linux/net/core/request_sock.c
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   1/*
   2 * NET          Generic infrastructure for Network protocols.
   3 *
   4 * Authors:     Arnaldo Carvalho de Melo <acme@conectiva.com.br>
   5 *
   6 *              From code originally in include/net/tcp.h
   7 *
   8 *              This program is free software; you can redistribute it and/or
   9 *              modify it under the terms of the GNU General Public License
  10 *              as published by the Free Software Foundation; either version
  11 *              2 of the License, or (at your option) any later version.
  12 */
  13
  14#include <linux/module.h>
  15#include <linux/random.h>
  16#include <linux/slab.h>
  17#include <linux/string.h>
  18#include <linux/tcp.h>
  19#include <linux/vmalloc.h>
  20
  21#include <net/request_sock.h>
  22
  23/*
  24 * Maximum number of SYN_RECV sockets in queue per LISTEN socket.
  25 * One SYN_RECV socket costs about 80bytes on a 32bit machine.
  26 * It would be better to replace it with a global counter for all sockets
  27 * but then some measure against one socket starving all other sockets
  28 * would be needed.
  29 *
  30 * The minimum value of it is 128. Experiments with real servers show that
  31 * it is absolutely not enough even at 100conn/sec. 256 cures most
  32 * of problems.
  33 * This value is adjusted to 128 for low memory machines,
  34 * and it will increase in proportion to the memory of machine.
  35 * Note : Dont forget somaxconn that may limit backlog too.
  36 */
  37int sysctl_max_syn_backlog = 256;
  38EXPORT_SYMBOL(sysctl_max_syn_backlog);
  39
  40int reqsk_queue_alloc(struct request_sock_queue *queue,
  41                      unsigned int nr_table_entries)
  42{
  43        size_t lopt_size = sizeof(struct listen_sock);
  44        struct listen_sock *lopt;
  45
  46        nr_table_entries = min_t(u32, nr_table_entries, sysctl_max_syn_backlog);
  47        nr_table_entries = max_t(u32, nr_table_entries, 8);
  48        nr_table_entries = roundup_pow_of_two(nr_table_entries + 1);
  49        lopt_size += nr_table_entries * sizeof(struct request_sock *);
  50        if (lopt_size > PAGE_SIZE)
  51                lopt = vzalloc(lopt_size);
  52        else
  53                lopt = kzalloc(lopt_size, GFP_KERNEL);
  54        if (lopt == NULL)
  55                return -ENOMEM;
  56
  57        for (lopt->max_qlen_log = 3;
  58             (1 << lopt->max_qlen_log) < nr_table_entries;
  59             lopt->max_qlen_log++);
  60
  61        get_random_bytes(&lopt->hash_rnd, sizeof(lopt->hash_rnd));
  62        rwlock_init(&queue->syn_wait_lock);
  63        queue->rskq_accept_head = NULL;
  64        lopt->nr_table_entries = nr_table_entries;
  65
  66        write_lock_bh(&queue->syn_wait_lock);
  67        queue->listen_opt = lopt;
  68        write_unlock_bh(&queue->syn_wait_lock);
  69
  70        return 0;
  71}
  72
  73void __reqsk_queue_destroy(struct request_sock_queue *queue)
  74{
  75        struct listen_sock *lopt;
  76        size_t lopt_size;
  77
  78        /*
  79         * this is an error recovery path only
  80         * no locking needed and the lopt is not NULL
  81         */
  82
  83        lopt = queue->listen_opt;
  84        lopt_size = sizeof(struct listen_sock) +
  85                lopt->nr_table_entries * sizeof(struct request_sock *);
  86
  87        if (lopt_size > PAGE_SIZE)
  88                vfree(lopt);
  89        else
  90                kfree(lopt);
  91}
  92
  93static inline struct listen_sock *reqsk_queue_yank_listen_sk(
  94                struct request_sock_queue *queue)
  95{
  96        struct listen_sock *lopt;
  97
  98        write_lock_bh(&queue->syn_wait_lock);
  99        lopt = queue->listen_opt;
 100        queue->listen_opt = NULL;
 101        write_unlock_bh(&queue->syn_wait_lock);
 102
 103        return lopt;
 104}
 105
 106void reqsk_queue_destroy(struct request_sock_queue *queue)
 107{
 108        /* make all the listen_opt local to us */
 109        struct listen_sock *lopt = reqsk_queue_yank_listen_sk(queue);
 110        size_t lopt_size = sizeof(struct listen_sock) +
 111                lopt->nr_table_entries * sizeof(struct request_sock *);
 112
 113        if (lopt->qlen != 0) {
 114                unsigned int i;
 115
 116                for (i = 0; i < lopt->nr_table_entries; i++) {
 117                        struct request_sock *req;
 118
 119                        while ((req = lopt->syn_table[i]) != NULL) {
 120                                lopt->syn_table[i] = req->dl_next;
 121                                lopt->qlen--;
 122                                reqsk_free(req);
 123                        }
 124                }
 125        }
 126
 127        WARN_ON(lopt->qlen != 0);
 128        if (lopt_size > PAGE_SIZE)
 129                vfree(lopt);
 130        else
 131                kfree(lopt);
 132}
 133
 134/*
 135 * This function is called to set a Fast Open socket's "fastopen_rsk" field
 136 * to NULL when a TFO socket no longer needs to access the request_sock.
 137 * This happens only after 3WHS has been either completed or aborted (e.g.,
 138 * RST is received).
 139 *
 140 * Before TFO, a child socket is created only after 3WHS is completed,
 141 * hence it never needs to access the request_sock. things get a lot more
 142 * complex with TFO. A child socket, accepted or not, has to access its
 143 * request_sock for 3WHS processing, e.g., to retransmit SYN-ACK pkts,
 144 * until 3WHS is either completed or aborted. Afterwards the req will stay
 145 * until either the child socket is accepted, or in the rare case when the
 146 * listener is closed before the child is accepted.
 147 *
 148 * In short, a request socket is only freed after BOTH 3WHS has completed
 149 * (or aborted) and the child socket has been accepted (or listener closed).
 150 * When a child socket is accepted, its corresponding req->sk is set to
 151 * NULL since it's no longer needed. More importantly, "req->sk == NULL"
 152 * will be used by the code below to determine if a child socket has been
 153 * accepted or not, and the check is protected by the fastopenq->lock
 154 * described below.
 155 *
 156 * Note that fastopen_rsk is only accessed from the child socket's context
 157 * with its socket lock held. But a request_sock (req) can be accessed by
 158 * both its child socket through fastopen_rsk, and a listener socket through
 159 * icsk_accept_queue.rskq_accept_head. To protect the access a simple spin
 160 * lock per listener "icsk->icsk_accept_queue.fastopenq->lock" is created.
 161 * only in the rare case when both the listener and the child locks are held,
 162 * e.g., in inet_csk_listen_stop() do we not need to acquire the lock.
 163 * The lock also protects other fields such as fastopenq->qlen, which is
 164 * decremented by this function when fastopen_rsk is no longer needed.
 165 *
 166 * Note that another solution was to simply use the existing socket lock
 167 * from the listener. But first socket lock is difficult to use. It is not
 168 * a simple spin lock - one must consider sock_owned_by_user() and arrange
 169 * to use sk_add_backlog() stuff. But what really makes it infeasible is the
 170 * locking hierarchy violation. E.g., inet_csk_listen_stop() may try to
 171 * acquire a child's lock while holding listener's socket lock. A corner
 172 * case might also exist in tcp_v4_hnd_req() that will trigger this locking
 173 * order.
 174 *
 175 * When a TFO req is created, it needs to sock_hold its listener to prevent
 176 * the latter data structure from going away.
 177 *
 178 * This function also sets "treq->listener" to NULL and unreference listener
 179 * socket. treq->listener is used by the listener so it is protected by the
 180 * fastopenq->lock in this function.
 181 */
 182void reqsk_fastopen_remove(struct sock *sk, struct request_sock *req,
 183                           bool reset)
 184{
 185        struct sock *lsk = tcp_rsk(req)->listener;
 186        struct fastopen_queue *fastopenq =
 187            inet_csk(lsk)->icsk_accept_queue.fastopenq;
 188
 189        BUG_ON(!spin_is_locked(&sk->sk_lock.slock) && !sock_owned_by_user(sk));
 190
 191        tcp_sk(sk)->fastopen_rsk = NULL;
 192        spin_lock_bh(&fastopenq->lock);
 193        fastopenq->qlen--;
 194        tcp_rsk(req)->listener = NULL;
 195        if (req->sk)    /* the child socket hasn't been accepted yet */
 196                goto out;
 197
 198        if (!reset || lsk->sk_state != TCP_LISTEN) {
 199                /* If the listener has been closed don't bother with the
 200                 * special RST handling below.
 201                 */
 202                spin_unlock_bh(&fastopenq->lock);
 203                sock_put(lsk);
 204                reqsk_free(req);
 205                return;
 206        }
 207        /* Wait for 60secs before removing a req that has triggered RST.
 208         * This is a simple defense against TFO spoofing attack - by
 209         * counting the req against fastopen.max_qlen, and disabling
 210         * TFO when the qlen exceeds max_qlen.
 211         *
 212         * For more details see CoNext'11 "TCP Fast Open" paper.
 213         */
 214        req->expires = jiffies + 60*HZ;
 215        if (fastopenq->rskq_rst_head == NULL)
 216                fastopenq->rskq_rst_head = req;
 217        else
 218                fastopenq->rskq_rst_tail->dl_next = req;
 219
 220        req->dl_next = NULL;
 221        fastopenq->rskq_rst_tail = req;
 222        fastopenq->qlen++;
 223out:
 224        spin_unlock_bh(&fastopenq->lock);
 225        sock_put(lsk);
 226        return;
 227}
 228
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