cregit-Linux how code gets into the kernel

Release 4.8 net/core/request_sock.c

Directory: net/core
 * NET          Generic infrastructure for Network protocols.
 * Authors:     Arnaldo Carvalho de Melo <>
 *              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.

int sysctl_max_syn_backlog = 256;


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; }


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arnaldo carvalho de meloarnaldo carvalho de melo1729.31%233.33%

/* * 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); }


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Directory: net/core
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