xref: /openbmc/linux/net/ipv4/tcp_minisocks.c (revision b6dcefde)
1 /*
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.
5  *
6  *		Implementation of the Transmission Control Protocol(TCP).
7  *
8  * Authors:	Ross Biro
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>
19  */
20 
21 #include <linux/mm.h>
22 #include <linux/module.h>
23 #include <linux/sysctl.h>
24 #include <linux/workqueue.h>
25 #include <net/tcp.h>
26 #include <net/inet_common.h>
27 #include <net/xfrm.h>
28 
29 int sysctl_tcp_syncookies __read_mostly = 1;
30 EXPORT_SYMBOL(sysctl_tcp_syncookies);
31 
32 int sysctl_tcp_abort_on_overflow __read_mostly;
33 
34 struct inet_timewait_death_row tcp_death_row = {
35 	.sysctl_max_tw_buckets = NR_FILE * 2,
36 	.period		= TCP_TIMEWAIT_LEN / INET_TWDR_TWKILL_SLOTS,
37 	.death_lock	= __SPIN_LOCK_UNLOCKED(tcp_death_row.death_lock),
38 	.hashinfo	= &tcp_hashinfo,
39 	.tw_timer	= TIMER_INITIALIZER(inet_twdr_hangman, 0,
40 					    (unsigned long)&tcp_death_row),
41 	.twkill_work	= __WORK_INITIALIZER(tcp_death_row.twkill_work,
42 					     inet_twdr_twkill_work),
43 /* Short-time timewait calendar */
44 
45 	.twcal_hand	= -1,
46 	.twcal_timer	= TIMER_INITIALIZER(inet_twdr_twcal_tick, 0,
47 					    (unsigned long)&tcp_death_row),
48 };
49 
50 EXPORT_SYMBOL_GPL(tcp_death_row);
51 
52 static __inline__ int tcp_in_window(u32 seq, u32 end_seq, u32 s_win, u32 e_win)
53 {
54 	if (seq == s_win)
55 		return 1;
56 	if (after(end_seq, s_win) && before(seq, e_win))
57 		return 1;
58 	return (seq == e_win && seq == end_seq);
59 }
60 
61 /*
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.
78  *
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.
82  *
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
88  */
89 enum tcp_tw_status
90 tcp_timewait_state_process(struct inet_timewait_sock *tw, struct sk_buff *skb,
91 			   const struct tcphdr *th)
92 {
93 	struct tcp_options_received tmp_opt;
94 	u8 *hash_location;
95 	struct tcp_timewait_sock *tcptw = tcp_twsk((struct sock *)tw);
96 	int paws_reject = 0;
97 
98 	tmp_opt.saw_tstamp = 0;
99 	if (th->doff > (sizeof(*th) >> 2) && tcptw->tw_ts_recent_stamp) {
100 		tcp_parse_options(skb, &tmp_opt, &hash_location, 0);
101 
102 		if (tmp_opt.saw_tstamp) {
103 			tmp_opt.ts_recent	= tcptw->tw_ts_recent;
104 			tmp_opt.ts_recent_stamp	= tcptw->tw_ts_recent_stamp;
105 			paws_reject = tcp_paws_reject(&tmp_opt, th->rst);
106 		}
107 	}
108 
109 	if (tw->tw_substate == TCP_FIN_WAIT2) {
110 		/* Just repeat all the checks of tcp_rcv_state_process() */
111 
112 		/* Out of window, send ACK */
113 		if (paws_reject ||
114 		    !tcp_in_window(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq,
115 				   tcptw->tw_rcv_nxt,
116 				   tcptw->tw_rcv_nxt + tcptw->tw_rcv_wnd))
117 			return TCP_TW_ACK;
118 
119 		if (th->rst)
120 			goto kill;
121 
122 		if (th->syn && !before(TCP_SKB_CB(skb)->seq, tcptw->tw_rcv_nxt))
123 			goto kill_with_rst;
124 
125 		/* Dup ACK? */
126 		if (!th->ack ||
127 		    !after(TCP_SKB_CB(skb)->end_seq, tcptw->tw_rcv_nxt) ||
128 		    TCP_SKB_CB(skb)->end_seq == TCP_SKB_CB(skb)->seq) {
129 			inet_twsk_put(tw);
130 			return TCP_TW_SUCCESS;
131 		}
132 
133 		/* New data or FIN. If new data arrive after half-duplex close,
134 		 * reset.
135 		 */
136 		if (!th->fin ||
137 		    TCP_SKB_CB(skb)->end_seq != tcptw->tw_rcv_nxt + 1) {
138 kill_with_rst:
139 			inet_twsk_deschedule(tw, &tcp_death_row);
140 			inet_twsk_put(tw);
141 			return TCP_TW_RST;
142 		}
143 
144 		/* FIN arrived, enter true time-wait state. */
145 		tw->tw_substate	  = TCP_TIME_WAIT;
146 		tcptw->tw_rcv_nxt = TCP_SKB_CB(skb)->end_seq;
147 		if (tmp_opt.saw_tstamp) {
148 			tcptw->tw_ts_recent_stamp = get_seconds();
149 			tcptw->tw_ts_recent	  = tmp_opt.rcv_tsval;
150 		}
151 
152 		/* I am shamed, but failed to make it more elegant.
153 		 * Yes, it is direct reference to IP, which is impossible
154 		 * to generalize to IPv6. Taking into account that IPv6
155 		 * do not understand recycling in any case, it not
156 		 * a big problem in practice. --ANK */
157 		if (tw->tw_family == AF_INET &&
158 		    tcp_death_row.sysctl_tw_recycle && tcptw->tw_ts_recent_stamp &&
159 		    tcp_v4_tw_remember_stamp(tw))
160 			inet_twsk_schedule(tw, &tcp_death_row, tw->tw_timeout,
161 					   TCP_TIMEWAIT_LEN);
162 		else
163 			inet_twsk_schedule(tw, &tcp_death_row, TCP_TIMEWAIT_LEN,
164 					   TCP_TIMEWAIT_LEN);
165 		return TCP_TW_ACK;
166 	}
167 
168 	/*
169 	 *	Now real TIME-WAIT state.
170 	 *
171 	 *	RFC 1122:
172 	 *	"When a connection is [...] on TIME-WAIT state [...]
173 	 *	[a TCP] MAY accept a new SYN from the remote TCP to
174 	 *	reopen the connection directly, if it:
175 	 *
176 	 *	(1)  assigns its initial sequence number for the new
177 	 *	connection to be larger than the largest sequence
178 	 *	number it used on the previous connection incarnation,
179 	 *	and
180 	 *
181 	 *	(2)  returns to TIME-WAIT state if the SYN turns out
182 	 *	to be an old duplicate".
183 	 */
184 
185 	if (!paws_reject &&
186 	    (TCP_SKB_CB(skb)->seq == tcptw->tw_rcv_nxt &&
187 	     (TCP_SKB_CB(skb)->seq == TCP_SKB_CB(skb)->end_seq || th->rst))) {
188 		/* In window segment, it may be only reset or bare ack. */
189 
190 		if (th->rst) {
191 			/* This is TIME_WAIT assassination, in two flavors.
192 			 * Oh well... nobody has a sufficient solution to this
193 			 * protocol bug yet.
194 			 */
195 			if (sysctl_tcp_rfc1337 == 0) {
196 kill:
197 				inet_twsk_deschedule(tw, &tcp_death_row);
198 				inet_twsk_put(tw);
199 				return TCP_TW_SUCCESS;
200 			}
201 		}
202 		inet_twsk_schedule(tw, &tcp_death_row, TCP_TIMEWAIT_LEN,
203 				   TCP_TIMEWAIT_LEN);
204 
205 		if (tmp_opt.saw_tstamp) {
206 			tcptw->tw_ts_recent	  = tmp_opt.rcv_tsval;
207 			tcptw->tw_ts_recent_stamp = get_seconds();
208 		}
209 
210 		inet_twsk_put(tw);
211 		return TCP_TW_SUCCESS;
212 	}
213 
214 	/* Out of window segment.
215 
216 	   All the segments are ACKed immediately.
217 
218 	   The only exception is new SYN. We accept it, if it is
219 	   not old duplicate and we are not in danger to be killed
220 	   by delayed old duplicates. RFC check is that it has
221 	   newer sequence number works at rates <40Mbit/sec.
222 	   However, if paws works, it is reliable AND even more,
223 	   we even may relax silly seq space cutoff.
224 
225 	   RED-PEN: we violate main RFC requirement, if this SYN will appear
226 	   old duplicate (i.e. we receive RST in reply to SYN-ACK),
227 	   we must return socket to time-wait state. It is not good,
228 	   but not fatal yet.
229 	 */
230 
231 	if (th->syn && !th->rst && !th->ack && !paws_reject &&
232 	    (after(TCP_SKB_CB(skb)->seq, tcptw->tw_rcv_nxt) ||
233 	     (tmp_opt.saw_tstamp &&
234 	      (s32)(tcptw->tw_ts_recent - tmp_opt.rcv_tsval) < 0))) {
235 		u32 isn = tcptw->tw_snd_nxt + 65535 + 2;
236 		if (isn == 0)
237 			isn++;
238 		TCP_SKB_CB(skb)->when = isn;
239 		return TCP_TW_SYN;
240 	}
241 
242 	if (paws_reject)
243 		NET_INC_STATS_BH(twsk_net(tw), LINUX_MIB_PAWSESTABREJECTED);
244 
245 	if (!th->rst) {
246 		/* In this case we must reset the TIMEWAIT timer.
247 		 *
248 		 * If it is ACKless SYN it may be both old duplicate
249 		 * and new good SYN with random sequence number <rcv_nxt.
250 		 * Do not reschedule in the last case.
251 		 */
252 		if (paws_reject || th->ack)
253 			inet_twsk_schedule(tw, &tcp_death_row, TCP_TIMEWAIT_LEN,
254 					   TCP_TIMEWAIT_LEN);
255 
256 		/* Send ACK. Note, we do not put the bucket,
257 		 * it will be released by caller.
258 		 */
259 		return TCP_TW_ACK;
260 	}
261 	inet_twsk_put(tw);
262 	return TCP_TW_SUCCESS;
263 }
264 
265 /*
266  * Move a socket to time-wait or dead fin-wait-2 state.
267  */
268 void tcp_time_wait(struct sock *sk, int state, int timeo)
269 {
270 	struct inet_timewait_sock *tw = NULL;
271 	const struct inet_connection_sock *icsk = inet_csk(sk);
272 	const struct tcp_sock *tp = tcp_sk(sk);
273 	int recycle_ok = 0;
274 
275 	if (tcp_death_row.sysctl_tw_recycle && tp->rx_opt.ts_recent_stamp)
276 		recycle_ok = icsk->icsk_af_ops->remember_stamp(sk);
277 
278 	if (tcp_death_row.tw_count < tcp_death_row.sysctl_max_tw_buckets)
279 		tw = inet_twsk_alloc(sk, state);
280 
281 	if (tw != NULL) {
282 		struct tcp_timewait_sock *tcptw = tcp_twsk((struct sock *)tw);
283 		const int rto = (icsk->icsk_rto << 2) - (icsk->icsk_rto >> 1);
284 
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 
292 #if defined(CONFIG_IPV6) || defined(CONFIG_IPV6_MODULE)
293 		if (tw->tw_family == PF_INET6) {
294 			struct ipv6_pinfo *np = inet6_sk(sk);
295 			struct inet6_timewait_sock *tw6;
296 
297 			tw->tw_ipv6_offset = inet6_tw_offset(sk->sk_prot);
298 			tw6 = inet6_twsk((struct sock *)tw);
299 			ipv6_addr_copy(&tw6->tw_v6_daddr, &np->daddr);
300 			ipv6_addr_copy(&tw6->tw_v6_rcv_saddr, &np->rcv_saddr);
301 			tw->tw_ipv6only = np->ipv6only;
302 		}
303 #endif
304 
305 #ifdef CONFIG_TCP_MD5SIG
306 		/*
307 		 * The timewait bucket does not have the key DB from the
308 		 * sock structure. We just make a quick copy of the
309 		 * md5 key being used (if indeed we are using one)
310 		 * so the timewait ack generating code has the key.
311 		 */
312 		do {
313 			struct tcp_md5sig_key *key;
314 			memset(tcptw->tw_md5_key, 0, sizeof(tcptw->tw_md5_key));
315 			tcptw->tw_md5_keylen = 0;
316 			key = tp->af_specific->md5_lookup(sk, sk);
317 			if (key != NULL) {
318 				memcpy(&tcptw->tw_md5_key, key->key, key->keylen);
319 				tcptw->tw_md5_keylen = key->keylen;
320 				if (tcp_alloc_md5sig_pool(sk) == NULL)
321 					BUG();
322 			}
323 		} while (0);
324 #endif
325 
326 		/* Linkage updates. */
327 		__inet_twsk_hashdance(tw, sk, &tcp_hashinfo);
328 
329 		/* Get the TIME_WAIT timeout firing. */
330 		if (timeo < rto)
331 			timeo = rto;
332 
333 		if (recycle_ok) {
334 			tw->tw_timeout = rto;
335 		} else {
336 			tw->tw_timeout = TCP_TIMEWAIT_LEN;
337 			if (state == TCP_TIME_WAIT)
338 				timeo = TCP_TIMEWAIT_LEN;
339 		}
340 
341 		inet_twsk_schedule(tw, &tcp_death_row, timeo,
342 				   TCP_TIMEWAIT_LEN);
343 		inet_twsk_put(tw);
344 	} else {
345 		/* Sorry, if we're out of memory, just CLOSE this
346 		 * socket up.  We've got bigger problems than
347 		 * non-graceful socket closings.
348 		 */
349 		LIMIT_NETDEBUG(KERN_INFO "TCP: time wait bucket table overflow\n");
350 	}
351 
352 	tcp_update_metrics(sk);
353 	tcp_done(sk);
354 }
355 
356 void tcp_twsk_destructor(struct sock *sk)
357 {
358 #ifdef CONFIG_TCP_MD5SIG
359 	struct tcp_timewait_sock *twsk = tcp_twsk(sk);
360 	if (twsk->tw_md5_keylen)
361 		tcp_free_md5sig_pool();
362 #endif
363 }
364 
365 EXPORT_SYMBOL_GPL(tcp_twsk_destructor);
366 
367 static inline void TCP_ECN_openreq_child(struct tcp_sock *tp,
368 					 struct request_sock *req)
369 {
370 	tp->ecn_flags = inet_rsk(req)->ecn_ok ? TCP_ECN_OK : 0;
371 }
372 
373 /* This is not only more efficient than what we used to do, it eliminates
374  * a lot of code duplication between IPv4/IPv6 SYN recv processing. -DaveM
375  *
376  * Actually, we could lots of memory writes here. tp of listening
377  * socket contains all necessary default parameters.
378  */
379 struct sock *tcp_create_openreq_child(struct sock *sk, struct request_sock *req, struct sk_buff *skb)
380 {
381 	struct sock *newsk = inet_csk_clone(sk, req, GFP_ATOMIC);
382 
383 	if (newsk != NULL) {
384 		const struct inet_request_sock *ireq = inet_rsk(req);
385 		struct tcp_request_sock *treq = tcp_rsk(req);
386 		struct inet_connection_sock *newicsk = inet_csk(newsk);
387 		struct tcp_sock *newtp = tcp_sk(newsk);
388 		struct tcp_sock *oldtp = tcp_sk(sk);
389 		struct tcp_cookie_values *oldcvp = oldtp->cookie_values;
390 
391 		/* TCP Cookie Transactions require space for the cookie pair,
392 		 * as it differs for each connection.  There is no need to
393 		 * copy any s_data_payload stored at the original socket.
394 		 * Failure will prevent resuming the connection.
395 		 *
396 		 * Presumed copied, in order of appearance:
397 		 *	cookie_in_always, cookie_out_never
398 		 */
399 		if (oldcvp != NULL) {
400 			struct tcp_cookie_values *newcvp =
401 				kzalloc(sizeof(*newtp->cookie_values),
402 					GFP_ATOMIC);
403 
404 			if (newcvp != NULL) {
405 				kref_init(&newcvp->kref);
406 				newcvp->cookie_desired =
407 						oldcvp->cookie_desired;
408 				newtp->cookie_values = newcvp;
409 			} else {
410 				/* Not Yet Implemented */
411 				newtp->cookie_values = NULL;
412 			}
413 		}
414 
415 		/* Now setup tcp_sock */
416 		newtp->pred_flags = 0;
417 
418 		newtp->rcv_wup = newtp->copied_seq =
419 		newtp->rcv_nxt = treq->rcv_isn + 1;
420 
421 		newtp->snd_sml = newtp->snd_una =
422 		newtp->snd_nxt = newtp->snd_up =
423 			treq->snt_isn + 1 + tcp_s_data_size(oldtp);
424 
425 		tcp_prequeue_init(newtp);
426 
427 		tcp_init_wl(newtp, treq->rcv_isn);
428 
429 		newtp->srtt = 0;
430 		newtp->mdev = TCP_TIMEOUT_INIT;
431 		newicsk->icsk_rto = TCP_TIMEOUT_INIT;
432 
433 		newtp->packets_out = 0;
434 		newtp->retrans_out = 0;
435 		newtp->sacked_out = 0;
436 		newtp->fackets_out = 0;
437 		newtp->snd_ssthresh = TCP_INFINITE_SSTHRESH;
438 
439 		/* So many TCP implementations out there (incorrectly) count the
440 		 * initial SYN frame in their delayed-ACK and congestion control
441 		 * algorithms that we must have the following bandaid to talk
442 		 * efficiently to them.  -DaveM
443 		 */
444 		newtp->snd_cwnd = 2;
445 		newtp->snd_cwnd_cnt = 0;
446 		newtp->bytes_acked = 0;
447 
448 		newtp->frto_counter = 0;
449 		newtp->frto_highmark = 0;
450 
451 		newicsk->icsk_ca_ops = &tcp_init_congestion_ops;
452 
453 		tcp_set_ca_state(newsk, TCP_CA_Open);
454 		tcp_init_xmit_timers(newsk);
455 		skb_queue_head_init(&newtp->out_of_order_queue);
456 		newtp->write_seq = newtp->pushed_seq =
457 			treq->snt_isn + 1 + tcp_s_data_size(oldtp);
458 
459 		newtp->rx_opt.saw_tstamp = 0;
460 
461 		newtp->rx_opt.dsack = 0;
462 		newtp->rx_opt.num_sacks = 0;
463 
464 		newtp->urg_data = 0;
465 
466 		if (sock_flag(newsk, SOCK_KEEPOPEN))
467 			inet_csk_reset_keepalive_timer(newsk,
468 						       keepalive_time_when(newtp));
469 
470 		newtp->rx_opt.tstamp_ok = ireq->tstamp_ok;
471 		if ((newtp->rx_opt.sack_ok = ireq->sack_ok) != 0) {
472 			if (sysctl_tcp_fack)
473 				tcp_enable_fack(newtp);
474 		}
475 		newtp->window_clamp = req->window_clamp;
476 		newtp->rcv_ssthresh = req->rcv_wnd;
477 		newtp->rcv_wnd = req->rcv_wnd;
478 		newtp->rx_opt.wscale_ok = ireq->wscale_ok;
479 		if (newtp->rx_opt.wscale_ok) {
480 			newtp->rx_opt.snd_wscale = ireq->snd_wscale;
481 			newtp->rx_opt.rcv_wscale = ireq->rcv_wscale;
482 		} else {
483 			newtp->rx_opt.snd_wscale = newtp->rx_opt.rcv_wscale = 0;
484 			newtp->window_clamp = min(newtp->window_clamp, 65535U);
485 		}
486 		newtp->snd_wnd = (ntohs(tcp_hdr(skb)->window) <<
487 				  newtp->rx_opt.snd_wscale);
488 		newtp->max_window = newtp->snd_wnd;
489 
490 		if (newtp->rx_opt.tstamp_ok) {
491 			newtp->rx_opt.ts_recent = req->ts_recent;
492 			newtp->rx_opt.ts_recent_stamp = get_seconds();
493 			newtp->tcp_header_len = sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED;
494 		} else {
495 			newtp->rx_opt.ts_recent_stamp = 0;
496 			newtp->tcp_header_len = sizeof(struct tcphdr);
497 		}
498 #ifdef CONFIG_TCP_MD5SIG
499 		newtp->md5sig_info = NULL;	/*XXX*/
500 		if (newtp->af_specific->md5_lookup(sk, newsk))
501 			newtp->tcp_header_len += TCPOLEN_MD5SIG_ALIGNED;
502 #endif
503 		if (skb->len >= TCP_MSS_DEFAULT + newtp->tcp_header_len)
504 			newicsk->icsk_ack.last_seg_size = skb->len - newtp->tcp_header_len;
505 		newtp->rx_opt.mss_clamp = req->mss;
506 		TCP_ECN_openreq_child(newtp, req);
507 
508 		TCP_INC_STATS_BH(sock_net(sk), TCP_MIB_PASSIVEOPENS);
509 	}
510 	return newsk;
511 }
512 
513 /*
514  *	Process an incoming packet for SYN_RECV sockets represented
515  *	as a request_sock.
516  */
517 
518 struct sock *tcp_check_req(struct sock *sk, struct sk_buff *skb,
519 			   struct request_sock *req,
520 			   struct request_sock **prev)
521 {
522 	struct tcp_options_received tmp_opt;
523 	u8 *hash_location;
524 	struct sock *child;
525 	const struct tcphdr *th = tcp_hdr(skb);
526 	__be32 flg = tcp_flag_word(th) & (TCP_FLAG_RST|TCP_FLAG_SYN|TCP_FLAG_ACK);
527 	int paws_reject = 0;
528 
529 	tmp_opt.saw_tstamp = 0;
530 	if (th->doff > (sizeof(struct tcphdr)>>2)) {
531 		tcp_parse_options(skb, &tmp_opt, &hash_location, 0);
532 
533 		if (tmp_opt.saw_tstamp) {
534 			tmp_opt.ts_recent = req->ts_recent;
535 			/* We do not store true stamp, but it is not required,
536 			 * it can be estimated (approximately)
537 			 * from another data.
538 			 */
539 			tmp_opt.ts_recent_stamp = get_seconds() - ((TCP_TIMEOUT_INIT/HZ)<<req->retrans);
540 			paws_reject = tcp_paws_reject(&tmp_opt, th->rst);
541 		}
542 	}
543 
544 	/* Check for pure retransmitted SYN. */
545 	if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn &&
546 	    flg == TCP_FLAG_SYN &&
547 	    !paws_reject) {
548 		/*
549 		 * RFC793 draws (Incorrectly! It was fixed in RFC1122)
550 		 * this case on figure 6 and figure 8, but formal
551 		 * protocol description says NOTHING.
552 		 * To be more exact, it says that we should send ACK,
553 		 * because this segment (at least, if it has no data)
554 		 * is out of window.
555 		 *
556 		 *  CONCLUSION: RFC793 (even with RFC1122) DOES NOT
557 		 *  describe SYN-RECV state. All the description
558 		 *  is wrong, we cannot believe to it and should
559 		 *  rely only on common sense and implementation
560 		 *  experience.
561 		 *
562 		 * Enforce "SYN-ACK" according to figure 8, figure 6
563 		 * of RFC793, fixed by RFC1122.
564 		 */
565 		req->rsk_ops->rtx_syn_ack(sk, req, NULL);
566 		return NULL;
567 	}
568 
569 	/* Further reproduces section "SEGMENT ARRIVES"
570 	   for state SYN-RECEIVED of RFC793.
571 	   It is broken, however, it does not work only
572 	   when SYNs are crossed.
573 
574 	   You would think that SYN crossing is impossible here, since
575 	   we should have a SYN_SENT socket (from connect()) on our end,
576 	   but this is not true if the crossed SYNs were sent to both
577 	   ends by a malicious third party.  We must defend against this,
578 	   and to do that we first verify the ACK (as per RFC793, page
579 	   36) and reset if it is invalid.  Is this a true full defense?
580 	   To convince ourselves, let us consider a way in which the ACK
581 	   test can still pass in this 'malicious crossed SYNs' case.
582 	   Malicious sender sends identical SYNs (and thus identical sequence
583 	   numbers) to both A and B:
584 
585 		A: gets SYN, seq=7
586 		B: gets SYN, seq=7
587 
588 	   By our good fortune, both A and B select the same initial
589 	   send sequence number of seven :-)
590 
591 		A: sends SYN|ACK, seq=7, ack_seq=8
592 		B: sends SYN|ACK, seq=7, ack_seq=8
593 
594 	   So we are now A eating this SYN|ACK, ACK test passes.  So
595 	   does sequence test, SYN is truncated, and thus we consider
596 	   it a bare ACK.
597 
598 	   If icsk->icsk_accept_queue.rskq_defer_accept, we silently drop this
599 	   bare ACK.  Otherwise, we create an established connection.  Both
600 	   ends (listening sockets) accept the new incoming connection and try
601 	   to talk to each other. 8-)
602 
603 	   Note: This case is both harmless, and rare.  Possibility is about the
604 	   same as us discovering intelligent life on another plant tomorrow.
605 
606 	   But generally, we should (RFC lies!) to accept ACK
607 	   from SYNACK both here and in tcp_rcv_state_process().
608 	   tcp_rcv_state_process() does not, hence, we do not too.
609 
610 	   Note that the case is absolutely generic:
611 	   we cannot optimize anything here without
612 	   violating protocol. All the checks must be made
613 	   before attempt to create socket.
614 	 */
615 
616 	/* RFC793 page 36: "If the connection is in any non-synchronized state ...
617 	 *                  and the incoming segment acknowledges something not yet
618 	 *                  sent (the segment carries an unacceptable ACK) ...
619 	 *                  a reset is sent."
620 	 *
621 	 * Invalid ACK: reset will be sent by listening socket
622 	 */
623 	if ((flg & TCP_FLAG_ACK) &&
624 	    (TCP_SKB_CB(skb)->ack_seq !=
625 	     tcp_rsk(req)->snt_isn + 1 + tcp_s_data_size(tcp_sk(sk))))
626 		return sk;
627 
628 	/* Also, it would be not so bad idea to check rcv_tsecr, which
629 	 * is essentially ACK extension and too early or too late values
630 	 * should cause reset in unsynchronized states.
631 	 */
632 
633 	/* RFC793: "first check sequence number". */
634 
635 	if (paws_reject || !tcp_in_window(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq,
636 					  tcp_rsk(req)->rcv_isn + 1, tcp_rsk(req)->rcv_isn + 1 + req->rcv_wnd)) {
637 		/* Out of window: send ACK and drop. */
638 		if (!(flg & TCP_FLAG_RST))
639 			req->rsk_ops->send_ack(sk, skb, req);
640 		if (paws_reject)
641 			NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_PAWSESTABREJECTED);
642 		return NULL;
643 	}
644 
645 	/* In sequence, PAWS is OK. */
646 
647 	if (tmp_opt.saw_tstamp && !after(TCP_SKB_CB(skb)->seq, tcp_rsk(req)->rcv_isn + 1))
648 		req->ts_recent = tmp_opt.rcv_tsval;
649 
650 	if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn) {
651 		/* Truncate SYN, it is out of window starting
652 		   at tcp_rsk(req)->rcv_isn + 1. */
653 		flg &= ~TCP_FLAG_SYN;
654 	}
655 
656 	/* RFC793: "second check the RST bit" and
657 	 *	   "fourth, check the SYN bit"
658 	 */
659 	if (flg & (TCP_FLAG_RST|TCP_FLAG_SYN)) {
660 		TCP_INC_STATS_BH(sock_net(sk), TCP_MIB_ATTEMPTFAILS);
661 		goto embryonic_reset;
662 	}
663 
664 	/* ACK sequence verified above, just make sure ACK is
665 	 * set.  If ACK not set, just silently drop the packet.
666 	 */
667 	if (!(flg & TCP_FLAG_ACK))
668 		return NULL;
669 
670 	/* While TCP_DEFER_ACCEPT is active, drop bare ACK. */
671 	if (req->retrans < inet_csk(sk)->icsk_accept_queue.rskq_defer_accept &&
672 	    TCP_SKB_CB(skb)->end_seq == tcp_rsk(req)->rcv_isn + 1) {
673 		inet_rsk(req)->acked = 1;
674 		return NULL;
675 	}
676 
677 	/* OK, ACK is valid, create big socket and
678 	 * feed this segment to it. It will repeat all
679 	 * the tests. THIS SEGMENT MUST MOVE SOCKET TO
680 	 * ESTABLISHED STATE. If it will be dropped after
681 	 * socket is created, wait for troubles.
682 	 */
683 	child = inet_csk(sk)->icsk_af_ops->syn_recv_sock(sk, skb, req, NULL);
684 	if (child == NULL)
685 		goto listen_overflow;
686 
687 	inet_csk_reqsk_queue_unlink(sk, req, prev);
688 	inet_csk_reqsk_queue_removed(sk, req);
689 
690 	inet_csk_reqsk_queue_add(sk, req, child);
691 	return child;
692 
693 listen_overflow:
694 	if (!sysctl_tcp_abort_on_overflow) {
695 		inet_rsk(req)->acked = 1;
696 		return NULL;
697 	}
698 
699 embryonic_reset:
700 	NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_EMBRYONICRSTS);
701 	if (!(flg & TCP_FLAG_RST))
702 		req->rsk_ops->send_reset(sk, skb);
703 
704 	inet_csk_reqsk_queue_drop(sk, req, prev);
705 	return NULL;
706 }
707 
708 /*
709  * Queue segment on the new socket if the new socket is active,
710  * otherwise we just shortcircuit this and continue with
711  * the new socket.
712  */
713 
714 int tcp_child_process(struct sock *parent, struct sock *child,
715 		      struct sk_buff *skb)
716 {
717 	int ret = 0;
718 	int state = child->sk_state;
719 
720 	if (!sock_owned_by_user(child)) {
721 		ret = tcp_rcv_state_process(child, skb, tcp_hdr(skb),
722 					    skb->len);
723 		/* Wakeup parent, send SIGIO */
724 		if (state == TCP_SYN_RECV && child->sk_state != state)
725 			parent->sk_data_ready(parent, 0);
726 	} else {
727 		/* Alas, it is possible again, because we do lookup
728 		 * in main socket hash table and lock on listening
729 		 * socket does not protect us more.
730 		 */
731 		sk_add_backlog(child, skb);
732 	}
733 
734 	bh_unlock_sock(child);
735 	sock_put(child);
736 	return ret;
737 }
738 
739 EXPORT_SYMBOL(tcp_check_req);
740 EXPORT_SYMBOL(tcp_child_process);
741 EXPORT_SYMBOL(tcp_create_openreq_child);
742 EXPORT_SYMBOL(tcp_timewait_state_process);
743