The Design and Implementation of the FreeBSD Operating System, Second Edition
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sys/net/slcompress.c

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    1 /*      $NetBSD: slcompress.c,v 1.41 2019/04/06 08:38:23 msaitoh Exp $   */
    2 /*      Id: slcompress.c,v 1.3 1996/05/24 07:04:47 paulus Exp   */
    3 
    4 /*
    5  * Copyright (c) 1989, 1993, 1994
    6  *      The Regents of the University of California.  All rights reserved.
    7  *
    8  * Redistribution and use in source and binary forms, with or without
    9  * modification, are permitted provided that the following conditions
   10  * are met:
   11  * 1. Redistributions of source code must retain the above copyright
   12  *    notice, this list of conditions and the following disclaimer.
   13  * 2. Redistributions in binary form must reproduce the above copyright
   14  *    notice, this list of conditions and the following disclaimer in the
   15  *    documentation and/or other materials provided with the distribution.
   16  * 3. Neither the name of the University nor the names of its contributors
   17  *    may be used to endorse or promote products derived from this software
   18  *    without specific prior written permission.
   19  *
   20  * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
   21  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
   22  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
   23  * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
   24  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
   25  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
   26  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
   27  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
   28  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
   29  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
   30  * SUCH DAMAGE.
   31  *
   32  *      @(#)slcompress.c        8.2 (Berkeley) 4/16/94
   33  */
   34 
   35 /*
   36  * Routines to compress and uncompess tcp packets (for transmission
   37  * over low speed serial lines.
   38  *
   39  * Van Jacobson (van@helios.ee.lbl.gov), Dec 31, 1989:
   40  *      - Initial distribution.
   41  */
   42 
   43 #include <sys/cdefs.h>
   44 __KERNEL_RCSID(0, "$NetBSD: slcompress.c,v 1.41 2019/04/06 08:38:23 msaitoh Exp $");
   45 
   46 #ifdef _KERNEL_OPT
   47 #include "opt_inet.h"
   48 #endif
   49 
   50 #ifdef INET
   51 #include <sys/param.h>
   52 #include <sys/mbuf.h>
   53 #include <sys/systm.h>
   54 #include <sys/module.h>
   55 
   56 #include <netinet/in.h>
   57 #include <netinet/in_systm.h>
   58 #include <netinet/ip.h>
   59 #include <netinet/tcp.h>
   60 
   61 #include <net/slcompress.h>
   62 
   63 #ifndef SL_NO_STATS
   64 #define INCR(counter) ++comp->counter;
   65 #else
   66 #define INCR(counter)
   67 #endif
   68 
   69 
   70 void
   71 sl_compress_init(struct slcompress *comp)
   72 {
   73         u_int i;
   74         struct cstate *tstate = comp->tstate;
   75 
   76         memset(comp, 0, sizeof(*comp));
   77         for (i = MAX_STATES - 1; i > 0; --i) {
   78                 tstate[i].cs_id = i;
   79                 tstate[i].cs_next = &tstate[i - 1];
   80         }
   81         tstate[0].cs_next = &tstate[MAX_STATES - 1];
   82         tstate[0].cs_id = 0;
   83         comp->last_cs = &tstate[0];
   84         comp->last_recv = 255;
   85         comp->last_xmit = 255;
   86         comp->flags = SLF_TOSS;
   87 }
   88 
   89 
   90 /*
   91  * Like sl_compress_init, but we get to specify the maximum connection
   92  * ID to use on transmission.
   93  */
   94 void
   95 sl_compress_setup(struct slcompress *comp, int max_state)
   96 {
   97         u_int i;
   98         struct cstate *tstate = comp->tstate;
   99 
  100         if (max_state == -1) {
  101                 max_state = MAX_STATES - 1;
  102                 memset(comp, 0, sizeof(*comp));
  103         } else {
  104                 /* Don't reset statistics */
  105                 memset(comp->tstate, 0, sizeof(comp->tstate));
  106                 memset(comp->rstate, 0, sizeof(comp->rstate));
  107         }
  108         for (i = max_state; i > 0; --i) {
  109                 tstate[i].cs_id = i;
  110                 tstate[i].cs_next = &tstate[i - 1];
  111         }
  112         tstate[0].cs_next = &tstate[max_state];
  113         tstate[0].cs_id = 0;
  114         comp->last_cs = &tstate[0];
  115         comp->last_recv = 255;
  116         comp->last_xmit = 255;
  117         comp->flags = SLF_TOSS;
  118 }
  119 
  120 
  121 /*
  122  * ENCODE encodes a number that is known to be non-zero.  ENCODEZ checks for
  123  * zero (since zero has to be encoded in the long, 3 byte form).
  124  */
  125 #define ENCODE(n) { \
  126         if ((uint16_t)(n) >= 256) { \
  127                 *cp++ = 0; \
  128                 cp[1] = (n); \
  129                 cp[0] = (n) >> 8; \
  130                 cp += 2; \
  131         } else { \
  132                 *cp++ = (n); \
  133         } \
  134 }
  135 #define ENCODEZ(n) { \
  136         if ((uint16_t)(n) >= 256 || (uint16_t)(n) == 0) { \
  137                 *cp++ = 0; \
  138                 cp[1] = (n); \
  139                 cp[0] = (n) >> 8; \
  140                 cp += 2; \
  141         } else { \
  142                 *cp++ = (n); \
  143         } \
  144 }
  145 
  146 #define DECODEL(f) { \
  147         if (*cp == 0) {\
  148                 (f) = htonl(ntohl(f) + ((cp[1] << 8) | cp[2])); \
  149                 cp += 3; \
  150         } else { \
  151                 (f) = htonl(ntohl(f) + (uint32_t)*cp++); \
  152         } \
  153 }
  154 
  155 #define DECODES(f) { \
  156         if (*cp == 0) {\
  157                 (f) = htons(ntohs(f) + ((cp[1] << 8) | cp[2])); \
  158                 cp += 3; \
  159         } else { \
  160                 (f) = htons(ntohs(f) + (uint32_t)*cp++); \
  161         } \
  162 }
  163 
  164 #define DECODEU(f) { \
  165         if (*cp == 0) {\
  166                 (f) = htons((cp[1] << 8) | cp[2]); \
  167                 cp += 3; \
  168         } else { \
  169                 (f) = htons((uint32_t)*cp++); \
  170         } \
  171 }
  172 
  173 u_int
  174 sl_compress_tcp(struct mbuf *m, struct ip *ip, struct slcompress *comp,
  175     int compress_cid)
  176 {
  177         struct cstate *cs = comp->last_cs->cs_next;
  178         u_int hlen = ip->ip_hl;
  179         struct tcphdr *oth;
  180         struct tcphdr *th;
  181         u_int deltaS, deltaA;
  182         u_int changes = 0;
  183         u_char new_seq[16];
  184         u_char *cp = new_seq;
  185 
  186         /*
  187          * Bail if this is an IP fragment or if the TCP packet isn't
  188          * `compressible' (i.e., ACK isn't set or some other control bit is
  189          * set).  (We assume that the caller has already made sure the
  190          * packet is IP proto TCP).
  191          */
  192         if ((ip->ip_off & htons(0x3fff)) || m->m_len < 40)
  193                 return TYPE_IP;
  194 
  195         th = (struct tcphdr *)&((int32_t *)ip)[hlen];
  196         if ((th->th_flags & (TH_SYN|TH_FIN|TH_RST|TH_ACK)) != TH_ACK)
  197                 return TYPE_IP;
  198         /*
  199          * Packet is compressible -- we're going to send either a
  200          * COMPRESSED_TCP or UNCOMPRESSED_TCP packet.  Either way we need
  201          * to locate (or create) the connection state.  Special case the
  202          * most recently used connection since it's most likely to be used
  203          * again & we don't have to do any reordering if it's used.
  204          */
  205         INCR(sls_packets)
  206         if (ip->ip_src.s_addr != cs->cs_ip.ip_src.s_addr ||
  207             ip->ip_dst.s_addr != cs->cs_ip.ip_dst.s_addr ||
  208             *(int32_t *)th != ((int32_t *)&cs->cs_ip)[cs->cs_ip.ip_hl]) {
  209                 /*
  210                  * Wasn't the first -- search for it.
  211                  *
  212                  * States are kept in a circularly linked list with last_cs
  213                  * pointing to the end of the list.  The list is kept in lru
  214                  * order by moving a state to the head of the list whenever it
  215                  * is referenced.  Since the list is short and, empirically,
  216                  * the connection we want is almost always near the front, we
  217                  * locate states via linear search.  If we don't find a state
  218                  * for the datagram, the oldest state is (re-)used.
  219                  */
  220                 struct cstate *lcs;
  221                 struct cstate *lastcs = comp->last_cs;
  222 
  223                 do {
  224                         lcs = cs; cs = cs->cs_next;
  225                         INCR(sls_searches)
  226                         if (ip->ip_src.s_addr == cs->cs_ip.ip_src.s_addr
  227                             && ip->ip_dst.s_addr == cs->cs_ip.ip_dst.s_addr
  228                             && *(int32_t *)th ==
  229                             ((int32_t *)&cs->cs_ip)[cs->cs_ip.ip_hl])
  230                                 goto found;
  231                 } while (cs != lastcs);
  232 
  233                 /*
  234                  * Didn't find it -- re-use oldest cstate.  Send an
  235                  * uncompressed packet that tells the other side what
  236                  * connection number we're using for this conversation.
  237                  * Note that since the state list is circular, the oldest
  238                  * state points to the newest and we only need to set
  239                  * last_cs to update the lru linkage.
  240                  */
  241                 INCR(sls_misses)
  242                 comp->last_cs = lcs;
  243                 hlen += th->th_off;
  244                 hlen <<= 2;
  245                 if (hlen > m->m_len)
  246                         return TYPE_IP;
  247                 goto uncompressed;
  248 
  249         found:
  250                 /* Found it -- move to the front on the connection list. */
  251                 if (cs == lastcs)
  252                         comp->last_cs = lcs;
  253                 else {
  254                         lcs->cs_next = cs->cs_next;
  255                         cs->cs_next = lastcs->cs_next;
  256                         lastcs->cs_next = cs;
  257                 }
  258         }
  259 
  260         /*
  261          * Make sure that only what we expect to change changed. The first
  262          * line of the `if' checks the IP protocol version, header length &
  263          * type of service.  The 2nd line checks the "Don't fragment" bit.
  264          * The 3rd line checks the time-to-live and protocol (the protocol
  265          * check is unnecessary but costless).  The 4th line checks the TCP
  266          * header length.  The 5th line checks IP options, if any.  The 6th
  267          * line checks TCP options, if any.  If any of these things are
  268          * different between the previous & current datagram, we send the
  269          * current datagram `uncompressed'.
  270          */
  271         oth = (struct tcphdr *)&((int32_t *)&cs->cs_ip)[hlen];
  272         deltaS = hlen;
  273         hlen += th->th_off;
  274         hlen <<= 2;
  275         if (hlen > m->m_len)
  276                 return TYPE_IP;
  277 
  278         if (((uint16_t *)ip)[0] != ((uint16_t *)&cs->cs_ip)[0] ||
  279             ((uint16_t *)ip)[3] != ((uint16_t *)&cs->cs_ip)[3] ||
  280             ((uint16_t *)ip)[4] != ((uint16_t *)&cs->cs_ip)[4] ||
  281             th->th_off != oth->th_off ||
  282             (deltaS > 5 &&
  283              memcmp(ip + 1, &cs->cs_ip + 1, (deltaS - 5) << 2)) ||
  284             (th->th_off > 5 &&
  285              memcmp(th + 1, oth + 1, (th->th_off - 5) << 2)))
  286                 goto uncompressed;
  287 
  288         /*
  289          * Figure out which of the changing fields changed.  The receiver
  290          * expects changes in the order: urgent, window, ack, seq (the order
  291          * minimizes the number of temporaries needed in this section of code).
  292          */
  293         if (th->th_flags & TH_URG) {
  294                 deltaS = ntohs(th->th_urp);
  295                 ENCODEZ(deltaS);
  296                 changes |= NEW_U;
  297         } else if (th->th_urp != oth->th_urp)
  298                 /*
  299                  * argh! URG not set but urp changed -- a sensible
  300                  * implementation should never do this but RFC793 doesn't
  301                  * prohibit the change so we have to deal with it.
  302                  */
  303                  goto uncompressed;
  304 
  305         deltaS = (uint16_t)(ntohs(th->th_win) - ntohs(oth->th_win));
  306         if (deltaS) {
  307                 ENCODE(deltaS);
  308                 changes |= NEW_W;
  309         }
  310 
  311         deltaA = ntohl(th->th_ack) - ntohl(oth->th_ack);
  312         if (deltaA) {
  313                 if (deltaA > 0xffff)
  314                         goto uncompressed;
  315                 ENCODE(deltaA);
  316                 changes |= NEW_A;
  317         }
  318 
  319         deltaS = ntohl(th->th_seq) - ntohl(oth->th_seq);
  320         if (deltaS) {
  321                 if (deltaS > 0xffff)
  322                         goto uncompressed;
  323                 ENCODE(deltaS);
  324                 changes |= NEW_S;
  325         }
  326 
  327         switch (changes) {
  328         case 0:
  329                 /*
  330                  * Nothing changed. If this packet contains data and the
  331                  * last one didn't, this is probably a data packet following
  332                  * an ack (normal on an interactive connection) and we send
  333                  * it compressed.  Otherwise it's probably a retransmit,
  334                  * retransmitted ack or window probe.  Send it uncompressed
  335                  * in case the other side missed the compressed version.
  336                  */
  337                 if (ip->ip_len != cs->cs_ip.ip_len &&
  338                     ntohs(cs->cs_ip.ip_len) == hlen)
  339                         break;
  340 
  341                 /* FALLTHROUGH */
  342 
  343         case SPECIAL_I:
  344         case SPECIAL_D:
  345                 /*
  346                  * actual changes match one of our special case encodings --
  347                  * send packet uncompressed.
  348                  */
  349                 goto uncompressed;
  350 
  351         case NEW_S|NEW_A:
  352                 if (deltaS == deltaA &&
  353                     deltaS == ntohs(cs->cs_ip.ip_len) - hlen) {
  354                         /* special case for echoed terminal traffic */
  355                         changes = SPECIAL_I;
  356                         cp = new_seq;
  357                 }
  358                 break;
  359 
  360         case NEW_S:
  361                 if (deltaS == ntohs(cs->cs_ip.ip_len) - hlen) {
  362                         /* special case for data xfer */
  363                         changes = SPECIAL_D;
  364                         cp = new_seq;
  365                 }
  366                 break;
  367         }
  368 
  369         deltaS = ntohs(ip->ip_id) - ntohs(cs->cs_ip.ip_id);
  370         if (deltaS != 1) {
  371                 ENCODEZ(deltaS);
  372                 changes |= NEW_I;
  373         }
  374         if (th->th_flags & TH_PUSH)
  375                 changes |= TCP_PUSH_BIT;
  376         /*
  377          * Grab the cksum before we overwrite it below.  Then update our
  378          * state with this packet's header.
  379          */
  380         deltaA = ntohs(th->th_sum);
  381         memcpy(&cs->cs_ip, ip, hlen);
  382 
  383         /*
  384          * We want to use the original packet as our compressed packet.
  385          * (cp - new_seq) is the number of bytes we need for compressed
  386          * sequence numbers.  In addition we need one byte for the change
  387          * mask, one for the connection id and two for the tcp checksum.
  388          * So, (cp - new_seq) + 4 bytes of header are needed.  hlen is how
  389          * many bytes of the original packet to toss so subtract the two to
  390          * get the new packet size.
  391          */
  392         deltaS = cp - new_seq;
  393         cp = (u_char *)ip;
  394         if (compress_cid == 0 || comp->last_xmit != cs->cs_id) {
  395                 comp->last_xmit = cs->cs_id;
  396                 hlen -= deltaS + 4;
  397                 cp += hlen;
  398                 *cp++ = changes | NEW_C;
  399                 *cp++ = cs->cs_id;
  400         } else {
  401                 hlen -= deltaS + 3;
  402                 cp += hlen;
  403                 *cp++ = changes;
  404         }
  405         m->m_len -= hlen;
  406         m->m_data += hlen;
  407         *cp++ = deltaA >> 8;
  408         *cp++ = deltaA;
  409         memcpy(cp, new_seq, deltaS);
  410         INCR(sls_compressed)
  411         return TYPE_COMPRESSED_TCP;
  412 
  413         /*
  414          * Update connection state cs & send uncompressed packet
  415          * ('uncompressed' means a regular ip/tcp packet but with the
  416          * 'conversation id' we hope to use on future compressed packets in the
  417          * protocol field).
  418          */
  419 uncompressed:
  420         memcpy(&cs->cs_ip, ip, hlen);
  421         ip->ip_p = cs->cs_id;
  422         comp->last_xmit = cs->cs_id;
  423         return TYPE_UNCOMPRESSED_TCP;
  424 }
  425 
  426 
  427 int
  428 sl_uncompress_tcp(u_char **bufp, int len, u_int type, struct slcompress *comp)
  429 {
  430         u_char *hdr, *cp;
  431         int vjlen;
  432         u_int hlen;
  433 
  434         cp = bufp ? *bufp : NULL;
  435         vjlen = sl_uncompress_tcp_core(cp, len, len, type, comp, &hdr, &hlen);
  436         if (vjlen < 0)
  437                 return 0;       /* error */
  438         if (vjlen == 0)
  439                 return len;     /* was uncompressed already */
  440 
  441         cp += vjlen;
  442         len -= vjlen;
  443 
  444         /*
  445          * At this point, cp points to the first byte of data in the
  446          * packet.  If we're not aligned on a 4-byte boundary, copy the
  447          * data down so the ip & tcp headers will be aligned.  Then back up
  448          * cp by the tcp/ip header length to make room for the reconstructed
  449          * header (we assume the packet we were handed has enough space to
  450          * prepend 128 bytes of header).
  451          */
  452         if ((long)cp & 3) {
  453                 if (len > 0)
  454                         memmove((void *)((long)cp &~ 3), cp, len);
  455                 cp = (u_char *)((long)cp &~ 3);
  456         }
  457         cp -= hlen;
  458         len += hlen;
  459         memcpy(cp, hdr, hlen);
  460 
  461         *bufp = cp;
  462         return len;
  463 }
  464 
  465 /*
  466  * Uncompress a packet of total length total_len.  The first buflen bytes are
  467  * at buf; this must include the entire (compressed or uncompressed) TCP/IP
  468  * header.  This procedure returns the length of the VJ header, with a pointer
  469  * to the uncompressed IP header in *hdrp and its length in *hlenp.
  470  */
  471 int
  472 sl_uncompress_tcp_core(u_char *buf, int buflen, int total_len, u_int type,
  473     struct slcompress *comp, u_char **hdrp, u_int *hlenp)
  474 {
  475         u_char *cp;
  476         u_int hlen, changes;
  477         struct tcphdr *th;
  478         struct cstate *cs;
  479         struct ip *ip;
  480         uint16_t *bp;
  481         u_int vjlen;
  482 
  483         switch (type) {
  484 
  485         case TYPE_UNCOMPRESSED_TCP:
  486                 if (buf == NULL)
  487                         goto bad;
  488                 ip = (struct ip *) buf;
  489                 if (ip->ip_p >= MAX_STATES)
  490                         goto bad;
  491                 cs = &comp->rstate[comp->last_recv = ip->ip_p];
  492                 comp->flags &=~ SLF_TOSS;
  493                 ip->ip_p = IPPROTO_TCP;
  494                 /*
  495                  * Calculate the size of the TCP/IP header and make sure that
  496                  * we don't overflow the space we have available for it.
  497                  */
  498                 hlen = ip->ip_hl << 2;
  499                 if (hlen + sizeof(struct tcphdr) > buflen)
  500                         goto bad;
  501                 hlen += ((struct tcphdr *)&((char *)ip)[hlen])->th_off << 2;
  502                 if (hlen > MAX_HDR || hlen > buflen)
  503                         goto bad;
  504                 memcpy(&cs->cs_ip, ip, hlen);
  505                 cs->cs_hlen = hlen;
  506                 INCR(sls_uncompressedin)
  507                 *hdrp = (u_char *)&cs->cs_ip;
  508                 *hlenp = hlen;
  509                 return 0;
  510 
  511         default:
  512                 goto bad;
  513 
  514         case TYPE_COMPRESSED_TCP:
  515                 break;
  516         }
  517         /* We've got a compressed packet. */
  518         INCR(sls_compressedin)
  519         if (buf == NULL)
  520                 goto bad;
  521         cp = buf;
  522         changes = *cp++;
  523         if (changes & NEW_C) {
  524                 /*
  525                  * Make sure the state index is in range, then grab the state.
  526                  * If we have a good state index, clear the 'discard' flag.
  527                  */
  528                 if (*cp >= MAX_STATES)
  529                         goto bad;
  530 
  531                 comp->flags &=~ SLF_TOSS;
  532                 comp->last_recv = *cp++;
  533         } else {
  534                 /*
  535                  * this packet has an implicit state index.  If we've had a
  536                  * line error since the last time we got an explicit state
  537                  * index, we have to toss the packet.
  538                  */
  539                 if (comp->flags & SLF_TOSS) {
  540                         INCR(sls_tossed)
  541                         return -1;
  542                 }
  543         }
  544         cs = &comp->rstate[comp->last_recv];
  545         hlen = cs->cs_ip.ip_hl << 2;
  546         th = (struct tcphdr *)&((u_char *)&cs->cs_ip)[hlen];
  547         th->th_sum = htons((*cp << 8) | cp[1]);
  548         cp += 2;
  549         if (changes & TCP_PUSH_BIT)
  550                 th->th_flags |= TH_PUSH;
  551         else
  552                 th->th_flags &=~ TH_PUSH;
  553 
  554         switch (changes & SPECIALS_MASK) {
  555         case SPECIAL_I:
  556                 {
  557                 u_int i = ntohs(cs->cs_ip.ip_len) - cs->cs_hlen;
  558                 th->th_ack = htonl(ntohl(th->th_ack) + i);
  559                 th->th_seq = htonl(ntohl(th->th_seq) + i);
  560                 }
  561                 break;
  562 
  563         case SPECIAL_D:
  564                 th->th_seq = htonl(ntohl(th->th_seq) + ntohs(cs->cs_ip.ip_len)
  565                                    - cs->cs_hlen);
  566                 break;
  567 
  568         default:
  569                 if (changes & NEW_U) {
  570                         th->th_flags |= TH_URG;
  571                         DECODEU(th->th_urp)
  572                 } else
  573                         th->th_flags &=~ TH_URG;
  574                 if (changes & NEW_W)
  575                         DECODES(th->th_win)
  576                 if (changes & NEW_A)
  577                         DECODEL(th->th_ack)
  578                 if (changes & NEW_S)
  579                         DECODEL(th->th_seq)
  580                 break;
  581         }
  582         if (changes & NEW_I) {
  583                 DECODES(cs->cs_ip.ip_id)
  584         } else
  585                 cs->cs_ip.ip_id = htons(ntohs(cs->cs_ip.ip_id) + 1);
  586 
  587         /*
  588          * At this point, cp points to the first byte of data in the packet.
  589          * Fill in the IP total length and update the IP header checksum.
  590          */
  591         vjlen = cp - buf;
  592         buflen -= vjlen;
  593         if (buflen < 0)
  594                 /*
  595                  * We must have dropped some characters (crc should detect
  596                  * this but the old slip framing won't)
  597                  */
  598                 goto bad;
  599 
  600         total_len += cs->cs_hlen - vjlen;
  601         cs->cs_ip.ip_len = htons(total_len);
  602 
  603         /* Recompute the ip header checksum */
  604         bp = (uint16_t *)&cs->cs_ip;
  605         cs->cs_ip.ip_sum = 0;
  606         for (changes = 0; hlen > 0; hlen -= 2)
  607                 changes += *bp++;
  608         changes = (changes & 0xffff) + (changes >> 16);
  609         changes = (changes & 0xffff) + (changes >> 16);
  610         cs->cs_ip.ip_sum = ~ changes;
  611 
  612         *hdrp = (u_char *)&cs->cs_ip;
  613         *hlenp = cs->cs_hlen;
  614         return vjlen;
  615 
  616 bad:
  617         comp->flags |= SLF_TOSS;
  618         INCR(sls_errorin)
  619         return -1;
  620 }
  621 #endif
  622 
  623 MODULE(MODULE_CLASS_MISC, slcompress, NULL);
  624 
  625 static int
  626 slcompress_modcmd(modcmd_t cmd, void *arg)
  627 {
  628         switch (cmd) {
  629         case MODULE_CMD_INIT:
  630         case MODULE_CMD_FINI:
  631 #ifdef INET
  632                 return 0;
  633 #endif
  634         case MODULE_CMD_STAT:
  635         case MODULE_CMD_AUTOUNLOAD:
  636         default:
  637                 return ENOTTY;
  638         }
  639 }

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