[NET]: dev_put/dev_hold cleanup
[linux-2.6.git] / net / ipv4 / tcp_minisocks.c
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  * Version:     $Id: tcp_minisocks.c,v 1.15 2002/02/01 22:01:04 davem Exp $
9  *
10  * Authors:     Ross Biro
11  *              Fred N. van Kempen, <waltje@uWalt.NL.Mugnet.ORG>
12  *              Mark Evans, <evansmp@uhura.aston.ac.uk>
13  *              Corey Minyard <wf-rch!minyard@relay.EU.net>
14  *              Florian La Roche, <flla@stud.uni-sb.de>
15  *              Charles Hedrick, <hedrick@klinzhai.rutgers.edu>
16  *              Linus Torvalds, <torvalds@cs.helsinki.fi>
17  *              Alan Cox, <gw4pts@gw4pts.ampr.org>
18  *              Matthew Dillon, <dillon@apollo.west.oic.com>
19  *              Arnt Gulbrandsen, <agulbra@nvg.unit.no>
20  *              Jorge Cwik, <jorge@laser.satlink.net>
21  */
22
23 #include <linux/config.h>
24 #include <linux/mm.h>
25 #include <linux/module.h>
26 #include <linux/sysctl.h>
27 #include <linux/workqueue.h>
28 #include <net/tcp.h>
29 #include <net/inet_common.h>
30 #include <net/xfrm.h>
31
32 #ifdef CONFIG_SYSCTL
33 #define SYNC_INIT 0 /* let the user enable it */
34 #else
35 #define SYNC_INIT 1
36 #endif
37
38 int sysctl_tcp_syncookies = SYNC_INIT; 
39 int sysctl_tcp_abort_on_overflow;
40
41 struct inet_timewait_death_row tcp_death_row = {
42         .sysctl_max_tw_buckets = NR_FILE * 2,
43         .period         = TCP_TIMEWAIT_LEN / INET_TWDR_TWKILL_SLOTS,
44         .death_lock     = SPIN_LOCK_UNLOCKED,
45         .hashinfo       = &tcp_hashinfo,
46         .tw_timer       = TIMER_INITIALIZER(inet_twdr_hangman, 0,
47                                             (unsigned long)&tcp_death_row),
48         .twkill_work    = __WORK_INITIALIZER(tcp_death_row.twkill_work,
49                                              inet_twdr_twkill_work,
50                                              &tcp_death_row),
51 /* Short-time timewait calendar */
52
53         .twcal_hand     = -1,
54         .twcal_timer    = TIMER_INITIALIZER(inet_twdr_twcal_tick, 0,
55                                             (unsigned long)&tcp_death_row),
56 };
57
58 EXPORT_SYMBOL_GPL(tcp_death_row);
59
60 static __inline__ int tcp_in_window(u32 seq, u32 end_seq, u32 s_win, u32 e_win)
61 {
62         if (seq == s_win)
63                 return 1;
64         if (after(end_seq, s_win) && before(seq, e_win))
65                 return 1;
66         return (seq == e_win && seq == end_seq);
67 }
68
69 /* 
70  * * Main purpose of TIME-WAIT state is to close connection gracefully,
71  *   when one of ends sits in LAST-ACK or CLOSING retransmitting FIN
72  *   (and, probably, tail of data) and one or more our ACKs are lost.
73  * * What is TIME-WAIT timeout? It is associated with maximal packet
74  *   lifetime in the internet, which results in wrong conclusion, that
75  *   it is set to catch "old duplicate segments" wandering out of their path.
76  *   It is not quite correct. This timeout is calculated so that it exceeds
77  *   maximal retransmission timeout enough to allow to lose one (or more)
78  *   segments sent by peer and our ACKs. This time may be calculated from RTO.
79  * * When TIME-WAIT socket receives RST, it means that another end
80  *   finally closed and we are allowed to kill TIME-WAIT too.
81  * * Second purpose of TIME-WAIT is catching old duplicate segments.
82  *   Well, certainly it is pure paranoia, but if we load TIME-WAIT
83  *   with this semantics, we MUST NOT kill TIME-WAIT state with RSTs.
84  * * If we invented some more clever way to catch duplicates
85  *   (f.e. based on PAWS), we could truncate TIME-WAIT to several RTOs.
86  *
87  * The algorithm below is based on FORMAL INTERPRETATION of RFCs.
88  * When you compare it to RFCs, please, read section SEGMENT ARRIVES
89  * from the very beginning.
90  *
91  * NOTE. With recycling (and later with fin-wait-2) TW bucket
92  * is _not_ stateless. It means, that strictly speaking we must
93  * spinlock it. I do not want! Well, probability of misbehaviour
94  * is ridiculously low and, seems, we could use some mb() tricks
95  * to avoid misread sequence numbers, states etc.  --ANK
96  */
97 enum tcp_tw_status
98 tcp_timewait_state_process(struct inet_timewait_sock *tw, struct sk_buff *skb,
99                            const struct tcphdr *th)
100 {
101         struct tcp_timewait_sock *tcptw = tcp_twsk((struct sock *)tw);
102         struct tcp_options_received tmp_opt;
103         int paws_reject = 0;
104
105         tmp_opt.saw_tstamp = 0;
106         if (th->doff > (sizeof(*th) >> 2) && tcptw->tw_ts_recent_stamp) {
107                 tcp_parse_options(skb, &tmp_opt, 0);
108
109                 if (tmp_opt.saw_tstamp) {
110                         tmp_opt.ts_recent       = tcptw->tw_ts_recent;
111                         tmp_opt.ts_recent_stamp = tcptw->tw_ts_recent_stamp;
112                         paws_reject = tcp_paws_check(&tmp_opt, th->rst);
113                 }
114         }
115
116         if (tw->tw_substate == TCP_FIN_WAIT2) {
117                 /* Just repeat all the checks of tcp_rcv_state_process() */
118
119                 /* Out of window, send ACK */
120                 if (paws_reject ||
121                     !tcp_in_window(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq,
122                                    tcptw->tw_rcv_nxt,
123                                    tcptw->tw_rcv_nxt + tcptw->tw_rcv_wnd))
124                         return TCP_TW_ACK;
125
126                 if (th->rst)
127                         goto kill;
128
129                 if (th->syn && !before(TCP_SKB_CB(skb)->seq, tcptw->tw_rcv_nxt))
130                         goto kill_with_rst;
131
132                 /* Dup ACK? */
133                 if (!after(TCP_SKB_CB(skb)->end_seq, tcptw->tw_rcv_nxt) ||
134                     TCP_SKB_CB(skb)->end_seq == TCP_SKB_CB(skb)->seq) {
135                         inet_twsk_put(tw);
136                         return TCP_TW_SUCCESS;
137                 }
138
139                 /* New data or FIN. If new data arrive after half-duplex close,
140                  * reset.
141                  */
142                 if (!th->fin ||
143                     TCP_SKB_CB(skb)->end_seq != tcptw->tw_rcv_nxt + 1) {
144 kill_with_rst:
145                         inet_twsk_deschedule(tw, &tcp_death_row);
146                         inet_twsk_put(tw);
147                         return TCP_TW_RST;
148                 }
149
150                 /* FIN arrived, enter true time-wait state. */
151                 tw->tw_substate   = TCP_TIME_WAIT;
152                 tcptw->tw_rcv_nxt = TCP_SKB_CB(skb)->end_seq;
153                 if (tmp_opt.saw_tstamp) {
154                         tcptw->tw_ts_recent_stamp = xtime.tv_sec;
155                         tcptw->tw_ts_recent       = tmp_opt.rcv_tsval;
156                 }
157
158                 /* I am shamed, but failed to make it more elegant.
159                  * Yes, it is direct reference to IP, which is impossible
160                  * to generalize to IPv6. Taking into account that IPv6
161                  * do not understand recycling in any case, it not
162                  * a big problem in practice. --ANK */
163                 if (tw->tw_family == AF_INET &&
164                     tcp_death_row.sysctl_tw_recycle && tcptw->tw_ts_recent_stamp &&
165                     tcp_v4_tw_remember_stamp(tw))
166                         inet_twsk_schedule(tw, &tcp_death_row, tw->tw_timeout,
167                                            TCP_TIMEWAIT_LEN);
168                 else
169                         inet_twsk_schedule(tw, &tcp_death_row, TCP_TIMEWAIT_LEN,
170                                            TCP_TIMEWAIT_LEN);
171                 return TCP_TW_ACK;
172         }
173
174         /*
175          *      Now real TIME-WAIT state.
176          *
177          *      RFC 1122:
178          *      "When a connection is [...] on TIME-WAIT state [...]
179          *      [a TCP] MAY accept a new SYN from the remote TCP to
180          *      reopen the connection directly, if it:
181          *      
182          *      (1)  assigns its initial sequence number for the new
183          *      connection to be larger than the largest sequence
184          *      number it used on the previous connection incarnation,
185          *      and
186          *
187          *      (2)  returns to TIME-WAIT state if the SYN turns out 
188          *      to be an old duplicate".
189          */
190
191         if (!paws_reject &&
192             (TCP_SKB_CB(skb)->seq == tcptw->tw_rcv_nxt &&
193              (TCP_SKB_CB(skb)->seq == TCP_SKB_CB(skb)->end_seq || th->rst))) {
194                 /* In window segment, it may be only reset or bare ack. */
195
196                 if (th->rst) {
197                         /* This is TIME_WAIT assassination, in two flavors.
198                          * Oh well... nobody has a sufficient solution to this
199                          * protocol bug yet.
200                          */
201                         if (sysctl_tcp_rfc1337 == 0) {
202 kill:
203                                 inet_twsk_deschedule(tw, &tcp_death_row);
204                                 inet_twsk_put(tw);
205                                 return TCP_TW_SUCCESS;
206                         }
207                 }
208                 inet_twsk_schedule(tw, &tcp_death_row, TCP_TIMEWAIT_LEN,
209                                    TCP_TIMEWAIT_LEN);
210
211                 if (tmp_opt.saw_tstamp) {
212                         tcptw->tw_ts_recent       = tmp_opt.rcv_tsval;
213                         tcptw->tw_ts_recent_stamp = xtime.tv_sec;
214                 }
215
216                 inet_twsk_put(tw);
217                 return TCP_TW_SUCCESS;
218         }
219
220         /* Out of window segment.
221
222            All the segments are ACKed immediately.
223
224            The only exception is new SYN. We accept it, if it is
225            not old duplicate and we are not in danger to be killed
226            by delayed old duplicates. RFC check is that it has
227            newer sequence number works at rates <40Mbit/sec.
228            However, if paws works, it is reliable AND even more,
229            we even may relax silly seq space cutoff.
230
231            RED-PEN: we violate main RFC requirement, if this SYN will appear
232            old duplicate (i.e. we receive RST in reply to SYN-ACK),
233            we must return socket to time-wait state. It is not good,
234            but not fatal yet.
235          */
236
237         if (th->syn && !th->rst && !th->ack && !paws_reject &&
238             (after(TCP_SKB_CB(skb)->seq, tcptw->tw_rcv_nxt) ||
239              (tmp_opt.saw_tstamp &&
240               (s32)(tcptw->tw_ts_recent - tmp_opt.rcv_tsval) < 0))) {
241                 u32 isn = tcptw->tw_snd_nxt + 65535 + 2;
242                 if (isn == 0)
243                         isn++;
244                 TCP_SKB_CB(skb)->when = isn;
245                 return TCP_TW_SYN;
246         }
247
248         if (paws_reject)
249                 NET_INC_STATS_BH(LINUX_MIB_PAWSESTABREJECTED);
250
251         if(!th->rst) {
252                 /* In this case we must reset the TIMEWAIT timer.
253                  *
254                  * If it is ACKless SYN it may be both old duplicate
255                  * and new good SYN with random sequence number <rcv_nxt.
256                  * Do not reschedule in the last case.
257                  */
258                 if (paws_reject || th->ack)
259                         inet_twsk_schedule(tw, &tcp_death_row, TCP_TIMEWAIT_LEN,
260                                            TCP_TIMEWAIT_LEN);
261
262                 /* Send ACK. Note, we do not put the bucket,
263                  * it will be released by caller.
264                  */
265                 return TCP_TW_ACK;
266         }
267         inet_twsk_put(tw);
268         return TCP_TW_SUCCESS;
269 }
270
271 /* 
272  * Move a socket to time-wait or dead fin-wait-2 state.
273  */ 
274 void tcp_time_wait(struct sock *sk, int state, int timeo)
275 {
276         struct inet_timewait_sock *tw = NULL;
277         const struct inet_connection_sock *icsk = inet_csk(sk);
278         const struct tcp_sock *tp = tcp_sk(sk);
279         int recycle_ok = 0;
280
281         if (tcp_death_row.sysctl_tw_recycle && tp->rx_opt.ts_recent_stamp)
282                 recycle_ok = icsk->icsk_af_ops->remember_stamp(sk);
283
284         if (tcp_death_row.tw_count < tcp_death_row.sysctl_max_tw_buckets)
285                 tw = inet_twsk_alloc(sk, state);
286
287         if (tw != NULL) {
288                 struct tcp_timewait_sock *tcptw = tcp_twsk((struct sock *)tw);
289                 const int rto = (icsk->icsk_rto << 2) - (icsk->icsk_rto >> 1);
290
291                 tw->tw_rcv_wscale       = tp->rx_opt.rcv_wscale;
292                 tcptw->tw_rcv_nxt       = tp->rcv_nxt;
293                 tcptw->tw_snd_nxt       = tp->snd_nxt;
294                 tcptw->tw_rcv_wnd       = tcp_receive_window(tp);
295                 tcptw->tw_ts_recent     = tp->rx_opt.ts_recent;
296                 tcptw->tw_ts_recent_stamp = tp->rx_opt.ts_recent_stamp;
297
298 #if defined(CONFIG_IPV6) || defined(CONFIG_IPV6_MODULE)
299                 if (tw->tw_family == PF_INET6) {
300                         struct ipv6_pinfo *np = inet6_sk(sk);
301                         struct inet6_timewait_sock *tw6;
302
303                         tw->tw_ipv6_offset = inet6_tw_offset(sk->sk_prot);
304                         tw6 = inet6_twsk((struct sock *)tw);
305                         ipv6_addr_copy(&tw6->tw_v6_daddr, &np->daddr);
306                         ipv6_addr_copy(&tw6->tw_v6_rcv_saddr, &np->rcv_saddr);
307                         tw->tw_ipv6only = np->ipv6only;
308                 }
309 #endif
310                 /* Linkage updates. */
311                 __inet_twsk_hashdance(tw, sk, &tcp_hashinfo);
312
313                 /* Get the TIME_WAIT timeout firing. */
314                 if (timeo < rto)
315                         timeo = rto;
316
317                 if (recycle_ok) {
318                         tw->tw_timeout = rto;
319                 } else {
320                         tw->tw_timeout = TCP_TIMEWAIT_LEN;
321                         if (state == TCP_TIME_WAIT)
322                                 timeo = TCP_TIMEWAIT_LEN;
323                 }
324
325                 inet_twsk_schedule(tw, &tcp_death_row, timeo,
326                                    TCP_TIMEWAIT_LEN);
327                 inet_twsk_put(tw);
328         } else {
329                 /* Sorry, if we're out of memory, just CLOSE this
330                  * socket up.  We've got bigger problems than
331                  * non-graceful socket closings.
332                  */
333                 if (net_ratelimit())
334                         printk(KERN_INFO "TCP: time wait bucket table overflow\n");
335         }
336
337         tcp_update_metrics(sk);
338         tcp_done(sk);
339 }
340
341 /* This is not only more efficient than what we used to do, it eliminates
342  * a lot of code duplication between IPv4/IPv6 SYN recv processing. -DaveM
343  *
344  * Actually, we could lots of memory writes here. tp of listening
345  * socket contains all necessary default parameters.
346  */
347 struct sock *tcp_create_openreq_child(struct sock *sk, struct request_sock *req, struct sk_buff *skb)
348 {
349         struct sock *newsk = inet_csk_clone(sk, req, GFP_ATOMIC);
350
351         if (newsk != NULL) {
352                 const struct inet_request_sock *ireq = inet_rsk(req);
353                 struct tcp_request_sock *treq = tcp_rsk(req);
354                 struct inet_connection_sock *newicsk = inet_csk(sk);
355                 struct tcp_sock *newtp;
356
357                 /* Now setup tcp_sock */
358                 newtp = tcp_sk(newsk);
359                 newtp->pred_flags = 0;
360                 newtp->rcv_nxt = treq->rcv_isn + 1;
361                 newtp->snd_nxt = newtp->snd_una = newtp->snd_sml = treq->snt_isn + 1;
362
363                 tcp_prequeue_init(newtp);
364
365                 tcp_init_wl(newtp, treq->snt_isn, treq->rcv_isn);
366
367                 newtp->srtt = 0;
368                 newtp->mdev = TCP_TIMEOUT_INIT;
369                 newicsk->icsk_rto = TCP_TIMEOUT_INIT;
370
371                 newtp->packets_out = 0;
372                 newtp->left_out = 0;
373                 newtp->retrans_out = 0;
374                 newtp->sacked_out = 0;
375                 newtp->fackets_out = 0;
376                 newtp->snd_ssthresh = 0x7fffffff;
377
378                 /* So many TCP implementations out there (incorrectly) count the
379                  * initial SYN frame in their delayed-ACK and congestion control
380                  * algorithms that we must have the following bandaid to talk
381                  * efficiently to them.  -DaveM
382                  */
383                 newtp->snd_cwnd = 2;
384                 newtp->snd_cwnd_cnt = 0;
385                 newtp->bytes_acked = 0;
386
387                 newtp->frto_counter = 0;
388                 newtp->frto_highmark = 0;
389
390                 newicsk->icsk_ca_ops = &tcp_init_congestion_ops;
391
392                 tcp_set_ca_state(newsk, TCP_CA_Open);
393                 tcp_init_xmit_timers(newsk);
394                 skb_queue_head_init(&newtp->out_of_order_queue);
395                 newtp->rcv_wup = treq->rcv_isn + 1;
396                 newtp->write_seq = treq->snt_isn + 1;
397                 newtp->pushed_seq = newtp->write_seq;
398                 newtp->copied_seq = treq->rcv_isn + 1;
399
400                 newtp->rx_opt.saw_tstamp = 0;
401
402                 newtp->rx_opt.dsack = 0;
403                 newtp->rx_opt.eff_sacks = 0;
404
405                 newtp->rx_opt.num_sacks = 0;
406                 newtp->urg_data = 0;
407
408                 if (sock_flag(newsk, SOCK_KEEPOPEN))
409                         inet_csk_reset_keepalive_timer(newsk,
410                                                        keepalive_time_when(newtp));
411
412                 newtp->rx_opt.tstamp_ok = ireq->tstamp_ok;
413                 if((newtp->rx_opt.sack_ok = ireq->sack_ok) != 0) {
414                         if (sysctl_tcp_fack)
415                                 newtp->rx_opt.sack_ok |= 2;
416                 }
417                 newtp->window_clamp = req->window_clamp;
418                 newtp->rcv_ssthresh = req->rcv_wnd;
419                 newtp->rcv_wnd = req->rcv_wnd;
420                 newtp->rx_opt.wscale_ok = ireq->wscale_ok;
421                 if (newtp->rx_opt.wscale_ok) {
422                         newtp->rx_opt.snd_wscale = ireq->snd_wscale;
423                         newtp->rx_opt.rcv_wscale = ireq->rcv_wscale;
424                 } else {
425                         newtp->rx_opt.snd_wscale = newtp->rx_opt.rcv_wscale = 0;
426                         newtp->window_clamp = min(newtp->window_clamp, 65535U);
427                 }
428                 newtp->snd_wnd = ntohs(skb->h.th->window) << newtp->rx_opt.snd_wscale;
429                 newtp->max_window = newtp->snd_wnd;
430
431                 if (newtp->rx_opt.tstamp_ok) {
432                         newtp->rx_opt.ts_recent = req->ts_recent;
433                         newtp->rx_opt.ts_recent_stamp = xtime.tv_sec;
434                         newtp->tcp_header_len = sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED;
435                 } else {
436                         newtp->rx_opt.ts_recent_stamp = 0;
437                         newtp->tcp_header_len = sizeof(struct tcphdr);
438                 }
439                 if (skb->len >= TCP_MIN_RCVMSS+newtp->tcp_header_len)
440                         newicsk->icsk_ack.last_seg_size = skb->len - newtp->tcp_header_len;
441                 newtp->rx_opt.mss_clamp = req->mss;
442                 TCP_ECN_openreq_child(newtp, req);
443                 if (newtp->ecn_flags&TCP_ECN_OK)
444                         sock_set_flag(newsk, SOCK_NO_LARGESEND);
445
446                 TCP_INC_STATS_BH(TCP_MIB_PASSIVEOPENS);
447         }
448         return newsk;
449 }
450
451 /* 
452  *      Process an incoming packet for SYN_RECV sockets represented
453  *      as a request_sock.
454  */
455
456 struct sock *tcp_check_req(struct sock *sk,struct sk_buff *skb,
457                            struct request_sock *req,
458                            struct request_sock **prev)
459 {
460         struct tcphdr *th = skb->h.th;
461         u32 flg = tcp_flag_word(th) & (TCP_FLAG_RST|TCP_FLAG_SYN|TCP_FLAG_ACK);
462         int paws_reject = 0;
463         struct tcp_options_received tmp_opt;
464         struct sock *child;
465
466         tmp_opt.saw_tstamp = 0;
467         if (th->doff > (sizeof(struct tcphdr)>>2)) {
468                 tcp_parse_options(skb, &tmp_opt, 0);
469
470                 if (tmp_opt.saw_tstamp) {
471                         tmp_opt.ts_recent = req->ts_recent;
472                         /* We do not store true stamp, but it is not required,
473                          * it can be estimated (approximately)
474                          * from another data.
475                          */
476                         tmp_opt.ts_recent_stamp = xtime.tv_sec - ((TCP_TIMEOUT_INIT/HZ)<<req->retrans);
477                         paws_reject = tcp_paws_check(&tmp_opt, th->rst);
478                 }
479         }
480
481         /* Check for pure retransmitted SYN. */
482         if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn &&
483             flg == TCP_FLAG_SYN &&
484             !paws_reject) {
485                 /*
486                  * RFC793 draws (Incorrectly! It was fixed in RFC1122)
487                  * this case on figure 6 and figure 8, but formal
488                  * protocol description says NOTHING.
489                  * To be more exact, it says that we should send ACK,
490                  * because this segment (at least, if it has no data)
491                  * is out of window.
492                  *
493                  *  CONCLUSION: RFC793 (even with RFC1122) DOES NOT
494                  *  describe SYN-RECV state. All the description
495                  *  is wrong, we cannot believe to it and should
496                  *  rely only on common sense and implementation
497                  *  experience.
498                  *
499                  * Enforce "SYN-ACK" according to figure 8, figure 6
500                  * of RFC793, fixed by RFC1122.
501                  */
502                 req->rsk_ops->rtx_syn_ack(sk, req, NULL);
503                 return NULL;
504         }
505
506         /* Further reproduces section "SEGMENT ARRIVES"
507            for state SYN-RECEIVED of RFC793.
508            It is broken, however, it does not work only
509            when SYNs are crossed.
510
511            You would think that SYN crossing is impossible here, since
512            we should have a SYN_SENT socket (from connect()) on our end,
513            but this is not true if the crossed SYNs were sent to both
514            ends by a malicious third party.  We must defend against this,
515            and to do that we first verify the ACK (as per RFC793, page
516            36) and reset if it is invalid.  Is this a true full defense?
517            To convince ourselves, let us consider a way in which the ACK
518            test can still pass in this 'malicious crossed SYNs' case.
519            Malicious sender sends identical SYNs (and thus identical sequence
520            numbers) to both A and B:
521
522                 A: gets SYN, seq=7
523                 B: gets SYN, seq=7
524
525            By our good fortune, both A and B select the same initial
526            send sequence number of seven :-)
527
528                 A: sends SYN|ACK, seq=7, ack_seq=8
529                 B: sends SYN|ACK, seq=7, ack_seq=8
530
531            So we are now A eating this SYN|ACK, ACK test passes.  So
532            does sequence test, SYN is truncated, and thus we consider
533            it a bare ACK.
534
535            If icsk->icsk_accept_queue.rskq_defer_accept, we silently drop this
536            bare ACK.  Otherwise, we create an established connection.  Both
537            ends (listening sockets) accept the new incoming connection and try
538            to talk to each other. 8-)
539
540            Note: This case is both harmless, and rare.  Possibility is about the
541            same as us discovering intelligent life on another plant tomorrow.
542
543            But generally, we should (RFC lies!) to accept ACK
544            from SYNACK both here and in tcp_rcv_state_process().
545            tcp_rcv_state_process() does not, hence, we do not too.
546
547            Note that the case is absolutely generic:
548            we cannot optimize anything here without
549            violating protocol. All the checks must be made
550            before attempt to create socket.
551          */
552
553         /* RFC793 page 36: "If the connection is in any non-synchronized state ...
554          *                  and the incoming segment acknowledges something not yet
555          *                  sent (the segment carries an unacceptable ACK) ...
556          *                  a reset is sent."
557          *
558          * Invalid ACK: reset will be sent by listening socket
559          */
560         if ((flg & TCP_FLAG_ACK) &&
561             (TCP_SKB_CB(skb)->ack_seq != tcp_rsk(req)->snt_isn + 1))
562                 return sk;
563
564         /* Also, it would be not so bad idea to check rcv_tsecr, which
565          * is essentially ACK extension and too early or too late values
566          * should cause reset in unsynchronized states.
567          */
568
569         /* RFC793: "first check sequence number". */
570
571         if (paws_reject || !tcp_in_window(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq,
572                                           tcp_rsk(req)->rcv_isn + 1, tcp_rsk(req)->rcv_isn + 1 + req->rcv_wnd)) {
573                 /* Out of window: send ACK and drop. */
574                 if (!(flg & TCP_FLAG_RST))
575                         req->rsk_ops->send_ack(skb, req);
576                 if (paws_reject)
577                         NET_INC_STATS_BH(LINUX_MIB_PAWSESTABREJECTED);
578                 return NULL;
579         }
580
581         /* In sequence, PAWS is OK. */
582
583         if (tmp_opt.saw_tstamp && !after(TCP_SKB_CB(skb)->seq, tcp_rsk(req)->rcv_isn + 1))
584                         req->ts_recent = tmp_opt.rcv_tsval;
585
586                 if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn) {
587                         /* Truncate SYN, it is out of window starting
588                            at tcp_rsk(req)->rcv_isn + 1. */
589                         flg &= ~TCP_FLAG_SYN;
590                 }
591
592                 /* RFC793: "second check the RST bit" and
593                  *         "fourth, check the SYN bit"
594                  */
595                 if (flg & (TCP_FLAG_RST|TCP_FLAG_SYN))
596                         goto embryonic_reset;
597
598                 /* ACK sequence verified above, just make sure ACK is
599                  * set.  If ACK not set, just silently drop the packet.
600                  */
601                 if (!(flg & TCP_FLAG_ACK))
602                         return NULL;
603
604                 /* If TCP_DEFER_ACCEPT is set, drop bare ACK. */
605                 if (inet_csk(sk)->icsk_accept_queue.rskq_defer_accept &&
606                     TCP_SKB_CB(skb)->end_seq == tcp_rsk(req)->rcv_isn + 1) {
607                         inet_rsk(req)->acked = 1;
608                         return NULL;
609                 }
610
611                 /* OK, ACK is valid, create big socket and
612                  * feed this segment to it. It will repeat all
613                  * the tests. THIS SEGMENT MUST MOVE SOCKET TO
614                  * ESTABLISHED STATE. If it will be dropped after
615                  * socket is created, wait for troubles.
616                  */
617                 child = inet_csk(sk)->icsk_af_ops->syn_recv_sock(sk, skb,
618                                                                  req, NULL);
619                 if (child == NULL)
620                         goto listen_overflow;
621
622                 inet_csk_reqsk_queue_unlink(sk, req, prev);
623                 inet_csk_reqsk_queue_removed(sk, req);
624
625                 inet_csk_reqsk_queue_add(sk, req, child);
626                 return child;
627
628         listen_overflow:
629                 if (!sysctl_tcp_abort_on_overflow) {
630                         inet_rsk(req)->acked = 1;
631                         return NULL;
632                 }
633
634         embryonic_reset:
635                 NET_INC_STATS_BH(LINUX_MIB_EMBRYONICRSTS);
636                 if (!(flg & TCP_FLAG_RST))
637                         req->rsk_ops->send_reset(skb);
638
639                 inet_csk_reqsk_queue_drop(sk, req, prev);
640                 return NULL;
641 }
642
643 /*
644  * Queue segment on the new socket if the new socket is active,
645  * otherwise we just shortcircuit this and continue with
646  * the new socket.
647  */
648
649 int tcp_child_process(struct sock *parent, struct sock *child,
650                       struct sk_buff *skb)
651 {
652         int ret = 0;
653         int state = child->sk_state;
654
655         if (!sock_owned_by_user(child)) {
656                 ret = tcp_rcv_state_process(child, skb, skb->h.th, skb->len);
657
658                 /* Wakeup parent, send SIGIO */
659                 if (state == TCP_SYN_RECV && child->sk_state != state)
660                         parent->sk_data_ready(parent, 0);
661         } else {
662                 /* Alas, it is possible again, because we do lookup
663                  * in main socket hash table and lock on listening
664                  * socket does not protect us more.
665                  */
666                 sk_add_backlog(child, skb);
667         }
668
669         bh_unlock_sock(child);
670         sock_put(child);
671         return ret;
672 }
673
674 EXPORT_SYMBOL(tcp_check_req);
675 EXPORT_SYMBOL(tcp_child_process);
676 EXPORT_SYMBOL(tcp_create_openreq_child);
677 EXPORT_SYMBOL(tcp_timewait_state_process);