnaldo Carvalho de Melo <acme@conectiva.com.br>
   *
   * 		From code originally in include/net/tcp.h
   *
   *		This program is free software; you can redistribute it and/or
   *		modify it under the terms of the GNU General Public License
   *		as published by the Free Software Foundation; either version
   *		2 of the License, or (at your option) any later version.
   */
  
  #include <linux/module.h>
  #include <linux/random.h>
  #include <linux/slab.h>
  #include <linux/string.h>

  #include <linux/tcp.h>

  #include <linux/vmalloc.h>

  
  #include <net/request_sock.h>

  /*
   * Maximum number of SYN_RECV sockets in queue per LISTEN socket.
   * One SYN_RECV socket costs about 80bytes on a 32bit machine.
   * It would be better to replace it with a global counter for all sockets
   * but then some measure against one socket starving all other sockets
   * would be needed.
   *

   * The minimum value of it is 128. Experiments with real servers show that

   * it is absolutely not enough even at 100conn/sec. 256 cures most

   * of problems.
   * This value is adjusted to 128 for low memory machines,
   * and it will increase in proportion to the memory of machine.

   * Note : Dont forget somaxconn that may limit backlog too.

   */

  void reqsk_queue_alloc(struct request_sock_queue *queue)

  {

  	spin_lock_init(&queue->rskq_lock);

  
  	spin_lock_init(&queue->fastopenq.lock);
  	queue->fastopenq.rskq_rst_head = NULL;
  	queue->fastopenq.rskq_rst_tail = NULL;
  	queue->fastopenq.qlen = 0;

  	queue->rskq_accept_head = NULL;

  }

  /*
   * This function is called to set a Fast Open socket's "fastopen_rsk" field
   * to NULL when a TFO socket no longer needs to access the request_sock.
   * This happens only after 3WHS has been either completed or aborted (e.g.,
   * RST is received).
   *
   * Before TFO, a child socket is created only after 3WHS is completed,
   * hence it never needs to access the request_sock. things get a lot more
   * complex with TFO. A child socket, accepted or not, has to access its
   * request_sock for 3WHS processing, e.g., to retransmit SYN-ACK pkts,
   * until 3WHS is either completed or aborted. Afterwards the req will stay
   * until either the child socket is accepted, or in the rare case when the
   * listener is closed before the child is accepted.
   *
   * In short, a request socket is only freed after BOTH 3WHS has completed
   * (or aborted) and the child socket has been accepted (or listener closed).
   * When a child socket is accepted, its corresponding req->sk is set to
   * NULL since it's no longer needed. More importantly, "req->sk == NULL"
   * will be used by the code below to determine if a child socket has been
   * accepted or not, and the check is protected by the fastopenq->lock
   * described below.
   *
   * Note that fastopen_rsk is only accessed from the child socket's context
   * with its socket lock held. But a request_sock (req) can be accessed by
   * both its child socket through fastopen_rsk, and a listener socket through
   * icsk_accept_queue.rskq_accept_head. To protect the access a simple spin
   * lock per listener "icsk->icsk_accept_queue.fastopenq->lock" is created.
   * only in the rare case when both the listener and the child locks are held,
   * e.g., in inet_csk_listen_stop() do we not need to acquire the lock.
   * The lock also protects other fields such as fastopenq->qlen, which is
   * decremented by this function when fastopen_rsk is no longer needed.
   *
   * Note that another solution was to simply use the existing socket lock
   * from the listener. But first socket lock is difficult to use. It is not
   * a simple spin lock - one must consider sock_owned_by_user() and arrange
   * to use sk_add_backlog() stuff. But what really makes it infeasible is the
   * locking hierarchy violation. E.g., inet_csk_listen_stop() may try to
   * acquire a child's lock while holding listener's socket lock. A corner
   * case might also exist in tcp_v4_hnd_req() that will trigger this locking
   * order.
   *

   * This function also sets "treq->tfo_listener" to false.
   * treq->tfo_listener is used by the listener so it is protected by the

   * fastopenq->lock in this function.
   */
  void reqsk_fastopen_remove(struct sock *sk, struct request_sock *req,
  			   bool reset)
  {

  	struct sock *lsk = req->rsk_listener;
  	struct fastopen_queue *fastopenq;

  	fastopenq = &inet_csk(lsk)->icsk_accept_queue.fastopenq;

  	tcp_sk(sk)->fastopen_rsk = NULL;
  	spin_lock_bh(&fastopenq->lock);
  	fastopenq->qlen--;

  	tcp_rsk(req)->tfo_listener = false;

  	if (req->sk)	/* the child socket hasn't been accepted yet */
  		goto out;
  
  	if (!reset || lsk->sk_state != TCP_LISTEN) {
  		/* If the listener has been closed don't bother with the
  		 * special RST handling below.
  		 */
  		spin_unlock_bh(&fastopenq->lock);

  		reqsk_put(req);

  		return;
  	}
  	/* Wait for 60secs before removing a req that has triggered RST.
  	 * This is a simple defense against TFO spoofing attack - by
  	 * counting the req against fastopen.max_qlen, and disabling
  	 * TFO when the qlen exceeds max_qlen.
  	 *
  	 * For more details see CoNext'11 "TCP Fast Open" paper.
  	 */

  	req->rsk_timer.expires = jiffies + 60*HZ;

  	if (fastopenq->rskq_rst_head == NULL)
  		fastopenq->rskq_rst_head = req;
  	else
  		fastopenq->rskq_rst_tail->dl_next = req;
  
  	req->dl_next = NULL;
  	fastopenq->rskq_rst_tail = req;
  	fastopenq->qlen++;
  out:
  	spin_unlock_bh(&fastopenq->lock);

  }