2 * INET An implementation of the TCP/IP protocol suite for the LINUX
3 * operating system. INET is implemented using the BSD Socket
4 * interface as the means of communication with the user level.
6 * Implementation of the Transmission Control Protocol(TCP).
9 * Fred N. van Kempen, <waltje@uWalt.NL.Mugnet.ORG>
10 * Mark Evans, <evansmp@uhura.aston.ac.uk>
11 * Corey Minyard <wf-rch!minyard@relay.EU.net>
12 * Florian La Roche, <flla@stud.uni-sb.de>
13 * Charles Hedrick, <hedrick@klinzhai.rutgers.edu>
14 * Linus Torvalds, <torvalds@cs.helsinki.fi>
15 * Alan Cox, <gw4pts@gw4pts.ampr.org>
16 * Matthew Dillon, <dillon@apollo.west.oic.com>
17 * Arnt Gulbrandsen, <agulbra@nvg.unit.no>
18 * Jorge Cwik, <jorge@laser.satlink.net>
22 #include <linux/module.h>
23 #include <linux/slab.h>
24 #include <linux/sysctl.h>
25 #include <linux/workqueue.h>
27 #include <net/inet_common.h>
30 int sysctl_tcp_syncookies __read_mostly = 1;
31 EXPORT_SYMBOL(sysctl_tcp_syncookies);
33 int sysctl_tcp_abort_on_overflow __read_mostly;
35 struct inet_timewait_death_row tcp_death_row = {
36 .sysctl_max_tw_buckets = NR_FILE * 2,
37 .period = TCP_TIMEWAIT_LEN / INET_TWDR_TWKILL_SLOTS,
38 .death_lock = __SPIN_LOCK_UNLOCKED(tcp_death_row.death_lock),
39 .hashinfo = &tcp_hashinfo,
40 .tw_timer = TIMER_INITIALIZER(inet_twdr_hangman, 0,
41 (unsigned long)&tcp_death_row),
42 .twkill_work = __WORK_INITIALIZER(tcp_death_row.twkill_work,
43 inet_twdr_twkill_work),
44 /* Short-time timewait calendar */
47 .twcal_timer = TIMER_INITIALIZER(inet_twdr_twcal_tick, 0,
48 (unsigned long)&tcp_death_row),
50 EXPORT_SYMBOL_GPL(tcp_death_row);
52 static bool tcp_in_window(u32 seq, u32 end_seq, u32 s_win, u32 e_win)
56 if (after(end_seq, s_win) && before(seq, e_win))
58 return seq == e_win && seq == end_seq;
62 * * Main purpose of TIME-WAIT state is to close connection gracefully,
63 * when one of ends sits in LAST-ACK or CLOSING retransmitting FIN
64 * (and, probably, tail of data) and one or more our ACKs are lost.
65 * * What is TIME-WAIT timeout? It is associated with maximal packet
66 * lifetime in the internet, which results in wrong conclusion, that
67 * it is set to catch "old duplicate segments" wandering out of their path.
68 * It is not quite correct. This timeout is calculated so that it exceeds
69 * maximal retransmission timeout enough to allow to lose one (or more)
70 * segments sent by peer and our ACKs. This time may be calculated from RTO.
71 * * When TIME-WAIT socket receives RST, it means that another end
72 * finally closed and we are allowed to kill TIME-WAIT too.
73 * * Second purpose of TIME-WAIT is catching old duplicate segments.
74 * Well, certainly it is pure paranoia, but if we load TIME-WAIT
75 * with this semantics, we MUST NOT kill TIME-WAIT state with RSTs.
76 * * If we invented some more clever way to catch duplicates
77 * (f.e. based on PAWS), we could truncate TIME-WAIT to several RTOs.
79 * The algorithm below is based on FORMAL INTERPRETATION of RFCs.
80 * When you compare it to RFCs, please, read section SEGMENT ARRIVES
81 * from the very beginning.
83 * NOTE. With recycling (and later with fin-wait-2) TW bucket
84 * is _not_ stateless. It means, that strictly speaking we must
85 * spinlock it. I do not want! Well, probability of misbehaviour
86 * is ridiculously low and, seems, we could use some mb() tricks
87 * to avoid misread sequence numbers, states etc. --ANK
89 * We don't need to initialize tmp_out.sack_ok as we don't use the results
92 tcp_timewait_state_process(struct inet_timewait_sock *tw, struct sk_buff *skb,
93 const struct tcphdr *th)
95 struct tcp_options_received tmp_opt;
96 struct tcp_timewait_sock *tcptw = tcp_twsk((struct sock *)tw);
97 bool paws_reject = false;
99 tmp_opt.saw_tstamp = 0;
100 if (th->doff > (sizeof(*th) >> 2) && tcptw->tw_ts_recent_stamp) {
101 tcp_parse_options(skb, &tmp_opt, 0, NULL);
103 if (tmp_opt.saw_tstamp) {
104 tmp_opt.rcv_tsecr -= tcptw->tw_ts_offset;
105 tmp_opt.ts_recent = tcptw->tw_ts_recent;
106 tmp_opt.ts_recent_stamp = tcptw->tw_ts_recent_stamp;
107 paws_reject = tcp_paws_reject(&tmp_opt, th->rst);
111 if (tw->tw_substate == TCP_FIN_WAIT2) {
112 /* Just repeat all the checks of tcp_rcv_state_process() */
114 /* Out of window, send ACK */
116 !tcp_in_window(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq,
118 tcptw->tw_rcv_nxt + tcptw->tw_rcv_wnd))
124 if (th->syn && !before(TCP_SKB_CB(skb)->seq, tcptw->tw_rcv_nxt))
129 !after(TCP_SKB_CB(skb)->end_seq, tcptw->tw_rcv_nxt) ||
130 TCP_SKB_CB(skb)->end_seq == TCP_SKB_CB(skb)->seq) {
132 return TCP_TW_SUCCESS;
135 /* New data or FIN. If new data arrive after half-duplex close,
139 TCP_SKB_CB(skb)->end_seq != tcptw->tw_rcv_nxt + 1) {
141 inet_twsk_deschedule(tw, &tcp_death_row);
146 /* FIN arrived, enter true time-wait state. */
147 tw->tw_substate = TCP_TIME_WAIT;
148 tcptw->tw_rcv_nxt = TCP_SKB_CB(skb)->end_seq;
149 if (tmp_opt.saw_tstamp) {
150 tcptw->tw_ts_recent_stamp = get_seconds();
151 tcptw->tw_ts_recent = tmp_opt.rcv_tsval;
154 if (tcp_death_row.sysctl_tw_recycle &&
155 tcptw->tw_ts_recent_stamp &&
156 tcp_tw_remember_stamp(tw))
157 inet_twsk_schedule(tw, &tcp_death_row, tw->tw_timeout,
160 inet_twsk_schedule(tw, &tcp_death_row, TCP_TIMEWAIT_LEN,
166 * Now real TIME-WAIT state.
169 * "When a connection is [...] on TIME-WAIT state [...]
170 * [a TCP] MAY accept a new SYN from the remote TCP to
171 * reopen the connection directly, if it:
173 * (1) assigns its initial sequence number for the new
174 * connection to be larger than the largest sequence
175 * number it used on the previous connection incarnation,
178 * (2) returns to TIME-WAIT state if the SYN turns out
179 * to be an old duplicate".
183 (TCP_SKB_CB(skb)->seq == tcptw->tw_rcv_nxt &&
184 (TCP_SKB_CB(skb)->seq == TCP_SKB_CB(skb)->end_seq || th->rst))) {
185 /* In window segment, it may be only reset or bare ack. */
188 /* This is TIME_WAIT assassination, in two flavors.
189 * Oh well... nobody has a sufficient solution to this
192 if (sysctl_tcp_rfc1337 == 0) {
194 inet_twsk_deschedule(tw, &tcp_death_row);
196 return TCP_TW_SUCCESS;
199 inet_twsk_schedule(tw, &tcp_death_row, TCP_TIMEWAIT_LEN,
202 if (tmp_opt.saw_tstamp) {
203 tcptw->tw_ts_recent = tmp_opt.rcv_tsval;
204 tcptw->tw_ts_recent_stamp = get_seconds();
208 return TCP_TW_SUCCESS;
211 /* Out of window segment.
213 All the segments are ACKed immediately.
215 The only exception is new SYN. We accept it, if it is
216 not old duplicate and we are not in danger to be killed
217 by delayed old duplicates. RFC check is that it has
218 newer sequence number works at rates <40Mbit/sec.
219 However, if paws works, it is reliable AND even more,
220 we even may relax silly seq space cutoff.
222 RED-PEN: we violate main RFC requirement, if this SYN will appear
223 old duplicate (i.e. we receive RST in reply to SYN-ACK),
224 we must return socket to time-wait state. It is not good,
228 if (th->syn && !th->rst && !th->ack && !paws_reject &&
229 (after(TCP_SKB_CB(skb)->seq, tcptw->tw_rcv_nxt) ||
230 (tmp_opt.saw_tstamp &&
231 (s32)(tcptw->tw_ts_recent - tmp_opt.rcv_tsval) < 0))) {
232 u32 isn = tcptw->tw_snd_nxt + 65535 + 2;
235 TCP_SKB_CB(skb)->when = isn;
240 NET_INC_STATS_BH(twsk_net(tw), LINUX_MIB_PAWSESTABREJECTED);
243 /* In this case we must reset the TIMEWAIT timer.
245 * If it is ACKless SYN it may be both old duplicate
246 * and new good SYN with random sequence number <rcv_nxt.
247 * Do not reschedule in the last case.
249 if (paws_reject || th->ack)
250 inet_twsk_schedule(tw, &tcp_death_row, TCP_TIMEWAIT_LEN,
253 /* Send ACK. Note, we do not put the bucket,
254 * it will be released by caller.
259 return TCP_TW_SUCCESS;
261 EXPORT_SYMBOL(tcp_timewait_state_process);
264 * Move a socket to time-wait or dead fin-wait-2 state.
266 void tcp_time_wait(struct sock *sk, int state, int timeo)
268 struct inet_timewait_sock *tw = NULL;
269 const struct inet_connection_sock *icsk = inet_csk(sk);
270 const struct tcp_sock *tp = tcp_sk(sk);
271 bool recycle_ok = false;
273 if (tcp_death_row.sysctl_tw_recycle && tp->rx_opt.ts_recent_stamp)
274 recycle_ok = tcp_remember_stamp(sk);
276 if (tcp_death_row.tw_count < tcp_death_row.sysctl_max_tw_buckets)
277 tw = inet_twsk_alloc(sk, state);
280 struct tcp_timewait_sock *tcptw = tcp_twsk((struct sock *)tw);
281 const int rto = (icsk->icsk_rto << 2) - (icsk->icsk_rto >> 1);
282 struct inet_sock *inet = inet_sk(sk);
284 tw->tw_transparent = inet->transparent;
285 tw->tw_rcv_wscale = tp->rx_opt.rcv_wscale;
286 tcptw->tw_rcv_nxt = tp->rcv_nxt;
287 tcptw->tw_snd_nxt = tp->snd_nxt;
288 tcptw->tw_rcv_wnd = tcp_receive_window(tp);
289 tcptw->tw_ts_recent = tp->rx_opt.ts_recent;
290 tcptw->tw_ts_recent_stamp = tp->rx_opt.ts_recent_stamp;
291 tcptw->tw_ts_offset = tp->tsoffset;
293 #if IS_ENABLED(CONFIG_IPV6)
294 if (tw->tw_family == PF_INET6) {
295 struct ipv6_pinfo *np = inet6_sk(sk);
297 tw->tw_v6_daddr = sk->sk_v6_daddr;
298 tw->tw_v6_rcv_saddr = sk->sk_v6_rcv_saddr;
299 tw->tw_tclass = np->tclass;
300 tw->tw_ipv6only = np->ipv6only;
304 #ifdef CONFIG_TCP_MD5SIG
306 * The timewait bucket does not have the key DB from the
307 * sock structure. We just make a quick copy of the
308 * md5 key being used (if indeed we are using one)
309 * so the timewait ack generating code has the key.
312 struct tcp_md5sig_key *key;
313 tcptw->tw_md5_key = NULL;
314 key = tp->af_specific->md5_lookup(sk, sk);
316 tcptw->tw_md5_key = kmemdup(key, sizeof(*key), GFP_ATOMIC);
317 if (tcptw->tw_md5_key && !tcp_alloc_md5sig_pool())
323 /* Linkage updates. */
324 __inet_twsk_hashdance(tw, sk, &tcp_hashinfo);
326 /* Get the TIME_WAIT timeout firing. */
331 tw->tw_timeout = rto;
333 tw->tw_timeout = TCP_TIMEWAIT_LEN;
334 if (state == TCP_TIME_WAIT)
335 timeo = TCP_TIMEWAIT_LEN;
338 inet_twsk_schedule(tw, &tcp_death_row, timeo,
342 /* Sorry, if we're out of memory, just CLOSE this
343 * socket up. We've got bigger problems than
344 * non-graceful socket closings.
346 NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPTIMEWAITOVERFLOW);
349 tcp_update_metrics(sk);
353 void tcp_twsk_destructor(struct sock *sk)
355 #ifdef CONFIG_TCP_MD5SIG
356 struct tcp_timewait_sock *twsk = tcp_twsk(sk);
358 if (twsk->tw_md5_key)
359 kfree_rcu(twsk->tw_md5_key, rcu);
362 EXPORT_SYMBOL_GPL(tcp_twsk_destructor);
364 static inline void TCP_ECN_openreq_child(struct tcp_sock *tp,
365 struct request_sock *req)
367 tp->ecn_flags = inet_rsk(req)->ecn_ok ? TCP_ECN_OK : 0;
370 /* This is not only more efficient than what we used to do, it eliminates
371 * a lot of code duplication between IPv4/IPv6 SYN recv processing. -DaveM
373 * Actually, we could lots of memory writes here. tp of listening
374 * socket contains all necessary default parameters.
376 struct sock *tcp_create_openreq_child(struct sock *sk, struct request_sock *req, struct sk_buff *skb)
378 struct sock *newsk = inet_csk_clone_lock(sk, req, GFP_ATOMIC);
381 const struct inet_request_sock *ireq = inet_rsk(req);
382 struct tcp_request_sock *treq = tcp_rsk(req);
383 struct inet_connection_sock *newicsk = inet_csk(newsk);
384 struct tcp_sock *newtp = tcp_sk(newsk);
386 /* Now setup tcp_sock */
387 newtp->pred_flags = 0;
389 newtp->rcv_wup = newtp->copied_seq =
390 newtp->rcv_nxt = treq->rcv_isn + 1;
392 newtp->snd_sml = newtp->snd_una =
393 newtp->snd_nxt = newtp->snd_up = treq->snt_isn + 1;
395 tcp_prequeue_init(newtp);
396 INIT_LIST_HEAD(&newtp->tsq_node);
398 tcp_init_wl(newtp, treq->rcv_isn);
401 newtp->mdev = TCP_TIMEOUT_INIT;
402 newicsk->icsk_rto = TCP_TIMEOUT_INIT;
404 newtp->packets_out = 0;
405 newtp->retrans_out = 0;
406 newtp->sacked_out = 0;
407 newtp->fackets_out = 0;
408 newtp->snd_ssthresh = TCP_INFINITE_SSTHRESH;
409 tcp_enable_early_retrans(newtp);
410 newtp->tlp_high_seq = 0;
411 newtp->lsndtime = treq->snt_synack;
412 newtp->total_retrans = req->num_retrans;
414 /* So many TCP implementations out there (incorrectly) count the
415 * initial SYN frame in their delayed-ACK and congestion control
416 * algorithms that we must have the following bandaid to talk
417 * efficiently to them. -DaveM
419 newtp->snd_cwnd = TCP_INIT_CWND;
420 newtp->snd_cwnd_cnt = 0;
422 if (newicsk->icsk_ca_ops != &tcp_init_congestion_ops &&
423 !try_module_get(newicsk->icsk_ca_ops->owner))
424 newicsk->icsk_ca_ops = &tcp_init_congestion_ops;
426 tcp_set_ca_state(newsk, TCP_CA_Open);
427 tcp_init_xmit_timers(newsk);
428 skb_queue_head_init(&newtp->out_of_order_queue);
429 newtp->write_seq = newtp->pushed_seq = treq->snt_isn + 1;
431 newtp->rx_opt.saw_tstamp = 0;
433 newtp->rx_opt.dsack = 0;
434 newtp->rx_opt.num_sacks = 0;
438 if (sock_flag(newsk, SOCK_KEEPOPEN))
439 inet_csk_reset_keepalive_timer(newsk,
440 keepalive_time_when(newtp));
442 newtp->rx_opt.tstamp_ok = ireq->tstamp_ok;
443 if ((newtp->rx_opt.sack_ok = ireq->sack_ok) != 0) {
445 tcp_enable_fack(newtp);
447 newtp->window_clamp = req->window_clamp;
448 newtp->rcv_ssthresh = req->rcv_wnd;
449 newtp->rcv_wnd = req->rcv_wnd;
450 newtp->rx_opt.wscale_ok = ireq->wscale_ok;
451 if (newtp->rx_opt.wscale_ok) {
452 newtp->rx_opt.snd_wscale = ireq->snd_wscale;
453 newtp->rx_opt.rcv_wscale = ireq->rcv_wscale;
455 newtp->rx_opt.snd_wscale = newtp->rx_opt.rcv_wscale = 0;
456 newtp->window_clamp = min(newtp->window_clamp, 65535U);
458 newtp->snd_wnd = (ntohs(tcp_hdr(skb)->window) <<
459 newtp->rx_opt.snd_wscale);
460 newtp->max_window = newtp->snd_wnd;
462 if (newtp->rx_opt.tstamp_ok) {
463 newtp->rx_opt.ts_recent = req->ts_recent;
464 newtp->rx_opt.ts_recent_stamp = get_seconds();
465 newtp->tcp_header_len = sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED;
467 newtp->rx_opt.ts_recent_stamp = 0;
468 newtp->tcp_header_len = sizeof(struct tcphdr);
471 #ifdef CONFIG_TCP_MD5SIG
472 newtp->md5sig_info = NULL; /*XXX*/
473 if (newtp->af_specific->md5_lookup(sk, newsk))
474 newtp->tcp_header_len += TCPOLEN_MD5SIG_ALIGNED;
476 if (skb->len >= TCP_MSS_DEFAULT + newtp->tcp_header_len)
477 newicsk->icsk_ack.last_seg_size = skb->len - newtp->tcp_header_len;
478 newtp->rx_opt.mss_clamp = req->mss;
479 TCP_ECN_openreq_child(newtp, req);
480 newtp->fastopen_rsk = NULL;
481 newtp->syn_data_acked = 0;
483 TCP_INC_STATS_BH(sock_net(sk), TCP_MIB_PASSIVEOPENS);
487 EXPORT_SYMBOL(tcp_create_openreq_child);
490 * Process an incoming packet for SYN_RECV sockets represented as a
491 * request_sock. Normally sk is the listener socket but for TFO it
492 * points to the child socket.
494 * XXX (TFO) - The current impl contains a special check for ack
495 * validation and inside tcp_v4_reqsk_send_ack(). Can we do better?
497 * We don't need to initialize tmp_opt.sack_ok as we don't use the results
500 struct sock *tcp_check_req(struct sock *sk, struct sk_buff *skb,
501 struct request_sock *req,
502 struct request_sock **prev,
505 struct tcp_options_received tmp_opt;
507 const struct tcphdr *th = tcp_hdr(skb);
508 __be32 flg = tcp_flag_word(th) & (TCP_FLAG_RST|TCP_FLAG_SYN|TCP_FLAG_ACK);
509 bool paws_reject = false;
511 BUG_ON(fastopen == (sk->sk_state == TCP_LISTEN));
513 tmp_opt.saw_tstamp = 0;
514 if (th->doff > (sizeof(struct tcphdr)>>2)) {
515 tcp_parse_options(skb, &tmp_opt, 0, NULL);
517 if (tmp_opt.saw_tstamp) {
518 tmp_opt.ts_recent = req->ts_recent;
519 /* We do not store true stamp, but it is not required,
520 * it can be estimated (approximately)
523 tmp_opt.ts_recent_stamp = get_seconds() - ((TCP_TIMEOUT_INIT/HZ)<<req->num_timeout);
524 paws_reject = tcp_paws_reject(&tmp_opt, th->rst);
528 /* Check for pure retransmitted SYN. */
529 if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn &&
530 flg == TCP_FLAG_SYN &&
533 * RFC793 draws (Incorrectly! It was fixed in RFC1122)
534 * this case on figure 6 and figure 8, but formal
535 * protocol description says NOTHING.
536 * To be more exact, it says that we should send ACK,
537 * because this segment (at least, if it has no data)
540 * CONCLUSION: RFC793 (even with RFC1122) DOES NOT
541 * describe SYN-RECV state. All the description
542 * is wrong, we cannot believe to it and should
543 * rely only on common sense and implementation
546 * Enforce "SYN-ACK" according to figure 8, figure 6
547 * of RFC793, fixed by RFC1122.
549 * Note that even if there is new data in the SYN packet
550 * they will be thrown away too.
552 * Reset timer after retransmitting SYNACK, similar to
553 * the idea of fast retransmit in recovery.
555 if (!inet_rtx_syn_ack(sk, req))
556 req->expires = min(TCP_TIMEOUT_INIT << req->num_timeout,
557 TCP_RTO_MAX) + jiffies;
561 /* Further reproduces section "SEGMENT ARRIVES"
562 for state SYN-RECEIVED of RFC793.
563 It is broken, however, it does not work only
564 when SYNs are crossed.
566 You would think that SYN crossing is impossible here, since
567 we should have a SYN_SENT socket (from connect()) on our end,
568 but this is not true if the crossed SYNs were sent to both
569 ends by a malicious third party. We must defend against this,
570 and to do that we first verify the ACK (as per RFC793, page
571 36) and reset if it is invalid. Is this a true full defense?
572 To convince ourselves, let us consider a way in which the ACK
573 test can still pass in this 'malicious crossed SYNs' case.
574 Malicious sender sends identical SYNs (and thus identical sequence
575 numbers) to both A and B:
580 By our good fortune, both A and B select the same initial
581 send sequence number of seven :-)
583 A: sends SYN|ACK, seq=7, ack_seq=8
584 B: sends SYN|ACK, seq=7, ack_seq=8
586 So we are now A eating this SYN|ACK, ACK test passes. So
587 does sequence test, SYN is truncated, and thus we consider
590 If icsk->icsk_accept_queue.rskq_defer_accept, we silently drop this
591 bare ACK. Otherwise, we create an established connection. Both
592 ends (listening sockets) accept the new incoming connection and try
593 to talk to each other. 8-)
595 Note: This case is both harmless, and rare. Possibility is about the
596 same as us discovering intelligent life on another plant tomorrow.
598 But generally, we should (RFC lies!) to accept ACK
599 from SYNACK both here and in tcp_rcv_state_process().
600 tcp_rcv_state_process() does not, hence, we do not too.
602 Note that the case is absolutely generic:
603 we cannot optimize anything here without
604 violating protocol. All the checks must be made
605 before attempt to create socket.
608 /* RFC793 page 36: "If the connection is in any non-synchronized state ...
609 * and the incoming segment acknowledges something not yet
610 * sent (the segment carries an unacceptable ACK) ...
613 * Invalid ACK: reset will be sent by listening socket.
614 * Note that the ACK validity check for a Fast Open socket is done
615 * elsewhere and is checked directly against the child socket rather
616 * than req because user data may have been sent out.
618 if ((flg & TCP_FLAG_ACK) && !fastopen &&
619 (TCP_SKB_CB(skb)->ack_seq !=
620 tcp_rsk(req)->snt_isn + 1))
623 /* Also, it would be not so bad idea to check rcv_tsecr, which
624 * is essentially ACK extension and too early or too late values
625 * should cause reset in unsynchronized states.
628 /* RFC793: "first check sequence number". */
630 if (paws_reject || !tcp_in_window(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq,
631 tcp_rsk(req)->rcv_nxt, tcp_rsk(req)->rcv_nxt + req->rcv_wnd)) {
632 /* Out of window: send ACK and drop. */
633 if (!(flg & TCP_FLAG_RST))
634 req->rsk_ops->send_ack(sk, skb, req);
636 NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_PAWSESTABREJECTED);
640 /* In sequence, PAWS is OK. */
642 if (tmp_opt.saw_tstamp && !after(TCP_SKB_CB(skb)->seq, tcp_rsk(req)->rcv_nxt))
643 req->ts_recent = tmp_opt.rcv_tsval;
645 if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn) {
646 /* Truncate SYN, it is out of window starting
647 at tcp_rsk(req)->rcv_isn + 1. */
648 flg &= ~TCP_FLAG_SYN;
651 /* RFC793: "second check the RST bit" and
652 * "fourth, check the SYN bit"
654 if (flg & (TCP_FLAG_RST|TCP_FLAG_SYN)) {
655 TCP_INC_STATS_BH(sock_net(sk), TCP_MIB_ATTEMPTFAILS);
656 goto embryonic_reset;
659 /* ACK sequence verified above, just make sure ACK is
660 * set. If ACK not set, just silently drop the packet.
662 * XXX (TFO) - if we ever allow "data after SYN", the
663 * following check needs to be removed.
665 if (!(flg & TCP_FLAG_ACK))
668 /* For Fast Open no more processing is needed (sk is the
674 /* While TCP_DEFER_ACCEPT is active, drop bare ACK. */
675 if (req->num_timeout < inet_csk(sk)->icsk_accept_queue.rskq_defer_accept &&
676 TCP_SKB_CB(skb)->end_seq == tcp_rsk(req)->rcv_isn + 1) {
677 inet_rsk(req)->acked = 1;
678 NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPDEFERACCEPTDROP);
682 /* OK, ACK is valid, create big socket and
683 * feed this segment to it. It will repeat all
684 * the tests. THIS SEGMENT MUST MOVE SOCKET TO
685 * ESTABLISHED STATE. If it will be dropped after
686 * socket is created, wait for troubles.
688 child = inet_csk(sk)->icsk_af_ops->syn_recv_sock(sk, skb, req, NULL);
690 goto listen_overflow;
692 inet_csk_reqsk_queue_unlink(sk, req, prev);
693 inet_csk_reqsk_queue_removed(sk, req);
695 inet_csk_reqsk_queue_add(sk, req, child);
699 if (!sysctl_tcp_abort_on_overflow) {
700 inet_rsk(req)->acked = 1;
705 if (!(flg & TCP_FLAG_RST)) {
706 /* Received a bad SYN pkt - for TFO We try not to reset
707 * the local connection unless it's really necessary to
708 * avoid becoming vulnerable to outside attack aiming at
709 * resetting legit local connections.
711 req->rsk_ops->send_reset(sk, skb);
712 } else if (fastopen) { /* received a valid RST pkt */
713 reqsk_fastopen_remove(sk, req, true);
717 inet_csk_reqsk_queue_drop(sk, req, prev);
718 NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_EMBRYONICRSTS);
722 EXPORT_SYMBOL(tcp_check_req);
725 * Queue segment on the new socket if the new socket is active,
726 * otherwise we just shortcircuit this and continue with
729 * For the vast majority of cases child->sk_state will be TCP_SYN_RECV
730 * when entering. But other states are possible due to a race condition
731 * where after __inet_lookup_established() fails but before the listener
732 * locked is obtained, other packets cause the same connection to
736 int tcp_child_process(struct sock *parent, struct sock *child,
740 int state = child->sk_state;
742 if (!sock_owned_by_user(child)) {
743 ret = tcp_rcv_state_process(child, skb, tcp_hdr(skb),
745 /* Wakeup parent, send SIGIO */
746 if (state == TCP_SYN_RECV && child->sk_state != state)
747 parent->sk_data_ready(parent, 0);
749 /* Alas, it is possible again, because we do lookup
750 * in main socket hash table and lock on listening
751 * socket does not protect us more.
753 __sk_add_backlog(child, skb);
756 bh_unlock_sock(child);
760 EXPORT_SYMBOL(tcp_child_process);