Bootlin logo

Elixir Cross Referencer

  1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
/*
 * INET		An implementation of the TCP/IP protocol suite for the LINUX
 *		operating system.  INET is implemented using the  BSD Socket
 *		interface as the means of communication with the user level.
 *
 *		Implementation of the Transmission Control Protocol(TCP).
 *
 * Authors:	Ross Biro
 *		Fred N. van Kempen, <waltje@uWalt.NL.Mugnet.ORG>
 *		Mark Evans, <evansmp@uhura.aston.ac.uk>
 *		Corey Minyard <wf-rch!minyard@relay.EU.net>
 *		Florian La Roche, <flla@stud.uni-sb.de>
 *		Charles Hedrick, <hedrick@klinzhai.rutgers.edu>
 *		Linus Torvalds, <torvalds@cs.helsinki.fi>
 *		Alan Cox, <gw4pts@gw4pts.ampr.org>
 *		Matthew Dillon, <dillon@apollo.west.oic.com>
 *		Arnt Gulbrandsen, <agulbra@nvg.unit.no>
 *		Jorge Cwik, <jorge@laser.satlink.net>
 */

#include <linux/mm.h>
#include <linux/module.h>
#include <linux/slab.h>
#include <linux/sysctl.h>
#include <linux/workqueue.h>
#include <linux/static_key.h>
#include <net/tcp.h>
#include <net/inet_common.h>
#include <net/xfrm.h>
#include <net/busy_poll.h>

static bool tcp_in_window(u32 seq, u32 end_seq, u32 s_win, u32 e_win)
{
	if (seq == s_win)
		return true;
	if (after(end_seq, s_win) && before(seq, e_win))
		return true;
	return seq == e_win && seq == end_seq;
}

static enum tcp_tw_status
tcp_timewait_check_oow_rate_limit(struct inet_timewait_sock *tw,
				  const struct sk_buff *skb, int mib_idx)
{
	struct tcp_timewait_sock *tcptw = tcp_twsk((struct sock *)tw);

	if (!tcp_oow_rate_limited(twsk_net(tw), skb, mib_idx,
				  &tcptw->tw_last_oow_ack_time)) {
		/* Send ACK. Note, we do not put the bucket,
		 * it will be released by caller.
		 */
		return TCP_TW_ACK;
	}

	/* We are rate-limiting, so just release the tw sock and drop skb. */
	inet_twsk_put(tw);
	return TCP_TW_SUCCESS;
}

/*
 * * Main purpose of TIME-WAIT state is to close connection gracefully,
 *   when one of ends sits in LAST-ACK or CLOSING retransmitting FIN
 *   (and, probably, tail of data) and one or more our ACKs are lost.
 * * What is TIME-WAIT timeout? It is associated with maximal packet
 *   lifetime in the internet, which results in wrong conclusion, that
 *   it is set to catch "old duplicate segments" wandering out of their path.
 *   It is not quite correct. This timeout is calculated so that it exceeds
 *   maximal retransmission timeout enough to allow to lose one (or more)
 *   segments sent by peer and our ACKs. This time may be calculated from RTO.
 * * When TIME-WAIT socket receives RST, it means that another end
 *   finally closed and we are allowed to kill TIME-WAIT too.
 * * Second purpose of TIME-WAIT is catching old duplicate segments.
 *   Well, certainly it is pure paranoia, but if we load TIME-WAIT
 *   with this semantics, we MUST NOT kill TIME-WAIT state with RSTs.
 * * If we invented some more clever way to catch duplicates
 *   (f.e. based on PAWS), we could truncate TIME-WAIT to several RTOs.
 *
 * The algorithm below is based on FORMAL INTERPRETATION of RFCs.
 * When you compare it to RFCs, please, read section SEGMENT ARRIVES
 * from the very beginning.
 *
 * NOTE. With recycling (and later with fin-wait-2) TW bucket
 * is _not_ stateless. It means, that strictly speaking we must
 * spinlock it. I do not want! Well, probability of misbehaviour
 * is ridiculously low and, seems, we could use some mb() tricks
 * to avoid misread sequence numbers, states etc.  --ANK
 *
 * We don't need to initialize tmp_out.sack_ok as we don't use the results
 */
enum tcp_tw_status
tcp_timewait_state_process(struct inet_timewait_sock *tw, struct sk_buff *skb,
			   const struct tcphdr *th)
{
	struct tcp_options_received tmp_opt;
	struct tcp_timewait_sock *tcptw = tcp_twsk((struct sock *)tw);
	bool paws_reject = false;

	tmp_opt.saw_tstamp = 0;
	if (th->doff > (sizeof(*th) >> 2) && tcptw->tw_ts_recent_stamp) {
		tcp_parse_options(twsk_net(tw), skb, &tmp_opt, 0, NULL);

		if (tmp_opt.saw_tstamp) {
			if (tmp_opt.rcv_tsecr)
				tmp_opt.rcv_tsecr -= tcptw->tw_ts_offset;
			tmp_opt.ts_recent	= tcptw->tw_ts_recent;
			tmp_opt.ts_recent_stamp	= tcptw->tw_ts_recent_stamp;
			paws_reject = tcp_paws_reject(&tmp_opt, th->rst);
		}
	}

	if (tw->tw_substate == TCP_FIN_WAIT2) {
		/* Just repeat all the checks of tcp_rcv_state_process() */

		/* Out of window, send ACK */
		if (paws_reject ||
		    !tcp_in_window(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq,
				   tcptw->tw_rcv_nxt,
				   tcptw->tw_rcv_nxt + tcptw->tw_rcv_wnd))
			return tcp_timewait_check_oow_rate_limit(
				tw, skb, LINUX_MIB_TCPACKSKIPPEDFINWAIT2);

		if (th->rst)
			goto kill;

		if (th->syn && !before(TCP_SKB_CB(skb)->seq, tcptw->tw_rcv_nxt))
			return TCP_TW_RST;

		/* Dup ACK? */
		if (!th->ack ||
		    !after(TCP_SKB_CB(skb)->end_seq, tcptw->tw_rcv_nxt) ||
		    TCP_SKB_CB(skb)->end_seq == TCP_SKB_CB(skb)->seq) {
			inet_twsk_put(tw);
			return TCP_TW_SUCCESS;
		}

		/* New data or FIN. If new data arrive after half-duplex close,
		 * reset.
		 */
		if (!th->fin ||
		    TCP_SKB_CB(skb)->end_seq != tcptw->tw_rcv_nxt + 1)
			return TCP_TW_RST;

		/* FIN arrived, enter true time-wait state. */
		tw->tw_substate	  = TCP_TIME_WAIT;
		tcptw->tw_rcv_nxt = TCP_SKB_CB(skb)->end_seq;
		if (tmp_opt.saw_tstamp) {
			tcptw->tw_ts_recent_stamp = get_seconds();
			tcptw->tw_ts_recent	  = tmp_opt.rcv_tsval;
		}

		inet_twsk_reschedule(tw, TCP_TIMEWAIT_LEN);
		return TCP_TW_ACK;
	}

	/*
	 *	Now real TIME-WAIT state.
	 *
	 *	RFC 1122:
	 *	"When a connection is [...] on TIME-WAIT state [...]
	 *	[a TCP] MAY accept a new SYN from the remote TCP to
	 *	reopen the connection directly, if it:
	 *
	 *	(1)  assigns its initial sequence number for the new
	 *	connection to be larger than the largest sequence
	 *	number it used on the previous connection incarnation,
	 *	and
	 *
	 *	(2)  returns to TIME-WAIT state if the SYN turns out
	 *	to be an old duplicate".
	 */

	if (!paws_reject &&
	    (TCP_SKB_CB(skb)->seq == tcptw->tw_rcv_nxt &&
	     (TCP_SKB_CB(skb)->seq == TCP_SKB_CB(skb)->end_seq || th->rst))) {
		/* In window segment, it may be only reset or bare ack. */

		if (th->rst) {
			/* This is TIME_WAIT assassination, in two flavors.
			 * Oh well... nobody has a sufficient solution to this
			 * protocol bug yet.
			 */
			if (twsk_net(tw)->ipv4.sysctl_tcp_rfc1337 == 0) {
kill:
				inet_twsk_deschedule_put(tw);
				return TCP_TW_SUCCESS;
			}
		}
		inet_twsk_reschedule(tw, TCP_TIMEWAIT_LEN);

		if (tmp_opt.saw_tstamp) {
			tcptw->tw_ts_recent	  = tmp_opt.rcv_tsval;
			tcptw->tw_ts_recent_stamp = get_seconds();
		}

		inet_twsk_put(tw);
		return TCP_TW_SUCCESS;
	}

	/* Out of window segment.

	   All the segments are ACKed immediately.

	   The only exception is new SYN. We accept it, if it is
	   not old duplicate and we are not in danger to be killed
	   by delayed old duplicates. RFC check is that it has
	   newer sequence number works at rates <40Mbit/sec.
	   However, if paws works, it is reliable AND even more,
	   we even may relax silly seq space cutoff.

	   RED-PEN: we violate main RFC requirement, if this SYN will appear
	   old duplicate (i.e. we receive RST in reply to SYN-ACK),
	   we must return socket to time-wait state. It is not good,
	   but not fatal yet.
	 */

	if (th->syn && !th->rst && !th->ack && !paws_reject &&
	    (after(TCP_SKB_CB(skb)->seq, tcptw->tw_rcv_nxt) ||
	     (tmp_opt.saw_tstamp &&
	      (s32)(tcptw->tw_ts_recent - tmp_opt.rcv_tsval) < 0))) {
		u32 isn = tcptw->tw_snd_nxt + 65535 + 2;
		if (isn == 0)
			isn++;
		TCP_SKB_CB(skb)->tcp_tw_isn = isn;
		return TCP_TW_SYN;
	}

	if (paws_reject)
		__NET_INC_STATS(twsk_net(tw), LINUX_MIB_PAWSESTABREJECTED);

	if (!th->rst) {
		/* In this case we must reset the TIMEWAIT timer.
		 *
		 * If it is ACKless SYN it may be both old duplicate
		 * and new good SYN with random sequence number <rcv_nxt.
		 * Do not reschedule in the last case.
		 */
		if (paws_reject || th->ack)
			inet_twsk_reschedule(tw, TCP_TIMEWAIT_LEN);

		return tcp_timewait_check_oow_rate_limit(
			tw, skb, LINUX_MIB_TCPACKSKIPPEDTIMEWAIT);
	}
	inet_twsk_put(tw);
	return TCP_TW_SUCCESS;
}
EXPORT_SYMBOL(tcp_timewait_state_process);

/*
 * Move a socket to time-wait or dead fin-wait-2 state.
 */
void tcp_time_wait(struct sock *sk, int state, int timeo)
{
	const struct inet_connection_sock *icsk = inet_csk(sk);
	const struct tcp_sock *tp = tcp_sk(sk);
	struct inet_timewait_sock *tw;
	struct inet_timewait_death_row *tcp_death_row = &sock_net(sk)->ipv4.tcp_death_row;

	tw = inet_twsk_alloc(sk, tcp_death_row, state);

	if (tw) {
		struct tcp_timewait_sock *tcptw = tcp_twsk((struct sock *)tw);
		const int rto = (icsk->icsk_rto << 2) - (icsk->icsk_rto >> 1);
		struct inet_sock *inet = inet_sk(sk);

		tw->tw_transparent	= inet->transparent;
		tw->tw_rcv_wscale	= tp->rx_opt.rcv_wscale;
		tcptw->tw_rcv_nxt	= tp->rcv_nxt;
		tcptw->tw_snd_nxt	= tp->snd_nxt;
		tcptw->tw_rcv_wnd	= tcp_receive_window(tp);
		tcptw->tw_ts_recent	= tp->rx_opt.ts_recent;
		tcptw->tw_ts_recent_stamp = tp->rx_opt.ts_recent_stamp;
		tcptw->tw_ts_offset	= tp->tsoffset;
		tcptw->tw_last_oow_ack_time = 0;

#if IS_ENABLED(CONFIG_IPV6)
		if (tw->tw_family == PF_INET6) {
			struct ipv6_pinfo *np = inet6_sk(sk);

			tw->tw_v6_daddr = sk->sk_v6_daddr;
			tw->tw_v6_rcv_saddr = sk->sk_v6_rcv_saddr;
			tw->tw_tclass = np->tclass;
			tw->tw_flowlabel = be32_to_cpu(np->flow_label & IPV6_FLOWLABEL_MASK);
			tw->tw_ipv6only = sk->sk_ipv6only;
		}
#endif

#ifdef CONFIG_TCP_MD5SIG
		/*
		 * The timewait bucket does not have the key DB from the
		 * sock structure. We just make a quick copy of the
		 * md5 key being used (if indeed we are using one)
		 * so the timewait ack generating code has the key.
		 */
		do {
			struct tcp_md5sig_key *key;
			tcptw->tw_md5_key = NULL;
			key = tp->af_specific->md5_lookup(sk, sk);
			if (key) {
				tcptw->tw_md5_key = kmemdup(key, sizeof(*key), GFP_ATOMIC);
				BUG_ON(tcptw->tw_md5_key && !tcp_alloc_md5sig_pool());
			}
		} while (0);
#endif

		/* Get the TIME_WAIT timeout firing. */
		if (timeo < rto)
			timeo = rto;

		tw->tw_timeout = TCP_TIMEWAIT_LEN;
		if (state == TCP_TIME_WAIT)
			timeo = TCP_TIMEWAIT_LEN;

		/* tw_timer is pinned, so we need to make sure BH are disabled
		 * in following section, otherwise timer handler could run before
		 * we complete the initialization.
		 */
		local_bh_disable();
		inet_twsk_schedule(tw, timeo);
		/* Linkage updates.
		 * Note that access to tw after this point is illegal.
		 */
		inet_twsk_hashdance(tw, sk, &tcp_hashinfo);
		local_bh_enable();
	} else {
		/* Sorry, if we're out of memory, just CLOSE this
		 * socket up.  We've got bigger problems than
		 * non-graceful socket closings.
		 */
		NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPTIMEWAITOVERFLOW);
	}

	tcp_update_metrics(sk);
	tcp_done(sk);
}
EXPORT_SYMBOL(tcp_time_wait);

void tcp_twsk_destructor(struct sock *sk)
{
#ifdef CONFIG_TCP_MD5SIG
	struct tcp_timewait_sock *twsk = tcp_twsk(sk);

	if (twsk->tw_md5_key)
		kfree_rcu(twsk->tw_md5_key, rcu);
#endif
}
EXPORT_SYMBOL_GPL(tcp_twsk_destructor);

/* Warning : This function is called without sk_listener being locked.
 * Be sure to read socket fields once, as their value could change under us.
 */
void tcp_openreq_init_rwin(struct request_sock *req,
			   const struct sock *sk_listener,
			   const struct dst_entry *dst)
{
	struct inet_request_sock *ireq = inet_rsk(req);
	const struct tcp_sock *tp = tcp_sk(sk_listener);
	int full_space = tcp_full_space(sk_listener);
	u32 window_clamp;
	__u8 rcv_wscale;
	u32 rcv_wnd;
	int mss;

	mss = tcp_mss_clamp(tp, dst_metric_advmss(dst));
	window_clamp = READ_ONCE(tp->window_clamp);
	/* Set this up on the first call only */
	req->rsk_window_clamp = window_clamp ? : dst_metric(dst, RTAX_WINDOW);

	/* limit the window selection if the user enforce a smaller rx buffer */
	if (sk_listener->sk_userlocks & SOCK_RCVBUF_LOCK &&
	    (req->rsk_window_clamp > full_space || req->rsk_window_clamp == 0))
		req->rsk_window_clamp = full_space;

	rcv_wnd = tcp_rwnd_init_bpf((struct sock *)req);
	if (rcv_wnd == 0)
		rcv_wnd = dst_metric(dst, RTAX_INITRWND);
	else if (full_space < rcv_wnd * mss)
		full_space = rcv_wnd * mss;

	/* tcp_full_space because it is guaranteed to be the first packet */
	tcp_select_initial_window(sk_listener, full_space,
		mss - (ireq->tstamp_ok ? TCPOLEN_TSTAMP_ALIGNED : 0),
		&req->rsk_rcv_wnd,
		&req->rsk_window_clamp,
		ireq->wscale_ok,
		&rcv_wscale,
		rcv_wnd);
	ireq->rcv_wscale = rcv_wscale;
}
EXPORT_SYMBOL(tcp_openreq_init_rwin);

static void tcp_ecn_openreq_child(struct tcp_sock *tp,
				  const struct request_sock *req)
{
	tp->ecn_flags = inet_rsk(req)->ecn_ok ? TCP_ECN_OK : 0;
}

void tcp_ca_openreq_child(struct sock *sk, const struct dst_entry *dst)
{
	struct inet_connection_sock *icsk = inet_csk(sk);
	u32 ca_key = dst_metric(dst, RTAX_CC_ALGO);
	bool ca_got_dst = false;

	if (ca_key != TCP_CA_UNSPEC) {
		const struct tcp_congestion_ops *ca;

		rcu_read_lock();
		ca = tcp_ca_find_key(ca_key);
		if (likely(ca && try_module_get(ca->owner))) {
			icsk->icsk_ca_dst_locked = tcp_ca_dst_locked(dst);
			icsk->icsk_ca_ops = ca;
			ca_got_dst = true;
		}
		rcu_read_unlock();
	}

	/* If no valid choice made yet, assign current system default ca. */
	if (!ca_got_dst &&
	    (!icsk->icsk_ca_setsockopt ||
	     !try_module_get(icsk->icsk_ca_ops->owner)))
		tcp_assign_congestion_control(sk);

	tcp_set_ca_state(sk, TCP_CA_Open);
}
EXPORT_SYMBOL_GPL(tcp_ca_openreq_child);

static void smc_check_reset_syn_req(struct tcp_sock *oldtp,
				    struct request_sock *req,
				    struct tcp_sock *newtp)
{
#if IS_ENABLED(CONFIG_SMC)
	struct inet_request_sock *ireq;

	if (static_branch_unlikely(&tcp_have_smc)) {
		ireq = inet_rsk(req);
		if (oldtp->syn_smc && !ireq->smc_ok)
			newtp->syn_smc = 0;
	}
#endif
}

/* This is not only more efficient than what we used to do, it eliminates
 * a lot of code duplication between IPv4/IPv6 SYN recv processing. -DaveM
 *
 * Actually, we could lots of memory writes here. tp of listening
 * socket contains all necessary default parameters.
 */
struct sock *tcp_create_openreq_child(const struct sock *sk,
				      struct request_sock *req,
				      struct sk_buff *skb)
{
	struct sock *newsk = inet_csk_clone_lock(sk, req, GFP_ATOMIC);

	if (newsk) {
		const struct inet_request_sock *ireq = inet_rsk(req);
		struct tcp_request_sock *treq = tcp_rsk(req);
		struct inet_connection_sock *newicsk = inet_csk(newsk);
		struct tcp_sock *newtp = tcp_sk(newsk);
		struct tcp_sock *oldtp = tcp_sk(sk);

		smc_check_reset_syn_req(oldtp, req, newtp);

		/* Now setup tcp_sock */
		newtp->pred_flags = 0;

		newtp->rcv_wup = newtp->copied_seq =
		newtp->rcv_nxt = treq->rcv_isn + 1;
		newtp->segs_in = 1;

		newtp->snd_sml = newtp->snd_una =
		newtp->snd_nxt = newtp->snd_up = treq->snt_isn + 1;

		INIT_LIST_HEAD(&newtp->tsq_node);
		INIT_LIST_HEAD(&newtp->tsorted_sent_queue);

		tcp_init_wl(newtp, treq->rcv_isn);

		newtp->srtt_us = 0;
		newtp->mdev_us = jiffies_to_usecs(TCP_TIMEOUT_INIT);
		minmax_reset(&newtp->rtt_min, tcp_jiffies32, ~0U);
		newicsk->icsk_rto = TCP_TIMEOUT_INIT;
		newicsk->icsk_ack.lrcvtime = tcp_jiffies32;

		newtp->packets_out = 0;
		newtp->retrans_out = 0;
		newtp->sacked_out = 0;
		newtp->snd_ssthresh = TCP_INFINITE_SSTHRESH;
		newtp->tlp_high_seq = 0;
		newtp->lsndtime = tcp_jiffies32;
		newsk->sk_txhash = treq->txhash;
		newtp->last_oow_ack_time = 0;
		newtp->total_retrans = req->num_retrans;

		/* So many TCP implementations out there (incorrectly) count the
		 * initial SYN frame in their delayed-ACK and congestion control
		 * algorithms that we must have the following bandaid to talk
		 * efficiently to them.  -DaveM
		 */
		newtp->snd_cwnd = TCP_INIT_CWND;
		newtp->snd_cwnd_cnt = 0;

		/* There's a bubble in the pipe until at least the first ACK. */
		newtp->app_limited = ~0U;

		tcp_init_xmit_timers(newsk);
		newtp->write_seq = newtp->pushed_seq = treq->snt_isn + 1;

		newtp->rx_opt.saw_tstamp = 0;

		newtp->rx_opt.dsack = 0;
		newtp->rx_opt.num_sacks = 0;

		newtp->urg_data = 0;

		if (sock_flag(newsk, SOCK_KEEPOPEN))
			inet_csk_reset_keepalive_timer(newsk,
						       keepalive_time_when(newtp));

		newtp->rx_opt.tstamp_ok = ireq->tstamp_ok;
		newtp->rx_opt.sack_ok = ireq->sack_ok;
		newtp->window_clamp = req->rsk_window_clamp;
		newtp->rcv_ssthresh = req->rsk_rcv_wnd;
		newtp->rcv_wnd = req->rsk_rcv_wnd;
		newtp->rx_opt.wscale_ok = ireq->wscale_ok;
		if (newtp->rx_opt.wscale_ok) {
			newtp->rx_opt.snd_wscale = ireq->snd_wscale;
			newtp->rx_opt.rcv_wscale = ireq->rcv_wscale;
		} else {
			newtp->rx_opt.snd_wscale = newtp->rx_opt.rcv_wscale = 0;
			newtp->window_clamp = min(newtp->window_clamp, 65535U);
		}
		newtp->snd_wnd = (ntohs(tcp_hdr(skb)->window) <<
				  newtp->rx_opt.snd_wscale);
		newtp->max_window = newtp->snd_wnd;

		if (newtp->rx_opt.tstamp_ok) {
			newtp->rx_opt.ts_recent = req->ts_recent;
			newtp->rx_opt.ts_recent_stamp = get_seconds();
			newtp->tcp_header_len = sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED;
		} else {
			newtp->rx_opt.ts_recent_stamp = 0;
			newtp->tcp_header_len = sizeof(struct tcphdr);
		}
		newtp->tsoffset = treq->ts_off;
#ifdef CONFIG_TCP_MD5SIG
		newtp->md5sig_info = NULL;	/*XXX*/
		if (newtp->af_specific->md5_lookup(sk, newsk))
			newtp->tcp_header_len += TCPOLEN_MD5SIG_ALIGNED;
#endif
		if (skb->len >= TCP_MSS_DEFAULT + newtp->tcp_header_len)
			newicsk->icsk_ack.last_seg_size = skb->len - newtp->tcp_header_len;
		newtp->rx_opt.mss_clamp = req->mss;
		tcp_ecn_openreq_child(newtp, req);
		newtp->fastopen_req = NULL;
		newtp->fastopen_rsk = NULL;
		newtp->syn_data_acked = 0;
		newtp->rack.mstamp = 0;
		newtp->rack.advanced = 0;
		newtp->rack.reo_wnd_steps = 1;
		newtp->rack.last_delivered = 0;
		newtp->rack.reo_wnd_persist = 0;
		newtp->rack.dsack_seen = 0;

		__TCP_INC_STATS(sock_net(sk), TCP_MIB_PASSIVEOPENS);
	}
	return newsk;
}
EXPORT_SYMBOL(tcp_create_openreq_child);

/*
 * Process an incoming packet for SYN_RECV sockets represented as a
 * request_sock. Normally sk is the listener socket but for TFO it
 * points to the child socket.
 *
 * XXX (TFO) - The current impl contains a special check for ack
 * validation and inside tcp_v4_reqsk_send_ack(). Can we do better?
 *
 * We don't need to initialize tmp_opt.sack_ok as we don't use the results
 */

struct sock *tcp_check_req(struct sock *sk, struct sk_buff *skb,
			   struct request_sock *req,
			   bool fastopen, bool *req_stolen)
{
	struct tcp_options_received tmp_opt;
	struct sock *child;
	const struct tcphdr *th = tcp_hdr(skb);
	__be32 flg = tcp_flag_word(th) & (TCP_FLAG_RST|TCP_FLAG_SYN|TCP_FLAG_ACK);
	bool paws_reject = false;
	bool own_req;

	tmp_opt.saw_tstamp = 0;
	if (th->doff > (sizeof(struct tcphdr)>>2)) {
		tcp_parse_options(sock_net(sk), skb, &tmp_opt, 0, NULL);

		if (tmp_opt.saw_tstamp) {
			tmp_opt.ts_recent = req->ts_recent;
			if (tmp_opt.rcv_tsecr)
				tmp_opt.rcv_tsecr -= tcp_rsk(req)->ts_off;
			/* We do not store true stamp, but it is not required,
			 * it can be estimated (approximately)
			 * from another data.
			 */
			tmp_opt.ts_recent_stamp = get_seconds() - ((TCP_TIMEOUT_INIT/HZ)<<req->num_timeout);
			paws_reject = tcp_paws_reject(&tmp_opt, th->rst);
		}
	}

	/* Check for pure retransmitted SYN. */
	if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn &&
	    flg == TCP_FLAG_SYN &&
	    !paws_reject) {
		/*
		 * RFC793 draws (Incorrectly! It was fixed in RFC1122)
		 * this case on figure 6 and figure 8, but formal
		 * protocol description says NOTHING.
		 * To be more exact, it says that we should send ACK,
		 * because this segment (at least, if it has no data)
		 * is out of window.
		 *
		 *  CONCLUSION: RFC793 (even with RFC1122) DOES NOT
		 *  describe SYN-RECV state. All the description
		 *  is wrong, we cannot believe to it and should
		 *  rely only on common sense and implementation
		 *  experience.
		 *
		 * Enforce "SYN-ACK" according to figure 8, figure 6
		 * of RFC793, fixed by RFC1122.
		 *
		 * Note that even if there is new data in the SYN packet
		 * they will be thrown away too.
		 *
		 * Reset timer after retransmitting SYNACK, similar to
		 * the idea of fast retransmit in recovery.
		 */
		if (!tcp_oow_rate_limited(sock_net(sk), skb,
					  LINUX_MIB_TCPACKSKIPPEDSYNRECV,
					  &tcp_rsk(req)->last_oow_ack_time) &&

		    !inet_rtx_syn_ack(sk, req)) {
			unsigned long expires = jiffies;

			expires += min(TCP_TIMEOUT_INIT << req->num_timeout,
				       TCP_RTO_MAX);
			if (!fastopen)
				mod_timer_pending(&req->rsk_timer, expires);
			else
				req->rsk_timer.expires = expires;
		}
		return NULL;
	}

	/* Further reproduces section "SEGMENT ARRIVES"
	   for state SYN-RECEIVED of RFC793.
	   It is broken, however, it does not work only
	   when SYNs are crossed.

	   You would think that SYN crossing is impossible here, since
	   we should have a SYN_SENT socket (from connect()) on our end,
	   but this is not true if the crossed SYNs were sent to both
	   ends by a malicious third party.  We must defend against this,
	   and to do that we first verify the ACK (as per RFC793, page
	   36) and reset if it is invalid.  Is this a true full defense?
	   To convince ourselves, let us consider a way in which the ACK
	   test can still pass in this 'malicious crossed SYNs' case.
	   Malicious sender sends identical SYNs (and thus identical sequence
	   numbers) to both A and B:

		A: gets SYN, seq=7
		B: gets SYN, seq=7

	   By our good fortune, both A and B select the same initial
	   send sequence number of seven :-)

		A: sends SYN|ACK, seq=7, ack_seq=8
		B: sends SYN|ACK, seq=7, ack_seq=8

	   So we are now A eating this SYN|ACK, ACK test passes.  So
	   does sequence test, SYN is truncated, and thus we consider
	   it a bare ACK.

	   If icsk->icsk_accept_queue.rskq_defer_accept, we silently drop this
	   bare ACK.  Otherwise, we create an established connection.  Both
	   ends (listening sockets) accept the new incoming connection and try
	   to talk to each other. 8-)

	   Note: This case is both harmless, and rare.  Possibility is about the
	   same as us discovering intelligent life on another plant tomorrow.

	   But generally, we should (RFC lies!) to accept ACK
	   from SYNACK both here and in tcp_rcv_state_process().
	   tcp_rcv_state_process() does not, hence, we do not too.

	   Note that the case is absolutely generic:
	   we cannot optimize anything here without
	   violating protocol. All the checks must be made
	   before attempt to create socket.
	 */

	/* RFC793 page 36: "If the connection is in any non-synchronized state ...
	 *                  and the incoming segment acknowledges something not yet
	 *                  sent (the segment carries an unacceptable ACK) ...
	 *                  a reset is sent."
	 *
	 * Invalid ACK: reset will be sent by listening socket.
	 * Note that the ACK validity check for a Fast Open socket is done
	 * elsewhere and is checked directly against the child socket rather
	 * than req because user data may have been sent out.
	 */
	if ((flg & TCP_FLAG_ACK) && !fastopen &&
	    (TCP_SKB_CB(skb)->ack_seq !=
	     tcp_rsk(req)->snt_isn + 1))
		return sk;

	/* Also, it would be not so bad idea to check rcv_tsecr, which
	 * is essentially ACK extension and too early or too late values
	 * should cause reset in unsynchronized states.
	 */

	/* RFC793: "first check sequence number". */

	if (paws_reject || !tcp_in_window(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq,
					  tcp_rsk(req)->rcv_nxt, tcp_rsk(req)->rcv_nxt + req->rsk_rcv_wnd)) {
		/* Out of window: send ACK and drop. */
		if (!(flg & TCP_FLAG_RST) &&
		    !tcp_oow_rate_limited(sock_net(sk), skb,
					  LINUX_MIB_TCPACKSKIPPEDSYNRECV,
					  &tcp_rsk(req)->last_oow_ack_time))
			req->rsk_ops->send_ack(sk, skb, req);
		if (paws_reject)
			__NET_INC_STATS(sock_net(sk), LINUX_MIB_PAWSESTABREJECTED);
		return NULL;
	}

	/* In sequence, PAWS is OK. */

	if (tmp_opt.saw_tstamp && !after(TCP_SKB_CB(skb)->seq, tcp_rsk(req)->rcv_nxt))
		req->ts_recent = tmp_opt.rcv_tsval;

	if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn) {
		/* Truncate SYN, it is out of window starting
		   at tcp_rsk(req)->rcv_isn + 1. */
		flg &= ~TCP_FLAG_SYN;
	}

	/* RFC793: "second check the RST bit" and
	 *	   "fourth, check the SYN bit"
	 */
	if (flg & (TCP_FLAG_RST|TCP_FLAG_SYN)) {
		__TCP_INC_STATS(sock_net(sk), TCP_MIB_ATTEMPTFAILS);
		goto embryonic_reset;
	}

	/* ACK sequence verified above, just make sure ACK is
	 * set.  If ACK not set, just silently drop the packet.
	 *
	 * XXX (TFO) - if we ever allow "data after SYN", the
	 * following check needs to be removed.
	 */
	if (!(flg & TCP_FLAG_ACK))
		return NULL;

	/* For Fast Open no more processing is needed (sk is the
	 * child socket).
	 */
	if (fastopen)
		return sk;

	/* While TCP_DEFER_ACCEPT is active, drop bare ACK. */
	if (req->num_timeout < inet_csk(sk)->icsk_accept_queue.rskq_defer_accept &&
	    TCP_SKB_CB(skb)->end_seq == tcp_rsk(req)->rcv_isn + 1) {
		inet_rsk(req)->acked = 1;
		__NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPDEFERACCEPTDROP);
		return NULL;
	}

	/* OK, ACK is valid, create big socket and
	 * feed this segment to it. It will repeat all
	 * the tests. THIS SEGMENT MUST MOVE SOCKET TO
	 * ESTABLISHED STATE. If it will be dropped after
	 * socket is created, wait for troubles.
	 */
	child = inet_csk(sk)->icsk_af_ops->syn_recv_sock(sk, skb, req, NULL,
							 req, &own_req);
	if (!child)
		goto listen_overflow;

	sock_rps_save_rxhash(child, skb);
	tcp_synack_rtt_meas(child, req);
	*req_stolen = !own_req;
	return inet_csk_complete_hashdance(sk, child, req, own_req);

listen_overflow:
	if (!sock_net(sk)->ipv4.sysctl_tcp_abort_on_overflow) {
		inet_rsk(req)->acked = 1;
		return NULL;
	}

embryonic_reset:
	if (!(flg & TCP_FLAG_RST)) {
		/* Received a bad SYN pkt - for TFO We try not to reset
		 * the local connection unless it's really necessary to
		 * avoid becoming vulnerable to outside attack aiming at
		 * resetting legit local connections.
		 */
		req->rsk_ops->send_reset(sk, skb);
	} else if (fastopen) { /* received a valid RST pkt */
		reqsk_fastopen_remove(sk, req, true);
		tcp_reset(sk);
	}
	if (!fastopen) {
		inet_csk_reqsk_queue_drop(sk, req);
		__NET_INC_STATS(sock_net(sk), LINUX_MIB_EMBRYONICRSTS);
	}
	return NULL;
}
EXPORT_SYMBOL(tcp_check_req);

/*
 * Queue segment on the new socket if the new socket is active,
 * otherwise we just shortcircuit this and continue with
 * the new socket.
 *
 * For the vast majority of cases child->sk_state will be TCP_SYN_RECV
 * when entering. But other states are possible due to a race condition
 * where after __inet_lookup_established() fails but before the listener
 * locked is obtained, other packets cause the same connection to
 * be created.
 */

int tcp_child_process(struct sock *parent, struct sock *child,
		      struct sk_buff *skb)
{
	int ret = 0;
	int state = child->sk_state;

	/* record NAPI ID of child */
	sk_mark_napi_id(child, skb);

	tcp_segs_in(tcp_sk(child), skb);
	if (!sock_owned_by_user(child)) {
		ret = tcp_rcv_state_process(child, skb);
		/* Wakeup parent, send SIGIO */
		if (state == TCP_SYN_RECV && child->sk_state != state)
			parent->sk_data_ready(parent);
	} else {
		/* Alas, it is possible again, because we do lookup
		 * in main socket hash table and lock on listening
		 * socket does not protect us more.
		 */
		__sk_add_backlog(child, skb);
	}

	bh_unlock_sock(child);
	sock_put(child);
	return ret;
}
EXPORT_SYMBOL(tcp_child_process);