The Design and Implementation of the FreeBSD Operating System, Second Edition
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FreeBSD/Linux Kernel Cross Reference
sys/netinet/tcp_timewait.c

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    1 /*-
    2  * SPDX-License-Identifier: BSD-3-Clause
    3  *
    4  * Copyright (c) 1982, 1986, 1988, 1990, 1993, 1995
    5  *      The Regents of the University of California.  All rights reserved.
    6  *
    7  * Redistribution and use in source and binary forms, with or without
    8  * modification, are permitted provided that the following conditions
    9  * are met:
   10  * 1. Redistributions of source code must retain the above copyright
   11  *    notice, this list of conditions and the following disclaimer.
   12  * 2. Redistributions in binary form must reproduce the above copyright
   13  *    notice, this list of conditions and the following disclaimer in the
   14  *    documentation and/or other materials provided with the distribution.
   15  * 3. Neither the name of the University nor the names of its contributors
   16  *    may be used to endorse or promote products derived from this software
   17  *    without specific prior written permission.
   18  *
   19  * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
   20  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
   21  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
   22  * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
   23  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
   24  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
   25  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
   26  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
   27  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
   28  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
   29  * SUCH DAMAGE.
   30  *
   31  *      @(#)tcp_subr.c  8.2 (Berkeley) 5/24/95
   32  */
   33 
   34 #include <sys/cdefs.h>
   35 __FBSDID("$FreeBSD: releng/12.0/sys/netinet/tcp_timewait.c 336937 2018-07-30 21:13:42Z tuexen $");
   36 
   37 #include "opt_inet.h"
   38 #include "opt_inet6.h"
   39 #include "opt_tcpdebug.h"
   40 
   41 #include <sys/param.h>
   42 #include <sys/systm.h>
   43 #include <sys/callout.h>
   44 #include <sys/kernel.h>
   45 #include <sys/sysctl.h>
   46 #include <sys/malloc.h>
   47 #include <sys/mbuf.h>
   48 #include <sys/priv.h>
   49 #include <sys/proc.h>
   50 #include <sys/socket.h>
   51 #include <sys/socketvar.h>
   52 #ifndef INVARIANTS
   53 #include <sys/syslog.h>
   54 #endif
   55 #include <sys/protosw.h>
   56 #include <sys/random.h>
   57 
   58 #include <vm/uma.h>
   59 
   60 #include <net/route.h>
   61 #include <net/if.h>
   62 #include <net/if_var.h>
   63 #include <net/vnet.h>
   64 
   65 #include <netinet/in.h>
   66 #include <netinet/in_kdtrace.h>
   67 #include <netinet/in_pcb.h>
   68 #include <netinet/in_systm.h>
   69 #include <netinet/in_var.h>
   70 #include <netinet/ip.h>
   71 #include <netinet/ip_icmp.h>
   72 #include <netinet/ip_var.h>
   73 #ifdef INET6
   74 #include <netinet/ip6.h>
   75 #include <netinet6/in6_pcb.h>
   76 #include <netinet6/ip6_var.h>
   77 #include <netinet6/scope6_var.h>
   78 #include <netinet6/nd6.h>
   79 #endif
   80 #include <netinet/tcp.h>
   81 #include <netinet/tcp_fsm.h>
   82 #include <netinet/tcp_seq.h>
   83 #include <netinet/tcp_timer.h>
   84 #include <netinet/tcp_var.h>
   85 #ifdef INET6
   86 #include <netinet6/tcp6_var.h>
   87 #endif
   88 #include <netinet/tcpip.h>
   89 #ifdef TCPDEBUG
   90 #include <netinet/tcp_debug.h>
   91 #endif
   92 #ifdef INET6
   93 #include <netinet6/ip6protosw.h>
   94 #endif
   95 
   96 #include <machine/in_cksum.h>
   97 
   98 #include <security/mac/mac_framework.h>
   99 
  100 VNET_DEFINE_STATIC(uma_zone_t, tcptw_zone);
  101 #define V_tcptw_zone            VNET(tcptw_zone)
  102 static int      maxtcptw;
  103 
  104 /*
  105  * The timed wait queue contains references to each of the TCP sessions
  106  * currently in the TIME_WAIT state.  The queue pointers, including the
  107  * queue pointers in each tcptw structure, are protected using the global
  108  * timewait lock, which must be held over queue iteration and modification.
  109  *
  110  * Rules on tcptw usage:
  111  *  - a inpcb is always freed _after_ its tcptw
  112  *  - a tcptw relies on its inpcb reference counting for memory stability
  113  *  - a tcptw is dereferenceable only while its inpcb is locked
  114  */
  115 VNET_DEFINE_STATIC(TAILQ_HEAD(, tcptw), twq_2msl);
  116 #define V_twq_2msl              VNET(twq_2msl)
  117 
  118 /* Global timewait lock */
  119 VNET_DEFINE_STATIC(struct rwlock, tw_lock);
  120 #define V_tw_lock               VNET(tw_lock)
  121 
  122 #define TW_LOCK_INIT(tw, d)     rw_init_flags(&(tw), (d), 0)
  123 #define TW_LOCK_DESTROY(tw)     rw_destroy(&(tw))
  124 #define TW_RLOCK(tw)            rw_rlock(&(tw))
  125 #define TW_WLOCK(tw)            rw_wlock(&(tw))
  126 #define TW_RUNLOCK(tw)          rw_runlock(&(tw))
  127 #define TW_WUNLOCK(tw)          rw_wunlock(&(tw))
  128 #define TW_LOCK_ASSERT(tw)      rw_assert(&(tw), RA_LOCKED)
  129 #define TW_RLOCK_ASSERT(tw)     rw_assert(&(tw), RA_RLOCKED)
  130 #define TW_WLOCK_ASSERT(tw)     rw_assert(&(tw), RA_WLOCKED)
  131 #define TW_UNLOCK_ASSERT(tw)    rw_assert(&(tw), RA_UNLOCKED)
  132 
  133 static void     tcp_tw_2msl_reset(struct tcptw *, int);
  134 static void     tcp_tw_2msl_stop(struct tcptw *, int);
  135 static int      tcp_twrespond(struct tcptw *, int);
  136 
  137 static int
  138 tcptw_auto_size(void)
  139 {
  140         int halfrange;
  141 
  142         /*
  143          * Max out at half the ephemeral port range so that TIME_WAIT
  144          * sockets don't tie up too many ephemeral ports.
  145          */
  146         if (V_ipport_lastauto > V_ipport_firstauto)
  147                 halfrange = (V_ipport_lastauto - V_ipport_firstauto) / 2;
  148         else
  149                 halfrange = (V_ipport_firstauto - V_ipport_lastauto) / 2;
  150         /* Protect against goofy port ranges smaller than 32. */
  151         return (imin(imax(halfrange, 32), maxsockets / 5));
  152 }
  153 
  154 static int
  155 sysctl_maxtcptw(SYSCTL_HANDLER_ARGS)
  156 {
  157         int error, new;
  158 
  159         if (maxtcptw == 0)
  160                 new = tcptw_auto_size();
  161         else
  162                 new = maxtcptw;
  163         error = sysctl_handle_int(oidp, &new, 0, req);
  164         if (error == 0 && req->newptr)
  165                 if (new >= 32) {
  166                         maxtcptw = new;
  167                         uma_zone_set_max(V_tcptw_zone, maxtcptw);
  168                 }
  169         return (error);
  170 }
  171 
  172 SYSCTL_PROC(_net_inet_tcp, OID_AUTO, maxtcptw, CTLTYPE_INT|CTLFLAG_RW,
  173     &maxtcptw, 0, sysctl_maxtcptw, "IU",
  174     "Maximum number of compressed TCP TIME_WAIT entries");
  175 
  176 VNET_DEFINE_STATIC(int, nolocaltimewait) = 0;
  177 #define V_nolocaltimewait       VNET(nolocaltimewait)
  178 SYSCTL_INT(_net_inet_tcp, OID_AUTO, nolocaltimewait, CTLFLAG_VNET | CTLFLAG_RW,
  179     &VNET_NAME(nolocaltimewait), 0,
  180     "Do not create compressed TCP TIME_WAIT entries for local connections");
  181 
  182 void
  183 tcp_tw_zone_change(void)
  184 {
  185 
  186         if (maxtcptw == 0)
  187                 uma_zone_set_max(V_tcptw_zone, tcptw_auto_size());
  188 }
  189 
  190 void
  191 tcp_tw_init(void)
  192 {
  193 
  194         V_tcptw_zone = uma_zcreate("tcptw", sizeof(struct tcptw),
  195             NULL, NULL, NULL, NULL, UMA_ALIGN_PTR, 0);
  196         TUNABLE_INT_FETCH("net.inet.tcp.maxtcptw", &maxtcptw);
  197         if (maxtcptw == 0)
  198                 uma_zone_set_max(V_tcptw_zone, tcptw_auto_size());
  199         else
  200                 uma_zone_set_max(V_tcptw_zone, maxtcptw);
  201         TAILQ_INIT(&V_twq_2msl);
  202         TW_LOCK_INIT(V_tw_lock, "tcptw");
  203 }
  204 
  205 #ifdef VIMAGE
  206 void
  207 tcp_tw_destroy(void)
  208 {
  209         struct tcptw *tw;
  210         struct epoch_tracker et;
  211 
  212         INP_INFO_RLOCK_ET(&V_tcbinfo, et);
  213         while ((tw = TAILQ_FIRST(&V_twq_2msl)) != NULL)
  214                 tcp_twclose(tw, 0);
  215         INP_INFO_RUNLOCK_ET(&V_tcbinfo, et);
  216 
  217         TW_LOCK_DESTROY(V_tw_lock);
  218         uma_zdestroy(V_tcptw_zone);
  219 }
  220 #endif
  221 
  222 /*
  223  * Move a TCP connection into TIME_WAIT state.
  224  *    tcbinfo is locked.
  225  *    inp is locked, and is unlocked before returning.
  226  */
  227 void
  228 tcp_twstart(struct tcpcb *tp)
  229 {
  230         struct tcptw twlocal, *tw;
  231         struct inpcb *inp = tp->t_inpcb;
  232         struct socket *so;
  233         uint32_t recwin;
  234         bool acknow, local;
  235 #ifdef INET6
  236         bool isipv6 = inp->inp_inc.inc_flags & INC_ISIPV6;
  237 #endif
  238 
  239         INP_INFO_RLOCK_ASSERT(&V_tcbinfo);
  240         INP_WLOCK_ASSERT(inp);
  241 
  242         /* A dropped inp should never transition to TIME_WAIT state. */
  243         KASSERT((inp->inp_flags & INP_DROPPED) == 0, ("tcp_twstart: "
  244             "(inp->inp_flags & INP_DROPPED) != 0"));
  245 
  246         if (V_nolocaltimewait) {
  247 #ifdef INET6
  248                 if (isipv6)
  249                         local = in6_localaddr(&inp->in6p_faddr);
  250                 else
  251 #endif
  252 #ifdef INET
  253                         local = in_localip(inp->inp_faddr);
  254 #else
  255                         local = false;
  256 #endif
  257         } else
  258                 local = false;
  259 
  260         /*
  261          * For use only by DTrace.  We do not reference the state
  262          * after this point so modifying it in place is not a problem.
  263          */
  264         tcp_state_change(tp, TCPS_TIME_WAIT);
  265 
  266         if (local)
  267                 tw = &twlocal;
  268         else
  269                 tw = uma_zalloc(V_tcptw_zone, M_NOWAIT);
  270         if (tw == NULL) {
  271                 /*
  272                  * Reached limit on total number of TIMEWAIT connections
  273                  * allowed. Remove a connection from TIMEWAIT queue in LRU
  274                  * fashion to make room for this connection.
  275                  *
  276                  * XXX:  Check if it possible to always have enough room
  277                  * in advance based on guarantees provided by uma_zalloc().
  278                  */
  279                 tw = tcp_tw_2msl_scan(1);
  280                 if (tw == NULL) {
  281                         tp = tcp_close(tp);
  282                         if (tp != NULL)
  283                                 INP_WUNLOCK(inp);
  284                         return;
  285                 }
  286         }
  287         /*
  288          * For !local case the tcptw will hold a reference on its inpcb
  289          * until tcp_twclose is called.
  290          */
  291         tw->tw_inpcb = inp;
  292 
  293         /*
  294          * Recover last window size sent.
  295          */
  296         so = inp->inp_socket;
  297         recwin = lmin(lmax(sbspace(&so->so_rcv), 0),
  298             (long)TCP_MAXWIN << tp->rcv_scale);
  299         if (recwin < (so->so_rcv.sb_hiwat / 4) &&
  300             recwin < tp->t_maxseg)
  301                 recwin = 0;
  302         if (SEQ_GT(tp->rcv_adv, tp->rcv_nxt) &&
  303             recwin < (tp->rcv_adv - tp->rcv_nxt))
  304                 recwin = (tp->rcv_adv - tp->rcv_nxt);
  305         tw->last_win = htons((u_short)(recwin >> tp->rcv_scale));
  306 
  307         /*
  308          * Set t_recent if timestamps are used on the connection.
  309          */
  310         if ((tp->t_flags & (TF_REQ_TSTMP|TF_RCVD_TSTMP|TF_NOOPT)) ==
  311             (TF_REQ_TSTMP|TF_RCVD_TSTMP)) {
  312                 tw->t_recent = tp->ts_recent;
  313                 tw->ts_offset = tp->ts_offset;
  314         } else {
  315                 tw->t_recent = 0;
  316                 tw->ts_offset = 0;
  317         }
  318 
  319         tw->snd_nxt = tp->snd_nxt;
  320         tw->rcv_nxt = tp->rcv_nxt;
  321         tw->iss     = tp->iss;
  322         tw->irs     = tp->irs;
  323         tw->t_starttime = tp->t_starttime;
  324         tw->tw_time = 0;
  325 
  326 /* XXX
  327  * If this code will
  328  * be used for fin-wait-2 state also, then we may need
  329  * a ts_recent from the last segment.
  330  */
  331         acknow = tp->t_flags & TF_ACKNOW;
  332 
  333         /*
  334          * First, discard tcpcb state, which includes stopping its timers and
  335          * freeing it.  tcp_discardcb() used to also release the inpcb, but
  336          * that work is now done in the caller.
  337          *
  338          * Note: soisdisconnected() call used to be made in tcp_discardcb(),
  339          * and might not be needed here any longer.
  340          */
  341         tcp_discardcb(tp);
  342         soisdisconnected(so);
  343         tw->tw_so_options = so->so_options;
  344         inp->inp_flags |= INP_TIMEWAIT;
  345         if (acknow)
  346                 tcp_twrespond(tw, TH_ACK);
  347         if (local)
  348                 in_pcbdrop(inp);
  349         else {
  350                 in_pcbref(inp); /* Reference from tw */
  351                 tw->tw_cred = crhold(so->so_cred);
  352                 inp->inp_ppcb = tw;
  353                 TCPSTATES_INC(TCPS_TIME_WAIT);
  354                 tcp_tw_2msl_reset(tw, 0);
  355         }
  356 
  357         /*
  358          * If the inpcb owns the sole reference to the socket, then we can
  359          * detach and free the socket as it is not needed in time wait.
  360          */
  361         if (inp->inp_flags & INP_SOCKREF) {
  362                 KASSERT(so->so_state & SS_PROTOREF,
  363                     ("tcp_twstart: !SS_PROTOREF"));
  364                 inp->inp_flags &= ~INP_SOCKREF;
  365                 INP_WUNLOCK(inp);
  366                 SOCK_LOCK(so);
  367                 so->so_state &= ~SS_PROTOREF;
  368                 sofree(so);
  369         } else
  370                 INP_WUNLOCK(inp);
  371 }
  372 
  373 /*
  374  * Returns 1 if the TIME_WAIT state was killed and we should start over,
  375  * looking for a pcb in the listen state.  Returns 0 otherwise.
  376  */
  377 int
  378 tcp_twcheck(struct inpcb *inp, struct tcpopt *to __unused, struct tcphdr *th,
  379     struct mbuf *m, int tlen)
  380 {
  381         struct tcptw *tw;
  382         int thflags;
  383         tcp_seq seq;
  384 
  385         INP_INFO_RLOCK_ASSERT(&V_tcbinfo);
  386         INP_WLOCK_ASSERT(inp);
  387 
  388         /*
  389          * XXXRW: Time wait state for inpcb has been recycled, but inpcb is
  390          * still present.  This is undesirable, but temporarily necessary
  391          * until we work out how to handle inpcb's who's timewait state has
  392          * been removed.
  393          */
  394         tw = intotw(inp);
  395         if (tw == NULL)
  396                 goto drop;
  397 
  398         thflags = th->th_flags;
  399 
  400         /*
  401          * NOTE: for FIN_WAIT_2 (to be added later),
  402          * must validate sequence number before accepting RST
  403          */
  404 
  405         /*
  406          * If the segment contains RST:
  407          *      Drop the segment - see Stevens, vol. 2, p. 964 and
  408          *      RFC 1337.
  409          */
  410         if (thflags & TH_RST)
  411                 goto drop;
  412 
  413 #if 0
  414 /* PAWS not needed at the moment */
  415         /*
  416          * RFC 1323 PAWS: If we have a timestamp reply on this segment
  417          * and it's less than ts_recent, drop it.
  418          */
  419         if ((to.to_flags & TOF_TS) != 0 && tp->ts_recent &&
  420             TSTMP_LT(to.to_tsval, tp->ts_recent)) {
  421                 if ((thflags & TH_ACK) == 0)
  422                         goto drop;
  423                 goto ack;
  424         }
  425         /*
  426          * ts_recent is never updated because we never accept new segments.
  427          */
  428 #endif
  429 
  430         /*
  431          * If a new connection request is received
  432          * while in TIME_WAIT, drop the old connection
  433          * and start over if the sequence numbers
  434          * are above the previous ones.
  435          */
  436         if ((thflags & TH_SYN) && SEQ_GT(th->th_seq, tw->rcv_nxt)) {
  437                 tcp_twclose(tw, 0);
  438                 return (1);
  439         }
  440 
  441         /*
  442          * Drop the segment if it does not contain an ACK.
  443          */
  444         if ((thflags & TH_ACK) == 0)
  445                 goto drop;
  446 
  447         /*
  448          * Reset the 2MSL timer if this is a duplicate FIN.
  449          */
  450         if (thflags & TH_FIN) {
  451                 seq = th->th_seq + tlen + (thflags & TH_SYN ? 1 : 0);
  452                 if (seq + 1 == tw->rcv_nxt)
  453                         tcp_tw_2msl_reset(tw, 1);
  454         }
  455 
  456         /*
  457          * Acknowledge the segment if it has data or is not a duplicate ACK.
  458          */
  459         if (thflags != TH_ACK || tlen != 0 ||
  460             th->th_seq != tw->rcv_nxt || th->th_ack != tw->snd_nxt) {
  461                 TCP_PROBE5(receive, NULL, NULL, m, NULL, th);
  462                 tcp_twrespond(tw, TH_ACK);
  463                 goto dropnoprobe;
  464         }
  465 drop:
  466         TCP_PROBE5(receive, NULL, NULL, m, NULL, th);
  467 dropnoprobe:
  468         INP_WUNLOCK(inp);
  469         m_freem(m);
  470         return (0);
  471 }
  472 
  473 void
  474 tcp_twclose(struct tcptw *tw, int reuse)
  475 {
  476         struct socket *so;
  477         struct inpcb *inp;
  478 
  479         /*
  480          * At this point, we are in one of two situations:
  481          *
  482          * (1) We have no socket, just an inpcb<->twtcp pair.  We can free
  483          *     all state.
  484          *
  485          * (2) We have a socket -- if we own a reference, release it and
  486          *     notify the socket layer.
  487          */
  488         inp = tw->tw_inpcb;
  489         KASSERT((inp->inp_flags & INP_TIMEWAIT), ("tcp_twclose: !timewait"));
  490         KASSERT(intotw(inp) == tw, ("tcp_twclose: inp_ppcb != tw"));
  491         INP_INFO_RLOCK_ASSERT(&V_tcbinfo);      /* in_pcbfree() */
  492         INP_WLOCK_ASSERT(inp);
  493 
  494         tcp_tw_2msl_stop(tw, reuse);
  495         inp->inp_ppcb = NULL;
  496         in_pcbdrop(inp);
  497 
  498         so = inp->inp_socket;
  499         if (so != NULL) {
  500                 /*
  501                  * If there's a socket, handle two cases: first, we own a
  502                  * strong reference, which we will now release, or we don't
  503                  * in which case another reference exists (XXXRW: think
  504                  * about this more), and we don't need to take action.
  505                  */
  506                 if (inp->inp_flags & INP_SOCKREF) {
  507                         inp->inp_flags &= ~INP_SOCKREF;
  508                         INP_WUNLOCK(inp);
  509                         SOCK_LOCK(so);
  510                         KASSERT(so->so_state & SS_PROTOREF,
  511                             ("tcp_twclose: INP_SOCKREF && !SS_PROTOREF"));
  512                         so->so_state &= ~SS_PROTOREF;
  513                         sofree(so);
  514                 } else {
  515                         /*
  516                          * If we don't own the only reference, the socket and
  517                          * inpcb need to be left around to be handled by
  518                          * tcp_usr_detach() later.
  519                          */
  520                         INP_WUNLOCK(inp);
  521                 }
  522         } else {
  523                 /*
  524                  * The socket has been already cleaned-up for us, only free the
  525                  * inpcb.
  526                  */
  527                 in_pcbfree(inp);
  528         }
  529         TCPSTAT_INC(tcps_closed);
  530 }
  531 
  532 static int
  533 tcp_twrespond(struct tcptw *tw, int flags)
  534 {
  535         struct inpcb *inp = tw->tw_inpcb;
  536 #if defined(INET6) || defined(INET)
  537         struct tcphdr *th = NULL;
  538 #endif
  539         struct mbuf *m;
  540 #ifdef INET
  541         struct ip *ip = NULL;
  542 #endif
  543         u_int hdrlen, optlen;
  544         int error = 0;                  /* Keep compiler happy */
  545         struct tcpopt to;
  546 #ifdef INET6
  547         struct ip6_hdr *ip6 = NULL;
  548         int isipv6 = inp->inp_inc.inc_flags & INC_ISIPV6;
  549 #endif
  550         hdrlen = 0;                     /* Keep compiler happy */
  551 
  552         INP_WLOCK_ASSERT(inp);
  553 
  554         m = m_gethdr(M_NOWAIT, MT_DATA);
  555         if (m == NULL)
  556                 return (ENOBUFS);
  557         m->m_data += max_linkhdr;
  558 
  559 #ifdef MAC
  560         mac_inpcb_create_mbuf(inp, m);
  561 #endif
  562 
  563 #ifdef INET6
  564         if (isipv6) {
  565                 hdrlen = sizeof(struct ip6_hdr) + sizeof(struct tcphdr);
  566                 ip6 = mtod(m, struct ip6_hdr *);
  567                 th = (struct tcphdr *)(ip6 + 1);
  568                 tcpip_fillheaders(inp, ip6, th);
  569         }
  570 #endif
  571 #if defined(INET6) && defined(INET)
  572         else
  573 #endif
  574 #ifdef INET
  575         {
  576                 hdrlen = sizeof(struct tcpiphdr);
  577                 ip = mtod(m, struct ip *);
  578                 th = (struct tcphdr *)(ip + 1);
  579                 tcpip_fillheaders(inp, ip, th);
  580         }
  581 #endif
  582         to.to_flags = 0;
  583 
  584         /*
  585          * Send a timestamp and echo-reply if both our side and our peer
  586          * have sent timestamps in our SYN's and this is not a RST.
  587          */
  588         if (tw->t_recent && flags == TH_ACK) {
  589                 to.to_flags |= TOF_TS;
  590                 to.to_tsval = tcp_ts_getticks() + tw->ts_offset;
  591                 to.to_tsecr = tw->t_recent;
  592         }
  593         optlen = tcp_addoptions(&to, (u_char *)(th + 1));
  594 
  595         m->m_len = hdrlen + optlen;
  596         m->m_pkthdr.len = m->m_len;
  597 
  598         KASSERT(max_linkhdr + m->m_len <= MHLEN, ("tcptw: mbuf too small"));
  599 
  600         th->th_seq = htonl(tw->snd_nxt);
  601         th->th_ack = htonl(tw->rcv_nxt);
  602         th->th_off = (sizeof(struct tcphdr) + optlen) >> 2;
  603         th->th_flags = flags;
  604         th->th_win = htons(tw->last_win);
  605 
  606         m->m_pkthdr.csum_data = offsetof(struct tcphdr, th_sum);
  607 #ifdef INET6
  608         if (isipv6) {
  609                 m->m_pkthdr.csum_flags = CSUM_TCP_IPV6;
  610                 th->th_sum = in6_cksum_pseudo(ip6,
  611                     sizeof(struct tcphdr) + optlen, IPPROTO_TCP, 0);
  612                 ip6->ip6_hlim = in6_selecthlim(inp, NULL);
  613                 TCP_PROBE5(send, NULL, NULL, ip6, NULL, th);
  614                 error = ip6_output(m, inp->in6p_outputopts, NULL,
  615                     (tw->tw_so_options & SO_DONTROUTE), NULL, NULL, inp);
  616         }
  617 #endif
  618 #if defined(INET6) && defined(INET)
  619         else
  620 #endif
  621 #ifdef INET
  622         {
  623                 m->m_pkthdr.csum_flags = CSUM_TCP;
  624                 th->th_sum = in_pseudo(ip->ip_src.s_addr, ip->ip_dst.s_addr,
  625                     htons(sizeof(struct tcphdr) + optlen + IPPROTO_TCP));
  626                 ip->ip_len = htons(m->m_pkthdr.len);
  627                 if (V_path_mtu_discovery)
  628                         ip->ip_off |= htons(IP_DF);
  629                 TCP_PROBE5(send, NULL, NULL, ip, NULL, th);
  630                 error = ip_output(m, inp->inp_options, NULL,
  631                     ((tw->tw_so_options & SO_DONTROUTE) ? IP_ROUTETOIF : 0),
  632                     NULL, inp);
  633         }
  634 #endif
  635         if (flags & TH_ACK)
  636                 TCPSTAT_INC(tcps_sndacks);
  637         else
  638                 TCPSTAT_INC(tcps_sndctrl);
  639         TCPSTAT_INC(tcps_sndtotal);
  640         return (error);
  641 }
  642 
  643 static void
  644 tcp_tw_2msl_reset(struct tcptw *tw, int rearm)
  645 {
  646 
  647         INP_INFO_RLOCK_ASSERT(&V_tcbinfo);
  648         INP_WLOCK_ASSERT(tw->tw_inpcb);
  649 
  650         TW_WLOCK(V_tw_lock);
  651         if (rearm)
  652                 TAILQ_REMOVE(&V_twq_2msl, tw, tw_2msl);
  653         tw->tw_time = ticks + 2 * tcp_msl;
  654         TAILQ_INSERT_TAIL(&V_twq_2msl, tw, tw_2msl);
  655         TW_WUNLOCK(V_tw_lock);
  656 }
  657 
  658 static void
  659 tcp_tw_2msl_stop(struct tcptw *tw, int reuse)
  660 {
  661         struct ucred *cred;
  662         struct inpcb *inp;
  663         int released __unused;
  664 
  665         INP_INFO_RLOCK_ASSERT(&V_tcbinfo);
  666 
  667         TW_WLOCK(V_tw_lock);
  668         inp = tw->tw_inpcb;
  669         tw->tw_inpcb = NULL;
  670 
  671         TAILQ_REMOVE(&V_twq_2msl, tw, tw_2msl);
  672         cred = tw->tw_cred;
  673         tw->tw_cred = NULL;
  674         TW_WUNLOCK(V_tw_lock);
  675 
  676         if (cred != NULL)
  677                 crfree(cred);
  678 
  679         released = in_pcbrele_wlocked(inp);
  680         KASSERT(!released, ("%s: inp should not be released here", __func__));
  681 
  682         if (!reuse)
  683                 uma_zfree(V_tcptw_zone, tw);
  684         TCPSTATES_DEC(TCPS_TIME_WAIT);
  685 }
  686 
  687 struct tcptw *
  688 tcp_tw_2msl_scan(int reuse)
  689 {
  690         struct tcptw *tw;
  691         struct inpcb *inp;
  692         struct epoch_tracker et;
  693 
  694 #ifdef INVARIANTS
  695         if (reuse) {
  696                 /*
  697                  * Exclusive pcbinfo lock is not required in reuse case even if
  698                  * two inpcb locks can be acquired simultaneously:
  699                  *  - the inpcb transitioning to TIME_WAIT state in
  700                  *    tcp_tw_start(),
  701                  *  - the inpcb closed by tcp_twclose().
  702                  *
  703                  * It is because only inpcbs in FIN_WAIT2 or CLOSING states can
  704                  * transition in TIME_WAIT state.  Then a pcbcb cannot be in
  705                  * TIME_WAIT list and transitioning to TIME_WAIT state at same
  706                  * time.
  707                  */
  708                 INP_INFO_RLOCK_ASSERT(&V_tcbinfo);
  709         }
  710 #endif
  711 
  712         for (;;) {
  713                 TW_RLOCK(V_tw_lock);
  714                 tw = TAILQ_FIRST(&V_twq_2msl);
  715                 if (tw == NULL || (!reuse && (tw->tw_time - ticks) > 0)) {
  716                         TW_RUNLOCK(V_tw_lock);
  717                         break;
  718                 }
  719                 KASSERT(tw->tw_inpcb != NULL, ("%s: tw->tw_inpcb == NULL",
  720                     __func__));
  721 
  722                 inp = tw->tw_inpcb;
  723                 in_pcbref(inp);
  724                 TW_RUNLOCK(V_tw_lock);
  725 
  726                 INP_INFO_RLOCK_ET(&V_tcbinfo, et);
  727                 INP_WLOCK(inp);
  728                 tw = intotw(inp);
  729                 if (in_pcbrele_wlocked(inp)) {
  730                         if (__predict_true(tw == NULL)) {
  731                                 INP_INFO_RUNLOCK_ET(&V_tcbinfo, et);
  732                                 continue;
  733                         } else {
  734                                 /* This should not happen as in TIMEWAIT
  735                                  * state the inp should not be destroyed
  736                                  * before its tcptw. If INVARIANTS is
  737                                  * defined panic.
  738                                  */
  739 #ifdef INVARIANTS
  740                                 panic("%s: Panic before an infinite "
  741                                           "loop: INP_TIMEWAIT && (INP_FREED "
  742                                           "|| inp last reference) && tw != "
  743                                           "NULL", __func__);
  744 #else
  745                                 log(LOG_ERR, "%s: Avoid an infinite "
  746                                         "loop: INP_TIMEWAIT && (INP_FREED "
  747                                         "|| inp last reference) && tw != "
  748                                         "NULL", __func__);
  749 #endif
  750                                 INP_INFO_RUNLOCK_ET(&V_tcbinfo, et);
  751                                 break;
  752                         }
  753                 }
  754 
  755                 if (tw == NULL) {
  756                         /* tcp_twclose() has already been called */
  757                         INP_WUNLOCK(inp);
  758                         INP_INFO_RUNLOCK_ET(&V_tcbinfo, et);
  759                         continue;
  760                 }
  761 
  762                 tcp_twclose(tw, reuse);
  763                 INP_INFO_RUNLOCK_ET(&V_tcbinfo, et);
  764                 if (reuse)
  765                         return tw;
  766         }
  767 
  768         return NULL;
  769 }

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