// SPDX-License-Identifier: GPL-2.0-or-later
/*
* NET Generic infrastructure for Network protocols.
*
* Authors: Arnaldo Carvalho de Melo <[email protected]>
*
* From code originally in include/net/tcp.h
*/
#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)
{
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;
RCU_INIT_POINTER(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);
}
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