Merge branch 'locks' of git://linux-nfs.org/~bfields/linux
[linux-2.6] / 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/mm.h>
24 #include <linux/module.h>
25 #include <linux/sysctl.h>
26 #include <linux/workqueue.h>
27 #include <net/tcp.h>
28 #include <net/inet_common.h>
29 #include <net/xfrm.h>
30
31 #ifdef CONFIG_SYSCTL
32 #define SYNC_INIT 0 /* let the user enable it */
33 #else
34 #define SYNC_INIT 1
35 #endif
36
37 int sysctl_tcp_syncookies __read_mostly = SYNC_INIT;
38 int sysctl_tcp_abort_on_overflow __read_mostly;
39
40 struct inet_timewait_death_row tcp_death_row = {
41         .sysctl_max_tw_buckets = NR_FILE * 2,
42         .period         = TCP_TIMEWAIT_LEN / INET_TWDR_TWKILL_SLOTS,
43         .death_lock     = __SPIN_LOCK_UNLOCKED(tcp_death_row.death_lock),
44         .hashinfo       = &tcp_hashinfo,
45         .tw_timer       = TIMER_INITIALIZER(inet_twdr_hangman, 0,
46                                             (unsigned long)&tcp_death_row),
47         .twkill_work    = __WORK_INITIALIZER(tcp_death_row.twkill_work,
48                                              inet_twdr_twkill_work),
49 /* Short-time timewait calendar */
50
51         .twcal_hand     = -1,
52         .twcal_timer    = TIMER_INITIALIZER(inet_twdr_twcal_tick, 0,
53                                             (unsigned long)&tcp_death_row),
54 };
55
56 EXPORT_SYMBOL_GPL(tcp_death_row);
57
58 static __inline__ int tcp_in_window(u32 seq, u32 end_seq, u32 s_win, u32 e_win)
59 {
60         if (seq == s_win)
61                 return 1;
62         if (after(end_seq, s_win) && before(seq, e_win))
63                 return 1;
64         return (seq == e_win && seq == end_seq);
65 }
66
67 /*
68  * * Main purpose of TIME-WAIT state is to close connection gracefully,
69  *   when one of ends sits in LAST-ACK or CLOSING retransmitting FIN
70  *   (and, probably, tail of data) and one or more our ACKs are lost.
71  * * What is TIME-WAIT timeout? It is associated with maximal packet
72  *   lifetime in the internet, which results in wrong conclusion, that
73  *   it is set to catch "old duplicate segments" wandering out of their path.
74  *   It is not quite correct. This timeout is calculated so that it exceeds
75  *   maximal retransmission timeout enough to allow to lose one (or more)
76  *   segments sent by peer and our ACKs. This time may be calculated from RTO.
77  * * When TIME-WAIT socket receives RST, it means that another end
78  *   finally closed and we are allowed to kill TIME-WAIT too.
79  * * Second purpose of TIME-WAIT is catching old duplicate segments.
80  *   Well, certainly it is pure paranoia, but if we load TIME-WAIT
81  *   with this semantics, we MUST NOT kill TIME-WAIT state with RSTs.
82  * * If we invented some more clever way to catch duplicates
83  *   (f.e. based on PAWS), we could truncate TIME-WAIT to several RTOs.
84  *
85  * The algorithm below is based on FORMAL INTERPRETATION of RFCs.
86  * When you compare it to RFCs, please, read section SEGMENT ARRIVES
87  * from the very beginning.
88  *
89  * NOTE. With recycling (and later with fin-wait-2) TW bucket
90  * is _not_ stateless. It means, that strictly speaking we must
91  * spinlock it. I do not want! Well, probability of misbehaviour
92  * is ridiculously low and, seems, we could use some mb() tricks
93  * to avoid misread sequence numbers, states etc.  --ANK
94  */
95 enum tcp_tw_status
96 tcp_timewait_state_process(struct inet_timewait_sock *tw, struct sk_buff *skb,
97                            const struct tcphdr *th)
98 {
99         struct tcp_timewait_sock *tcptw = tcp_twsk((struct sock *)tw);
100         struct tcp_options_received tmp_opt;
101         int paws_reject = 0;
102
103         tmp_opt.saw_tstamp = 0;
104         if (th->doff > (sizeof(*th) >> 2) && tcptw->tw_ts_recent_stamp) {
105                 tcp_parse_options(skb, &tmp_opt, 0);
106
107                 if (tmp_opt.saw_tstamp) {
108                         tmp_opt.ts_recent       = tcptw->tw_ts_recent;
109                         tmp_opt.ts_recent_stamp = tcptw->tw_ts_recent_stamp;
110                         paws_reject = tcp_paws_check(&tmp_opt, th->rst);
111                 }
112         }
113
114         if (tw->tw_substate == TCP_FIN_WAIT2) {
115                 /* Just repeat all the checks of tcp_rcv_state_process() */
116
117                 /* Out of window, send ACK */
118                 if (paws_reject ||
119                     !tcp_in_window(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq,
120                                    tcptw->tw_rcv_nxt,
121                                    tcptw->tw_rcv_nxt + tcptw->tw_rcv_wnd))
122                         return TCP_TW_ACK;
123
124                 if (th->rst)
125                         goto kill;
126
127                 if (th->syn && !before(TCP_SKB_CB(skb)->seq, tcptw->tw_rcv_nxt))
128                         goto kill_with_rst;
129
130                 /* Dup ACK? */
131                 if (!after(TCP_SKB_CB(skb)->end_seq, tcptw->tw_rcv_nxt) ||
132                     TCP_SKB_CB(skb)->end_seq == TCP_SKB_CB(skb)->seq) {
133                         inet_twsk_put(tw);
134                         return TCP_TW_SUCCESS;
135                 }
136
137                 /* New data or FIN. If new data arrive after half-duplex close,
138                  * reset.
139                  */
140                 if (!th->fin ||
141                     TCP_SKB_CB(skb)->end_seq != tcptw->tw_rcv_nxt + 1) {
142 kill_with_rst:
143                         inet_twsk_deschedule(tw, &tcp_death_row);
144                         inet_twsk_put(tw);
145                         return TCP_TW_RST;
146                 }
147
148                 /* FIN arrived, enter true time-wait state. */
149                 tw->tw_substate   = TCP_TIME_WAIT;
150                 tcptw->tw_rcv_nxt = TCP_SKB_CB(skb)->end_seq;
151                 if (tmp_opt.saw_tstamp) {
152                         tcptw->tw_ts_recent_stamp = get_seconds();
153                         tcptw->tw_ts_recent       = tmp_opt.rcv_tsval;
154                 }
155
156                 /* I am shamed, but failed to make it more elegant.
157                  * Yes, it is direct reference to IP, which is impossible
158                  * to generalize to IPv6. Taking into account that IPv6
159                  * do not understand recycling in any case, it not
160                  * a big problem in practice. --ANK */
161                 if (tw->tw_family == AF_INET &&
162                     tcp_death_row.sysctl_tw_recycle && tcptw->tw_ts_recent_stamp &&
163                     tcp_v4_tw_remember_stamp(tw))
164                         inet_twsk_schedule(tw, &tcp_death_row, tw->tw_timeout,
165                                            TCP_TIMEWAIT_LEN);
166                 else
167                         inet_twsk_schedule(tw, &tcp_death_row, TCP_TIMEWAIT_LEN,
168                                            TCP_TIMEWAIT_LEN);
169                 return TCP_TW_ACK;
170         }
171
172         /*
173          *      Now real TIME-WAIT state.
174          *
175          *      RFC 1122:
176          *      "When a connection is [...] on TIME-WAIT state [...]
177          *      [a TCP] MAY accept a new SYN from the remote TCP to
178          *      reopen the connection directly, if it:
179          *
180          *      (1)  assigns its initial sequence number for the new
181          *      connection to be larger than the largest sequence
182          *      number it used on the previous connection incarnation,
183          *      and
184          *
185          *      (2)  returns to TIME-WAIT state if the SYN turns out
186          *      to be an old duplicate".
187          */
188
189         if (!paws_reject &&
190             (TCP_SKB_CB(skb)->seq == tcptw->tw_rcv_nxt &&
191              (TCP_SKB_CB(skb)->seq == TCP_SKB_CB(skb)->end_seq || th->rst))) {
192                 /* In window segment, it may be only reset or bare ack. */
193
194                 if (th->rst) {
195                         /* This is TIME_WAIT assassination, in two flavors.
196                          * Oh well... nobody has a sufficient solution to this
197                          * protocol bug yet.
198                          */
199                         if (sysctl_tcp_rfc1337 == 0) {
200 kill:
201                                 inet_twsk_deschedule(tw, &tcp_death_row);
202                                 inet_twsk_put(tw);
203                                 return TCP_TW_SUCCESS;
204                         }
205                 }
206                 inet_twsk_schedule(tw, &tcp_death_row, TCP_TIMEWAIT_LEN,
207                                    TCP_TIMEWAIT_LEN);
208
209                 if (tmp_opt.saw_tstamp) {
210                         tcptw->tw_ts_recent       = tmp_opt.rcv_tsval;
211                         tcptw->tw_ts_recent_stamp = get_seconds();
212                 }
213
214                 inet_twsk_put(tw);
215                 return TCP_TW_SUCCESS;
216         }
217
218         /* Out of window segment.
219
220            All the segments are ACKed immediately.
221
222            The only exception is new SYN. We accept it, if it is
223            not old duplicate and we are not in danger to be killed
224            by delayed old duplicates. RFC check is that it has
225            newer sequence number works at rates <40Mbit/sec.
226            However, if paws works, it is reliable AND even more,
227            we even may relax silly seq space cutoff.
228
229            RED-PEN: we violate main RFC requirement, if this SYN will appear
230            old duplicate (i.e. we receive RST in reply to SYN-ACK),
231            we must return socket to time-wait state. It is not good,
232            but not fatal yet.
233          */
234
235         if (th->syn && !th->rst && !th->ack && !paws_reject &&
236             (after(TCP_SKB_CB(skb)->seq, tcptw->tw_rcv_nxt) ||
237              (tmp_opt.saw_tstamp &&
238               (s32)(tcptw->tw_ts_recent - tmp_opt.rcv_tsval) < 0))) {
239                 u32 isn = tcptw->tw_snd_nxt + 65535 + 2;
240                 if (isn == 0)
241                         isn++;
242                 TCP_SKB_CB(skb)->when = isn;
243                 return TCP_TW_SYN;
244         }
245
246         if (paws_reject)
247                 NET_INC_STATS_BH(LINUX_MIB_PAWSESTABREJECTED);
248
249         if (!th->rst) {
250                 /* In this case we must reset the TIMEWAIT timer.
251                  *
252                  * If it is ACKless SYN it may be both old duplicate
253                  * and new good SYN with random sequence number <rcv_nxt.
254                  * Do not reschedule in the last case.
255                  */
256                 if (paws_reject || th->ack)
257                         inet_twsk_schedule(tw, &tcp_death_row, TCP_TIMEWAIT_LEN,
258                                            TCP_TIMEWAIT_LEN);
259
260                 /* Send ACK. Note, we do not put the bucket,
261                  * it will be released by caller.
262                  */
263                 return TCP_TW_ACK;
264         }
265         inet_twsk_put(tw);
266         return TCP_TW_SUCCESS;
267 }
268
269 /*
270  * Move a socket to time-wait or dead fin-wait-2 state.
271  */
272 void tcp_time_wait(struct sock *sk, int state, int timeo)
273 {
274         struct inet_timewait_sock *tw = NULL;
275         const struct inet_connection_sock *icsk = inet_csk(sk);
276         const struct tcp_sock *tp = tcp_sk(sk);
277         int recycle_ok = 0;
278
279         if (tcp_death_row.sysctl_tw_recycle && tp->rx_opt.ts_recent_stamp)
280                 recycle_ok = icsk->icsk_af_ops->remember_stamp(sk);
281
282         if (tcp_death_row.tw_count < tcp_death_row.sysctl_max_tw_buckets)
283                 tw = inet_twsk_alloc(sk, state);
284
285         if (tw != NULL) {
286                 struct tcp_timewait_sock *tcptw = tcp_twsk((struct sock *)tw);
287                 const int rto = (icsk->icsk_rto << 2) - (icsk->icsk_rto >> 1);
288
289                 tw->tw_rcv_wscale       = tp->rx_opt.rcv_wscale;
290                 tcptw->tw_rcv_nxt       = tp->rcv_nxt;
291                 tcptw->tw_snd_nxt       = tp->snd_nxt;
292                 tcptw->tw_rcv_wnd       = tcp_receive_window(tp);
293                 tcptw->tw_ts_recent     = tp->rx_opt.ts_recent;
294                 tcptw->tw_ts_recent_stamp = tp->rx_opt.ts_recent_stamp;
295
296 #if defined(CONFIG_IPV6) || defined(CONFIG_IPV6_MODULE)
297                 if (tw->tw_family == PF_INET6) {
298                         struct ipv6_pinfo *np = inet6_sk(sk);
299                         struct inet6_timewait_sock *tw6;
300
301                         tw->tw_ipv6_offset = inet6_tw_offset(sk->sk_prot);
302                         tw6 = inet6_twsk((struct sock *)tw);
303                         ipv6_addr_copy(&tw6->tw_v6_daddr, &np->daddr);
304                         ipv6_addr_copy(&tw6->tw_v6_rcv_saddr, &np->rcv_saddr);
305                         tw->tw_ipv6only = np->ipv6only;
306                 }
307 #endif
308
309 #ifdef CONFIG_TCP_MD5SIG
310                 /*
311                  * The timewait bucket does not have the key DB from the
312                  * sock structure. We just make a quick copy of the
313                  * md5 key being used (if indeed we are using one)
314                  * so the timewait ack generating code has the key.
315                  */
316                 do {
317                         struct tcp_md5sig_key *key;
318                         memset(tcptw->tw_md5_key, 0, sizeof(tcptw->tw_md5_key));
319                         tcptw->tw_md5_keylen = 0;
320                         key = tp->af_specific->md5_lookup(sk, sk);
321                         if (key != NULL) {
322                                 memcpy(&tcptw->tw_md5_key, key->key, key->keylen);
323                                 tcptw->tw_md5_keylen = key->keylen;
324                                 if (tcp_alloc_md5sig_pool() == NULL)
325                                         BUG();
326                         }
327                 } while (0);
328 #endif
329
330                 /* Linkage updates. */
331                 __inet_twsk_hashdance(tw, sk, &tcp_hashinfo);
332
333                 /* Get the TIME_WAIT timeout firing. */
334                 if (timeo < rto)
335                         timeo = rto;
336
337                 if (recycle_ok) {
338                         tw->tw_timeout = rto;
339                 } else {
340                         tw->tw_timeout = TCP_TIMEWAIT_LEN;
341                         if (state == TCP_TIME_WAIT)
342                                 timeo = TCP_TIMEWAIT_LEN;
343                 }
344
345                 inet_twsk_schedule(tw, &tcp_death_row, timeo,
346                                    TCP_TIMEWAIT_LEN);
347                 inet_twsk_put(tw);
348         } else {
349                 /* Sorry, if we're out of memory, just CLOSE this
350                  * socket up.  We've got bigger problems than
351                  * non-graceful socket closings.
352                  */
353                 LIMIT_NETDEBUG(KERN_INFO "TCP: time wait bucket table overflow\n");
354         }
355
356         tcp_update_metrics(sk);
357         tcp_done(sk);
358 }
359
360 void tcp_twsk_destructor(struct sock *sk)
361 {
362 #ifdef CONFIG_TCP_MD5SIG
363         struct tcp_timewait_sock *twsk = tcp_twsk(sk);
364         if (twsk->tw_md5_keylen)
365                 tcp_put_md5sig_pool();
366 #endif
367 }
368
369 EXPORT_SYMBOL_GPL(tcp_twsk_destructor);
370
371 static inline void TCP_ECN_openreq_child(struct tcp_sock *tp,
372                                          struct request_sock *req)
373 {
374         tp->ecn_flags = inet_rsk(req)->ecn_ok ? TCP_ECN_OK : 0;
375 }
376
377 /* This is not only more efficient than what we used to do, it eliminates
378  * a lot of code duplication between IPv4/IPv6 SYN recv processing. -DaveM
379  *
380  * Actually, we could lots of memory writes here. tp of listening
381  * socket contains all necessary default parameters.
382  */
383 struct sock *tcp_create_openreq_child(struct sock *sk, struct request_sock *req, struct sk_buff *skb)
384 {
385         struct sock *newsk = inet_csk_clone(sk, req, GFP_ATOMIC);
386
387         if (newsk != NULL) {
388                 const struct inet_request_sock *ireq = inet_rsk(req);
389                 struct tcp_request_sock *treq = tcp_rsk(req);
390                 struct inet_connection_sock *newicsk = inet_csk(newsk);
391                 struct tcp_sock *newtp;
392
393                 /* Now setup tcp_sock */
394                 newtp = tcp_sk(newsk);
395                 newtp->pred_flags = 0;
396                 newtp->rcv_wup = newtp->copied_seq = newtp->rcv_nxt = treq->rcv_isn + 1;
397                 newtp->snd_sml = newtp->snd_una = newtp->snd_nxt = treq->snt_isn + 1;
398
399                 tcp_prequeue_init(newtp);
400
401                 tcp_init_wl(newtp, treq->snt_isn, treq->rcv_isn);
402
403                 newtp->srtt = 0;
404                 newtp->mdev = TCP_TIMEOUT_INIT;
405                 newicsk->icsk_rto = TCP_TIMEOUT_INIT;
406
407                 newtp->packets_out = 0;
408                 newtp->retrans_out = 0;
409                 newtp->sacked_out = 0;
410                 newtp->fackets_out = 0;
411                 newtp->snd_ssthresh = 0x7fffffff;
412
413                 /* So many TCP implementations out there (incorrectly) count the
414                  * initial SYN frame in their delayed-ACK and congestion control
415                  * algorithms that we must have the following bandaid to talk
416                  * efficiently to them.  -DaveM
417                  */
418                 newtp->snd_cwnd = 2;
419                 newtp->snd_cwnd_cnt = 0;
420                 newtp->bytes_acked = 0;
421
422                 newtp->frto_counter = 0;
423                 newtp->frto_highmark = 0;
424
425                 newicsk->icsk_ca_ops = &tcp_init_congestion_ops;
426
427                 tcp_set_ca_state(newsk, TCP_CA_Open);
428                 tcp_init_xmit_timers(newsk);
429                 skb_queue_head_init(&newtp->out_of_order_queue);
430                 newtp->write_seq = treq->snt_isn + 1;
431                 newtp->pushed_seq = newtp->write_seq;
432
433                 newtp->rx_opt.saw_tstamp = 0;
434
435                 newtp->rx_opt.dsack = 0;
436                 newtp->rx_opt.eff_sacks = 0;
437
438                 newtp->rx_opt.num_sacks = 0;
439                 newtp->urg_data = 0;
440
441                 if (sock_flag(newsk, SOCK_KEEPOPEN))
442                         inet_csk_reset_keepalive_timer(newsk,
443                                                        keepalive_time_when(newtp));
444
445                 newtp->rx_opt.tstamp_ok = ireq->tstamp_ok;
446                 if ((newtp->rx_opt.sack_ok = ireq->sack_ok) != 0) {
447                         if (sysctl_tcp_fack)
448                                 tcp_enable_fack(newtp);
449                 }
450                 newtp->window_clamp = req->window_clamp;
451                 newtp->rcv_ssthresh = req->rcv_wnd;
452                 newtp->rcv_wnd = req->rcv_wnd;
453                 newtp->rx_opt.wscale_ok = ireq->wscale_ok;
454                 if (newtp->rx_opt.wscale_ok) {
455                         newtp->rx_opt.snd_wscale = ireq->snd_wscale;
456                         newtp->rx_opt.rcv_wscale = ireq->rcv_wscale;
457                 } else {
458                         newtp->rx_opt.snd_wscale = newtp->rx_opt.rcv_wscale = 0;
459                         newtp->window_clamp = min(newtp->window_clamp, 65535U);
460                 }
461                 newtp->snd_wnd = (ntohs(tcp_hdr(skb)->window) <<
462                                   newtp->rx_opt.snd_wscale);
463                 newtp->max_window = newtp->snd_wnd;
464
465                 if (newtp->rx_opt.tstamp_ok) {
466                         newtp->rx_opt.ts_recent = req->ts_recent;
467                         newtp->rx_opt.ts_recent_stamp = get_seconds();
468                         newtp->tcp_header_len = sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED;
469                 } else {
470                         newtp->rx_opt.ts_recent_stamp = 0;
471                         newtp->tcp_header_len = sizeof(struct tcphdr);
472                 }
473 #ifdef CONFIG_TCP_MD5SIG
474                 newtp->md5sig_info = NULL;      /*XXX*/
475                 if (newtp->af_specific->md5_lookup(sk, newsk))
476                         newtp->tcp_header_len += TCPOLEN_MD5SIG_ALIGNED;
477 #endif
478                 if (skb->len >= TCP_MIN_RCVMSS+newtp->tcp_header_len)
479                         newicsk->icsk_ack.last_seg_size = skb->len - newtp->tcp_header_len;
480                 newtp->rx_opt.mss_clamp = req->mss;
481                 TCP_ECN_openreq_child(newtp, req);
482
483                 TCP_INC_STATS_BH(TCP_MIB_PASSIVEOPENS);
484         }
485         return newsk;
486 }
487
488 /*
489  *      Process an incoming packet for SYN_RECV sockets represented
490  *      as a request_sock.
491  */
492
493 struct sock *tcp_check_req(struct sock *sk,struct sk_buff *skb,
494                            struct request_sock *req,
495                            struct request_sock **prev)
496 {
497         const struct tcphdr *th = tcp_hdr(skb);
498         __be32 flg = tcp_flag_word(th) & (TCP_FLAG_RST|TCP_FLAG_SYN|TCP_FLAG_ACK);
499         int paws_reject = 0;
500         struct tcp_options_received tmp_opt;
501         struct sock *child;
502
503         tmp_opt.saw_tstamp = 0;
504         if (th->doff > (sizeof(struct tcphdr)>>2)) {
505                 tcp_parse_options(skb, &tmp_opt, 0);
506
507                 if (tmp_opt.saw_tstamp) {
508                         tmp_opt.ts_recent = req->ts_recent;
509                         /* We do not store true stamp, but it is not required,
510                          * it can be estimated (approximately)
511                          * from another data.
512                          */
513                         tmp_opt.ts_recent_stamp = get_seconds() - ((TCP_TIMEOUT_INIT/HZ)<<req->retrans);
514                         paws_reject = tcp_paws_check(&tmp_opt, th->rst);
515                 }
516         }
517
518         /* Check for pure retransmitted SYN. */
519         if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn &&
520             flg == TCP_FLAG_SYN &&
521             !paws_reject) {
522                 /*
523                  * RFC793 draws (Incorrectly! It was fixed in RFC1122)
524                  * this case on figure 6 and figure 8, but formal
525                  * protocol description says NOTHING.
526                  * To be more exact, it says that we should send ACK,
527                  * because this segment (at least, if it has no data)
528                  * is out of window.
529                  *
530                  *  CONCLUSION: RFC793 (even with RFC1122) DOES NOT
531                  *  describe SYN-RECV state. All the description
532                  *  is wrong, we cannot believe to it and should
533                  *  rely only on common sense and implementation
534                  *  experience.
535                  *
536                  * Enforce "SYN-ACK" according to figure 8, figure 6
537                  * of RFC793, fixed by RFC1122.
538                  */
539                 req->rsk_ops->rtx_syn_ack(sk, req, NULL);
540                 return NULL;
541         }
542
543         /* Further reproduces section "SEGMENT ARRIVES"
544            for state SYN-RECEIVED of RFC793.
545            It is broken, however, it does not work only
546            when SYNs are crossed.
547
548            You would think that SYN crossing is impossible here, since
549            we should have a SYN_SENT socket (from connect()) on our end,
550            but this is not true if the crossed SYNs were sent to both
551            ends by a malicious third party.  We must defend against this,
552            and to do that we first verify the ACK (as per RFC793, page
553            36) and reset if it is invalid.  Is this a true full defense?
554            To convince ourselves, let us consider a way in which the ACK
555            test can still pass in this 'malicious crossed SYNs' case.
556            Malicious sender sends identical SYNs (and thus identical sequence
557            numbers) to both A and B:
558
559                 A: gets SYN, seq=7
560                 B: gets SYN, seq=7
561
562            By our good fortune, both A and B select the same initial
563            send sequence number of seven :-)
564
565                 A: sends SYN|ACK, seq=7, ack_seq=8
566                 B: sends SYN|ACK, seq=7, ack_seq=8
567
568            So we are now A eating this SYN|ACK, ACK test passes.  So
569            does sequence test, SYN is truncated, and thus we consider
570            it a bare ACK.
571
572            If icsk->icsk_accept_queue.rskq_defer_accept, we silently drop this
573            bare ACK.  Otherwise, we create an established connection.  Both
574            ends (listening sockets) accept the new incoming connection and try
575            to talk to each other. 8-)
576
577            Note: This case is both harmless, and rare.  Possibility is about the
578            same as us discovering intelligent life on another plant tomorrow.
579
580            But generally, we should (RFC lies!) to accept ACK
581            from SYNACK both here and in tcp_rcv_state_process().
582            tcp_rcv_state_process() does not, hence, we do not too.
583
584            Note that the case is absolutely generic:
585            we cannot optimize anything here without
586            violating protocol. All the checks must be made
587            before attempt to create socket.
588          */
589
590         /* RFC793 page 36: "If the connection is in any non-synchronized state ...
591          *                  and the incoming segment acknowledges something not yet
592          *                  sent (the segment carries an unacceptable ACK) ...
593          *                  a reset is sent."
594          *
595          * Invalid ACK: reset will be sent by listening socket
596          */
597         if ((flg & TCP_FLAG_ACK) &&
598             (TCP_SKB_CB(skb)->ack_seq != tcp_rsk(req)->snt_isn + 1))
599                 return sk;
600
601         /* Also, it would be not so bad idea to check rcv_tsecr, which
602          * is essentially ACK extension and too early or too late values
603          * should cause reset in unsynchronized states.
604          */
605
606         /* RFC793: "first check sequence number". */
607
608         if (paws_reject || !tcp_in_window(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq,
609                                           tcp_rsk(req)->rcv_isn + 1, tcp_rsk(req)->rcv_isn + 1 + req->rcv_wnd)) {
610                 /* Out of window: send ACK and drop. */
611                 if (!(flg & TCP_FLAG_RST))
612                         req->rsk_ops->send_ack(skb, req);
613                 if (paws_reject)
614                         NET_INC_STATS_BH(LINUX_MIB_PAWSESTABREJECTED);
615                 return NULL;
616         }
617
618         /* In sequence, PAWS is OK. */
619
620         if (tmp_opt.saw_tstamp && !after(TCP_SKB_CB(skb)->seq, tcp_rsk(req)->rcv_isn + 1))
621                         req->ts_recent = tmp_opt.rcv_tsval;
622
623                 if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn) {
624                         /* Truncate SYN, it is out of window starting
625                            at tcp_rsk(req)->rcv_isn + 1. */
626                         flg &= ~TCP_FLAG_SYN;
627                 }
628
629                 /* RFC793: "second check the RST bit" and
630                  *         "fourth, check the SYN bit"
631                  */
632                 if (flg & (TCP_FLAG_RST|TCP_FLAG_SYN)) {
633                         TCP_INC_STATS_BH(TCP_MIB_ATTEMPTFAILS);
634                         goto embryonic_reset;
635                 }
636
637                 /* ACK sequence verified above, just make sure ACK is
638                  * set.  If ACK not set, just silently drop the packet.
639                  */
640                 if (!(flg & TCP_FLAG_ACK))
641                         return NULL;
642
643                 /* If TCP_DEFER_ACCEPT is set, drop bare ACK. */
644                 if (inet_csk(sk)->icsk_accept_queue.rskq_defer_accept &&
645                     TCP_SKB_CB(skb)->end_seq == tcp_rsk(req)->rcv_isn + 1) {
646                         inet_rsk(req)->acked = 1;
647                         return NULL;
648                 }
649
650                 /* OK, ACK is valid, create big socket and
651                  * feed this segment to it. It will repeat all
652                  * the tests. THIS SEGMENT MUST MOVE SOCKET TO
653                  * ESTABLISHED STATE. If it will be dropped after
654                  * socket is created, wait for troubles.
655                  */
656                 child = inet_csk(sk)->icsk_af_ops->syn_recv_sock(sk, skb,
657                                                                  req, NULL);
658                 if (child == NULL)
659                         goto listen_overflow;
660 #ifdef CONFIG_TCP_MD5SIG
661                 else {
662                         /* Copy over the MD5 key from the original socket */
663                         struct tcp_md5sig_key *key;
664                         struct tcp_sock *tp = tcp_sk(sk);
665                         key = tp->af_specific->md5_lookup(sk, child);
666                         if (key != NULL) {
667                                 /*
668                                  * We're using one, so create a matching key on the
669                                  * newsk structure. If we fail to get memory then we
670                                  * end up not copying the key across. Shucks.
671                                  */
672                                 char *newkey = kmemdup(key->key, key->keylen,
673                                                        GFP_ATOMIC);
674                                 if (newkey) {
675                                         if (!tcp_alloc_md5sig_pool())
676                                                 BUG();
677                                         tp->af_specific->md5_add(child, child,
678                                                                  newkey,
679                                                                  key->keylen);
680                                 }
681                         }
682                 }
683 #endif
684
685                 inet_csk_reqsk_queue_unlink(sk, req, prev);
686                 inet_csk_reqsk_queue_removed(sk, req);
687
688                 inet_csk_reqsk_queue_add(sk, req, child);
689                 return child;
690
691         listen_overflow:
692                 if (!sysctl_tcp_abort_on_overflow) {
693                         inet_rsk(req)->acked = 1;
694                         return NULL;
695                 }
696
697         embryonic_reset:
698                 NET_INC_STATS_BH(LINUX_MIB_EMBRYONICRSTS);
699                 if (!(flg & TCP_FLAG_RST))
700                         req->rsk_ops->send_reset(sk, skb);
701
702                 inet_csk_reqsk_queue_drop(sk, req, prev);
703                 return NULL;
704 }
705
706 /*
707  * Queue segment on the new socket if the new socket is active,
708  * otherwise we just shortcircuit this and continue with
709  * the new socket.
710  */
711
712 int tcp_child_process(struct sock *parent, struct sock *child,
713                       struct sk_buff *skb)
714 {
715         int ret = 0;
716         int state = child->sk_state;
717
718         if (!sock_owned_by_user(child)) {
719                 ret = tcp_rcv_state_process(child, skb, tcp_hdr(skb),
720                                             skb->len);
721                 /* Wakeup parent, send SIGIO */
722                 if (state == TCP_SYN_RECV && child->sk_state != state)
723                         parent->sk_data_ready(parent, 0);
724         } else {
725                 /* Alas, it is possible again, because we do lookup
726                  * in main socket hash table and lock on listening
727                  * socket does not protect us more.
728                  */
729                 sk_add_backlog(child, skb);
730         }
731
732         bh_unlock_sock(child);
733         sock_put(child);
734         return ret;
735 }
736
737 EXPORT_SYMBOL(tcp_check_req);
738 EXPORT_SYMBOL(tcp_child_process);
739 EXPORT_SYMBOL(tcp_create_openreq_child);
740 EXPORT_SYMBOL(tcp_timewait_state_process);