FreeBSD/Linux Kernel Cross Reference
sys/net/bsd-comp.c

     /*      $NetBSD: bsd-comp.c,v 1.16 2006/11/16 01:33:40 christos Exp $   */
     /*      Id: bsd-comp.c,v 1.6 1996/08/28 06:31:58 paulus Exp     */
     
     /* Because this code is derived from the 4.3BSD compress source:
      *
      *
      * Copyright (c) 1985, 1986 The Regents of the University of California.
      * All rights reserved.
      *
     * This code is derived from software contributed to Berkeley by
     * James A. Woods, derived from original work by Spencer Thomas
     * and Joseph Orost.
     *
     * Redistribution and use in source and binary forms, with or without
     * modification, are permitted provided that the following conditions
     * are met:
     * 1. Redistributions of source code must retain the above copyright
     *    notice, this list of conditions and the following disclaimer.
     * 2. Redistributions in binary form must reproduce the above copyright
     *    notice, this list of conditions and the following disclaimer in the
     *    documentation and/or other materials provided with the distribution.
     * 3. Neither the name of the University nor the names of its contributors
     *    may be used to endorse or promote products derived from this software
     *    without specific prior written permission.
     *
     * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
     * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
     * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
     * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
     * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
     * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
     * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
     * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
     * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
     * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
     * SUCH DAMAGE.
     */
    
    /*
     * This version is for use with mbufs on BSD-derived systems.
     */
    
    #include <sys/cdefs.h>
    __KERNEL_RCSID(0, "$NetBSD: bsd-comp.c,v 1.16 2006/11/16 01:33:40 christos Exp $");
    
    #include <sys/param.h>
    #include <sys/systm.h>
    #include <sys/mbuf.h>
    #include <sys/socket.h>
    #include <net/if.h>
    #include <net/if_types.h>
    #include <net/ppp_defs.h>
    #include <net/if_ppp.h>
    
    #define PACKETPTR       struct mbuf *
    #include <net/ppp-comp.h>
    
    #if DO_BSD_COMPRESS
    /*
     * PPP "BSD compress" compression
     *  The differences between this compression and the classic BSD LZW
     *  source are obvious from the requirement that the classic code worked
     *  with files while this handles arbitrarily long streams that
     *  are broken into packets.  They are:
     *
     *      When the code size expands, a block of junk is not emitted by
     *          the compressor and not expected by the decompressor.
     *
     *      New codes are not necessarily assigned every time an old
     *          code is output by the compressor.  This is because a packet
     *          end forces a code to be emitted, but does not imply that a
     *          new sequence has been seen.
     *
     *      The compression ratio is checked at the first end of a packet
     *          after the appropriate gap.  Besides simplifying and speeding
     *          things up, this makes it more likely that the transmitter
     *          and receiver will agree when the dictionary is cleared when
     *          compression is not going well.
     */
    
    /*
     * A dictionary for doing BSD compress.
     */
    struct bsd_db {
        int     totlen;                     /* length of this structure */
        u_int   hsize;                      /* size of the hash table */
        u_char  hshift;                     /* used in hash function */
        u_char  n_bits;                     /* current bits/code */
        u_char  maxbits;
        u_char  debug;
        u_char  unit;
        u_int16_t seqno;                    /* sequence # of next packet */
        u_int   hdrlen;                     /* header length to preallocate */
        u_int   mru;
        u_int   maxmaxcode;                 /* largest valid code */
        u_int   max_ent;                    /* largest code in use */
        u_int   in_count;                   /* uncompressed bytes, aged */
        u_int   bytes_out;                  /* compressed bytes, aged */
        u_int   ratio;                      /* recent compression ratio */
       u_int   checkpoint;                 /* when to next check the ratio */
       u_int   clear_count;                /* times dictionary cleared */
       u_int   incomp_count;               /* incompressible packets */
       u_int   incomp_bytes;               /* incompressible bytes */
       u_int   uncomp_count;               /* uncompressed packets */
       u_int   uncomp_bytes;               /* uncompressed bytes */
       u_int   comp_count;                 /* compressed packets */
       u_int   comp_bytes;                 /* compressed bytes */
       u_int16_t *lens;                    /* array of lengths of codes */
       struct bsd_dict {
           union {                         /* hash value */
               u_int32_t   fcode;
               struct {
   #if BYTE_ORDER == LITTLE_ENDIAN
                   u_int16_t prefix;       /* preceding code */
                   u_char  suffix;         /* last character of new code */
                   u_char  pad;
   #else
                   u_char  pad;
                   u_char  suffix;         /* last character of new code */
                   u_int16_t prefix;       /* preceding code */
   #endif
               } hs;
           } f;
           u_int16_t codem1;               /* output of hash table -1 */
           u_int16_t cptr;                 /* map code to hash table entry */
       } dict[1];
   };
   
   #define BSD_OVHD        2               /* BSD compress overhead/packet */
   #define BSD_INIT_BITS   BSD_MIN_BITS
   
   static void     *bsd_comp_alloc(u_char *options, int opt_len);
   static void     *bsd_decomp_alloc(u_char *options, int opt_len);
   static void     bsd_free(void *state);
   static int      bsd_comp_init(void *state, u_char *options, int opt_len,
                                 int unit, int hdrlen, int debug);
   static int      bsd_decomp_init(void *state, u_char *options, int opt_len,
                                   int unit, int hdrlen, int mru, int debug);
   static int      bsd_compress(void *state, struct mbuf **mret,
                                struct mbuf *mp, int slen, int maxolen);
   static void     bsd_incomp(void *state, struct mbuf *dmsg);
   static int      bsd_decompress(void *state, struct mbuf *cmp,
                                  struct mbuf **dmpp);
   static void     bsd_reset(void *state);
   static void     bsd_comp_stats(void *state, struct compstat *stats);
   
   /*
    * Procedures exported to if_ppp.c.
    */
   struct compressor ppp_bsd_compress = {
       CI_BSD_COMPRESS,            /* compress_proto */
       bsd_comp_alloc,             /* comp_alloc */
       bsd_free,                   /* comp_free */
       bsd_comp_init,              /* comp_init */
       bsd_reset,                  /* comp_reset */
       bsd_compress,               /* compress */
       bsd_comp_stats,             /* comp_stat */
       bsd_decomp_alloc,           /* decomp_alloc */
       bsd_free,                   /* decomp_free */
       bsd_decomp_init,            /* decomp_init */
       bsd_reset,                  /* decomp_reset */
       bsd_decompress,             /* decompress */
       bsd_incomp,                 /* incomp */
       bsd_comp_stats,             /* decomp_stat */
   };
   
   /*
    * the next two codes should not be changed lightly, as they must not
    * lie within the contiguous general code space.
    */
   #define CLEAR   256                     /* table clear output code */
   #define FIRST   257                     /* first free entry */
   #define LAST    255
   
   #define MAXCODE(b)      ((1 << (b)) - 1)
   #define BADCODEM1       MAXCODE(BSD_MAX_BITS)
   
   #define BSD_HASH(prefix,suffix,hshift)  ((((u_int32_t)(suffix)) << (hshift)) \
                                            ^ (u_int32_t)(prefix))
   #define BSD_KEY(prefix,suffix)          ((((u_int32_t)(suffix)) << 16) \
                                            + (u_int32_t)(prefix))
   
   #define CHECK_GAP       10000           /* Ratio check interval */
   
   #define RATIO_SCALE_LOG 8
   #define RATIO_SCALE     (1<<RATIO_SCALE_LOG)
   #define RATIO_MAX       (0x7fffffff>>RATIO_SCALE_LOG)
   
   static void     bsd_clear(struct bsd_db *);
   static int      bsd_check(struct bsd_db *);
   static void     *bsd_alloc(u_char *, int, int);
   static int      bsd_init(struct bsd_db *, u_char *, int, int, int, int,
                            int, int);
   
   /*
    * clear the dictionary
    */
   static void
   bsd_clear(struct bsd_db *db)
   {
       db->clear_count++;
       db->max_ent = FIRST-1;
       db->n_bits = BSD_INIT_BITS;
       db->ratio = 0;
       db->bytes_out = 0;
       db->in_count = 0;
       db->checkpoint = CHECK_GAP;
   }
   
   /*
    * If the dictionary is full, then see if it is time to reset it.
    *
    * Compute the compression ratio using fixed-point arithmetic
    * with 8 fractional bits.
    *
    * Since we have an infinite stream instead of a single file,
    * watch only the local compression ratio.
    *
    * Since both peers must reset the dictionary at the same time even in
    * the absence of CLEAR codes (while packets are incompressible), they
    * must compute the same ratio.
    */
   static int                              /* 1=output CLEAR */
   bsd_check(struct bsd_db *db)
   {
       u_int new_ratio;
   
       if (db->in_count >= db->checkpoint) {
           /* age the ratio by limiting the size of the counts */
           if (db->in_count >= RATIO_MAX
               || db->bytes_out >= RATIO_MAX) {
               db->in_count -= db->in_count/4;
               db->bytes_out -= db->bytes_out/4;
           }
   
           db->checkpoint = db->in_count + CHECK_GAP;
   
           if (db->max_ent >= db->maxmaxcode) {
               /* Reset the dictionary only if the ratio is worse,
                * or if it looks as if it has been poisoned
                * by incompressible data.
                *
                * This does not overflow, because
                *  db->in_count <= RATIO_MAX.
                */
               new_ratio = db->in_count << RATIO_SCALE_LOG;
               if (db->bytes_out != 0)
                   new_ratio /= db->bytes_out;
   
               if (new_ratio < db->ratio || new_ratio < 1 * RATIO_SCALE) {
                   bsd_clear(db);
                   return 1;
               }
               db->ratio = new_ratio;
           }
       }
       return 0;
   }
   
   /*
    * Return statistics.
    */
   static void
   bsd_comp_stats(void *state, struct compstat *stats)
   {
       struct bsd_db *db = (struct bsd_db *) state;
       u_int out;
   
       stats->unc_bytes = db->uncomp_bytes;
       stats->unc_packets = db->uncomp_count;
       stats->comp_bytes = db->comp_bytes;
       stats->comp_packets = db->comp_count;
       stats->inc_bytes = db->incomp_bytes;
       stats->inc_packets = db->incomp_count;
       stats->ratio = db->in_count;
       out = db->bytes_out;
       if (stats->ratio <= 0x7fffff)
           stats->ratio <<= 8;
       else
           out >>= 8;
       if (out != 0)
           stats->ratio /= out;
   }
   
   /*
    * Reset state, as on a CCP ResetReq.
    */
   static void
   bsd_reset(void *state)
   {
       struct bsd_db *db = (struct bsd_db *) state;
   
       db->seqno = 0;
       bsd_clear(db);
       db->clear_count = 0;
   }
   
   /*
    * Allocate space for a (de) compressor.
    */
   static void *
   bsd_alloc(u_char *options, int opt_len, int decomp)
   {
       int bits;
       u_int newlen, hsize, hshift, maxmaxcode;
       struct bsd_db *db;
   
       if (opt_len < CILEN_BSD_COMPRESS || options[0] != CI_BSD_COMPRESS
           || options[1] != CILEN_BSD_COMPRESS
           || BSD_VERSION(options[2]) != BSD_CURRENT_VERSION)
           return NULL;
       bits = BSD_NBITS(options[2]);
       switch (bits) {
       case 9:                     /* needs 82152 for both directions */
       case 10:                    /* needs 84144 */
       case 11:                    /* needs 88240 */
       case 12:                    /* needs 96432 */
           hsize = 5003;
           hshift = 4;
           break;
       case 13:                    /* needs 176784 */
           hsize = 9001;
           hshift = 5;
           break;
       case 14:                    /* needs 353744 */
           hsize = 18013;
           hshift = 6;
           break;
       case 15:                    /* needs 691440 */
           hsize = 35023;
           hshift = 7;
           break;
       case 16:                    /* needs 1366160--far too much, */
           /* hsize = 69001; */    /* and 69001 is too big for cptr */
           /* hshift = 8; */       /* in struct bsd_db */
           /* break; */
       default:
           return NULL;
       }
   
       maxmaxcode = MAXCODE(bits);
       newlen = sizeof(*db) + (hsize-1) * (sizeof(db->dict[0]));
       db = malloc(newlen, M_DEVBUF, M_NOWAIT);
       if (!db)
           return NULL;
       memset(db, 0, sizeof(*db) - sizeof(db->dict));
   
       if (!decomp) {
           db->lens = NULL;
       } else {
           db->lens = malloc((maxmaxcode+1) * sizeof(db->lens[0]),
               M_DEVBUF, M_NOWAIT);
           if (!db->lens) {
               free(db, M_DEVBUF);
               return NULL;
           }
       }
   
       db->totlen = newlen;
       db->hsize = hsize;
       db->hshift = hshift;
       db->maxmaxcode = maxmaxcode;
       db->maxbits = bits;
   
       return (void *) db;
   }
   
   static void
   bsd_free(void *state)
   {
       struct bsd_db *db = (struct bsd_db *) state;
   
       if (db->lens)
           free(db->lens, M_DEVBUF);
       free(db, M_DEVBUF);
   }
   
   static void *
   bsd_comp_alloc(u_char *options, int opt_len)
   {
       return bsd_alloc(options, opt_len, 0);
   }
   
   static void *
   bsd_decomp_alloc(u_char *options, int opt_len)
   {
       return bsd_alloc(options, opt_len, 1);
   }
   
   /*
    * Initialize the database.
    */
   static int
   bsd_init(struct bsd_db *db, u_char *options, int opt_len, int unit, int hdrlen,
       int mru, int debug, int decomp)
   {
       int i;
   
       if (opt_len < CILEN_BSD_COMPRESS || options[0] != CI_BSD_COMPRESS
           || options[1] != CILEN_BSD_COMPRESS
           || BSD_VERSION(options[2]) != BSD_CURRENT_VERSION
           || BSD_NBITS(options[2]) != db->maxbits
           || (decomp && db->lens == NULL))
           return 0;
   
       if (decomp) {
           i = LAST+1;
           while (i != 0)
               db->lens[--i] = 1;
       }
       i = db->hsize;
       while (i != 0) {
           db->dict[--i].codem1 = BADCODEM1;
           db->dict[i].cptr = 0;
       }
   
       db->unit = unit;
       db->hdrlen = hdrlen;
       db->mru = mru;
   #ifndef DEBUG
       if (debug)
   #endif
           db->debug = 1;
   
       bsd_reset(db);
   
       return 1;
   }
   
   static int
   bsd_comp_init(void *state, u_char *options, int opt_len, int unit, int hdrlen,
                 int debug)
   {
       return bsd_init((struct bsd_db *) state, options, opt_len,
                       unit, hdrlen, 0, debug, 0);
   }
   
   static int
   bsd_decomp_init(void *state, u_char *options, int opt_len, int unit, int hdrlen,
                   int mru, int debug)
   {
       return bsd_init((struct bsd_db *) state, options, opt_len,
                       unit, hdrlen, mru, debug, 1);
   }
   
   
   /*
    * compress a packet
    *      One change from the BSD compress command is that when the
    *      code size expands, we do not output a bunch of padding.
    */
   int                                     /* new slen */
   bsd_compress(void *state,
                struct mbuf **mret /* return compressed mbuf chain here */,
                struct mbuf *mp /* from here */,
                int slen, int maxolen)
   {
       struct bsd_db *db = (struct bsd_db *) state;
       int hshift = db->hshift;
       u_int max_ent = db->max_ent;
       u_int n_bits = db->n_bits;
       u_int bitno = 32;
       u_int32_t accm = 0, fcode;
       struct bsd_dict *dictp;
       u_char c;
       int hval, disp, ent, ilen;
       u_char *rptr, *wptr;
       u_char *cp_end;
       int olen;
       struct mbuf *m;
   
   #define PUTBYTE(v) {                                    \
       ++olen;                                             \
       if (wptr) {                                         \
           *wptr++ = (v);                                  \
           if (wptr >= cp_end) {                           \
               m->m_len = wptr - mtod(m, u_char *);        \
               MGET(m->m_next, M_DONTWAIT, MT_DATA);       \
               m = m->m_next;                              \
               if (m) {                                    \
                   m->m_len = 0;                           \
                   if (maxolen - olen > MLEN)              \
                       MCLGET(m, M_DONTWAIT);              \
                   wptr = mtod(m, u_char *);               \
                   cp_end = wptr + M_TRAILINGSPACE(m);     \
               } else                                      \
                   wptr = NULL;                            \
           }                                               \
       }                                                   \
   }
   
   #define OUTPUT(ent) {                                   \
       bitno -= n_bits;                                    \
       accm |= ((ent) << bitno);                           \
       do {                                                \
           PUTBYTE(accm >> 24);                            \
           accm <<= 8;                                     \
           bitno += 8;                                     \
       } while (bitno <= 24);                              \
   }
   
       /*
        * If the protocol is not in the range we're interested in,
        * just return without compressing the packet.  If it is,
        * the protocol becomes the first byte to compress.
        */
       rptr = mtod(mp, u_char *);
       ent = PPP_PROTOCOL(rptr);
       if (ent < 0x21 || ent > 0xf9) {
           *mret = NULL;
           return slen;
       }
   
       /* Don't generate compressed packets which are larger than
          the uncompressed packet. */
       if (maxolen > slen)
           maxolen = slen;
   
       /* Allocate one mbuf to start with. */
       MGET(m, M_DONTWAIT, MT_DATA);
       *mret = m;
       if (m != NULL) {
           m->m_len = 0;
           if (maxolen + db->hdrlen > MLEN)
               MCLGET(m, M_DONTWAIT);
           m->m_data += db->hdrlen;
           wptr = mtod(m, u_char *);
           cp_end = wptr + M_TRAILINGSPACE(m);
       } else
           wptr = cp_end = NULL;
   
       /*
        * Copy the PPP header over, changing the protocol,
        * and install the 2-byte packet sequence number.
        */
       if (wptr) {
           *wptr++ = PPP_ADDRESS(rptr);    /* assumes the ppp header is */
           *wptr++ = PPP_CONTROL(rptr);    /* all in one mbuf */
           *wptr++ = 0;                    /* change the protocol */
           *wptr++ = PPP_COMP;
           *wptr++ = db->seqno >> 8;
           *wptr++ = db->seqno;
       }
       ++db->seqno;
   
       olen = 0;
       rptr += PPP_HDRLEN;
       slen = mp->m_len - PPP_HDRLEN;
       ilen = slen + 1;
       for (;;) {
           if (slen <= 0) {
               mp = mp->m_next;
               if (!mp)
                   break;
               rptr = mtod(mp, u_char *);
               slen = mp->m_len;
               if (!slen)
                   continue;   /* handle 0-length buffers */
               ilen += slen;
           }
   
           slen--;
           c = *rptr++;
           fcode = BSD_KEY(ent, c);
           hval = BSD_HASH(ent, c, hshift);
           dictp = &db->dict[hval];
   
           /* Validate and then check the entry. */
           if (dictp->codem1 >= max_ent)
               goto nomatch;
           if (dictp->f.fcode == fcode) {
               ent = dictp->codem1+1;
               continue;   /* found (prefix,suffix) */
           }
   
           /* continue probing until a match or invalid entry */
           disp = (hval == 0) ? 1 : hval;
           do {
               hval += disp;
               if (hval >= db->hsize)
                   hval -= db->hsize;
               dictp = &db->dict[hval];
               if (dictp->codem1 >= max_ent)
                   goto nomatch;
           } while (dictp->f.fcode != fcode);
           ent = dictp->codem1 + 1;        /* finally found (prefix,suffix) */
           continue;
   
       nomatch:
           OUTPUT(ent);            /* output the prefix */
   
           /* code -> hashtable */
           if (max_ent < db->maxmaxcode) {
               struct bsd_dict *dictp2;
               /* expand code size if needed */
               if (max_ent >= MAXCODE(n_bits))
                   db->n_bits = ++n_bits;
   
               /* Invalidate old hash table entry using
                * this code, and then take it over.
                */
               dictp2 = &db->dict[max_ent+1];
               if (db->dict[dictp2->cptr].codem1 == max_ent)
                   db->dict[dictp2->cptr].codem1 = BADCODEM1;
               dictp2->cptr = hval;
               dictp->codem1 = max_ent;
               dictp->f.fcode = fcode;
   
               db->max_ent = ++max_ent;
           }
           ent = c;
       }
   
       OUTPUT(ent);                /* output the last code */
       db->bytes_out += olen;
       db->in_count += ilen;
       if (bitno < 32)
           ++db->bytes_out;        /* count complete bytes */
   
       if (bsd_check(db))
           OUTPUT(CLEAR);          /* do not count the CLEAR */
   
       /*
        * Pad dribble bits of last code with ones.
        * Do not emit a completely useless byte of ones.
        */
       if (bitno != 32)
           PUTBYTE((accm | (0xff << (bitno-8))) >> 24);
   
       if (m != NULL) {
           m->m_len = wptr - mtod(m, u_char *);
           m->m_next = NULL;
       }
   
       /*
        * Increase code size if we would have without the packet
        * boundary and as the decompressor will.
        */
       if (max_ent >= MAXCODE(n_bits) && max_ent < db->maxmaxcode)
           db->n_bits++;
   
       db->uncomp_bytes += ilen;
       ++db->uncomp_count;
       if (olen + PPP_HDRLEN + BSD_OVHD > maxolen) {
           /* throw away the compressed stuff if it is longer than uncompressed */
           if (*mret != NULL) {
               m_freem(*mret);
               *mret = NULL;
           }
           ++db->incomp_count;
           db->incomp_bytes += ilen;
       } else {
           ++db->comp_count;
           db->comp_bytes += olen + BSD_OVHD;
       }
   
       return olen + PPP_HDRLEN + BSD_OVHD;
   #undef OUTPUT
   #undef PUTBYTE
   }
   
   
   /*
    * Update the "BSD Compress" dictionary on the receiver for
    * incompressible data by pretending to compress the incoming data.
    */
   static void
   bsd_incomp(void *state, struct mbuf *dmsg)
   {
       struct bsd_db *db = (struct bsd_db *) state;
       u_int hshift = db->hshift;
       u_int max_ent = db->max_ent;
       u_int n_bits = db->n_bits;
       struct bsd_dict *dictp;
       u_int32_t fcode;
       u_char c;
       u_int32_t hval, disp;
       int slen, ilen;
       u_int bitno = 7;
       u_char *rptr;
       u_int ent;
   
       /*
        * If the protocol is not in the range we're interested in,
        * just return without looking at the packet.  If it is,
        * the protocol becomes the first byte to "compress".
        */
       rptr = mtod(dmsg, u_char *);
       ent = PPP_PROTOCOL(rptr);
       if (ent < 0x21 || ent > 0xf9)
           return;
   
       db->seqno++;
       ilen = 1;           /* count the protocol as 1 byte */
       rptr += PPP_HDRLEN;
       slen = dmsg->m_len - PPP_HDRLEN;
       for (;;) {
           if (slen <= 0) {
               dmsg = dmsg->m_next;
               if (!dmsg)
                   break;
               rptr = mtod(dmsg, u_char *);
               slen = dmsg->m_len;
               continue;
           }
           ilen += slen;
   
           do {
               c = *rptr++;
               fcode = BSD_KEY(ent, c);
               hval = BSD_HASH(ent, c, hshift);
               dictp = &db->dict[hval];
   
               /* validate and then check the entry */
               if (dictp->codem1 >= max_ent)
                   goto nomatch;
               if (dictp->f.fcode == fcode) {
                   ent = dictp->codem1+1;
                   continue;   /* found (prefix,suffix) */
               }
   
               /* continue probing until a match or invalid entry */
               disp = (hval == 0) ? 1 : hval;
               do {
                   hval += disp;
                   if (hval >= db->hsize)
                       hval -= db->hsize;
                   dictp = &db->dict[hval];
                   if (dictp->codem1 >= max_ent)
                       goto nomatch;
               } while (dictp->f.fcode != fcode);
               ent = dictp->codem1+1;
               continue;   /* finally found (prefix,suffix) */
   
           nomatch:                /* output (count) the prefix */
               bitno += n_bits;
   
               /* code -> hashtable */
               if (max_ent < db->maxmaxcode) {
                   struct bsd_dict *dictp2;
                   /* expand code size if needed */
                   if (max_ent >= MAXCODE(n_bits))
                       db->n_bits = ++n_bits;
   
                   /* Invalidate previous hash table entry
                    * assigned this code, and then take it over.
                    */
                   dictp2 = &db->dict[max_ent+1];
                   if (db->dict[dictp2->cptr].codem1 == max_ent)
                       db->dict[dictp2->cptr].codem1 = BADCODEM1;
                   dictp2->cptr = hval;
                   dictp->codem1 = max_ent;
                   dictp->f.fcode = fcode;
   
                   db->max_ent = ++max_ent;
                   db->lens[max_ent] = db->lens[ent]+1;
               }
               ent = c;
           } while (--slen != 0);
       }
       bitno += n_bits;            /* output (count) the last code */
       db->bytes_out += bitno/8;
       db->in_count += ilen;
       (void)bsd_check(db);
   
       ++db->incomp_count;
       db->incomp_bytes += ilen;
       ++db->uncomp_count;
       db->uncomp_bytes += ilen;
   
       /* Increase code size if we would have without the packet
        * boundary and as the decompressor will.
        */
       if (max_ent >= MAXCODE(n_bits) && max_ent < db->maxmaxcode)
           db->n_bits++;
   }
   
   
   /*
    * Decompress "BSD Compress".
    *
    * Because of patent problems, we return DECOMP_ERROR for errors
    * found by inspecting the input data and for system problems, but
    * DECOMP_FATALERROR for any errors which could possibly be said to
    * be being detected "after" decompression.  For DECOMP_ERROR,
    * we can issue a CCP reset-request; for DECOMP_FATALERROR, we may be
    * infringing a patent of Motorola's if we do, so we take CCP down
    * instead.
    *
    * Given that the frame has the correct sequence number and a good FCS,
    * errors such as invalid codes in the input most likely indicate a
    * bug, so we return DECOMP_FATALERROR for them in order to turn off
    * compression, even though they are detected by inspecting the input.
    */
   int
   bsd_decompress(void *state, struct mbuf *cmp, struct mbuf **dmpp)
   {
       struct bsd_db *db = (struct bsd_db *) state;
       u_int max_ent = db->max_ent;
       u_int32_t accm = 0;
       u_int bitno = 32;           /* 1st valid bit in accm */
       u_int n_bits = db->n_bits;
       u_int tgtbitno = 32-n_bits; /* bitno when we have a code */
       struct bsd_dict *dictp;
       int explen, i, seq, len;
       u_int incode, oldcode, finchar;
       u_char *p, *rptr, *wptr;
       struct mbuf *m, *dmp, *mret;
       int adrs, ctrl, ilen;
       int space, codelen, extra;
   
       /*
        * Save the address/control from the PPP header
        * and then get the sequence number.
        */
       *dmpp = NULL;
       rptr = mtod(cmp, u_char *);
       adrs = PPP_ADDRESS(rptr);
       ctrl = PPP_CONTROL(rptr);
       rptr += PPP_HDRLEN;
       len = cmp->m_len - PPP_HDRLEN;
       seq = 0;
       for (i = 0; i < 2; ++i) {
           while (len <= 0) {
               cmp = cmp->m_next;
               if (cmp == NULL)
                   return DECOMP_ERROR;
               rptr = mtod(cmp, u_char *);
               len = cmp->m_len;
           }
           seq = (seq << 8) + *rptr++;
           --len;
       }
   
       /*
        * Check the sequence number and give up if it differs from
        * the value we're expecting.
        */
       if (seq != db->seqno) {
           if (db->debug)
               printf("bsd_decomp%d: bad sequence # %d, expected %d\n",
                      db->unit, seq, db->seqno - 1);
           return DECOMP_ERROR;
       }
       ++db->seqno;
   
       /*
        * Allocate one mbuf to start with.
        */
       MGETHDR(dmp, M_DONTWAIT, MT_DATA);
       if (dmp == NULL)
           return DECOMP_ERROR;
       mret = dmp;
       dmp->m_len = 0;
       dmp->m_next = NULL;
       MCLGET(dmp, M_DONTWAIT);
       dmp->m_data += db->hdrlen;
       wptr = mtod(dmp, u_char *);
       space = M_TRAILINGSPACE(dmp) - PPP_HDRLEN + 1;
   
       /*
        * Fill in the ppp header, but not the last byte of the protocol
        * (that comes from the decompressed data).
        */
       wptr[0] = adrs;
       wptr[1] = ctrl;
       wptr[2] = 0;
       wptr += PPP_HDRLEN - 1;
   
       ilen = len;
       oldcode = CLEAR;
       explen = 0;
       for (;;) {
           if (len == 0) {
               cmp = cmp->m_next;
               if (!cmp)           /* quit at end of message */
                   break;
               rptr = mtod(cmp, u_char *);
               len = cmp->m_len;
               ilen += len;
               continue;           /* handle 0-length buffers */
           }
   
           /*
            * Accumulate bytes until we have a complete code.
            * Then get the next code, relying on the 32-bit,
            * unsigned accm to mask the result.
            */
           bitno -= 8;
           accm |= *rptr++ << bitno;
           --len;
           if (tgtbitno < bitno)
               continue;
           incode = accm >> tgtbitno;
           accm <<= n_bits;
           bitno += n_bits;
   
           if (incode == CLEAR) {
               /*
                * The dictionary must only be cleared at
                * the end of a packet.  But there could be an
                * empty mbuf at the end.
                */
               if (len > 0 || cmp->m_next != NULL) {
                   while ((cmp = cmp->m_next) != NULL)
                       len += cmp->m_len;
                   if (len > 0) {
                       m_freem(mret);
                       if (db->debug)
                           printf("bsd_decomp%d: bad CLEAR\n", db->unit);
                       return DECOMP_FATALERROR;   /* probably a bug */
                   }
               }
               bsd_clear(db);
               explen = ilen = 0;
               break;
           }
   
           if (incode > max_ent + 2 || incode > db->maxmaxcode
               || (incode > max_ent && oldcode == CLEAR)) {
               m_freem(mret);
               if (db->debug) {
                   printf("bsd_decomp%d: bad code 0x%x oldcode=0x%x ",
                          db->unit, incode, oldcode);
                   printf("max_ent=0x%x explen=%d seqno=%d\n",
                          max_ent, explen, db->seqno);
               }
               return DECOMP_FATALERROR;   /* probably a bug */
           }
   
           /* Special case for KwKwK string. */
           if (incode > max_ent) {
               finchar = oldcode;
               extra = 1;
           } else {
               finchar = incode;
               extra = 0;
           }
   
           codelen = db->lens[finchar];
           explen += codelen + extra;
           if (explen > db->mru + 1) {
               m_freem(mret);
               if (db->debug) {
                   printf("bsd_decomp%d: ran out of mru\n", db->unit);
   #ifdef DEBUG
                   while ((cmp = cmp->m_next) != NULL)
                       len += cmp->m_len;
                   printf("  len=%d, finchar=0x%x, codelen=%d, explen=%d\n",
                          len, finchar, codelen, explen);
   #endif
               }
               return DECOMP_FATALERROR;
           }
   
           /*
            * For simplicity, the decoded characters go in a single mbuf,
            * so we allocate a single extra cluster mbuf if necessary.
            */
           if ((space -= codelen + extra) < 0) {
               dmp->m_len = wptr - mtod(dmp, u_char *);
               MGET(m, M_DONTWAIT, MT_DATA);
               if (m == NULL) {
                   m_freem(mret);
                   return DECOMP_ERROR;
               }
               m->m_len = 0;
               m->m_next = NULL;
               dmp->m_next = m;
               MCLGET(m, M_DONTWAIT);
               space = M_TRAILINGSPACE(m) - (codelen + extra);
               if (space < 0) {
                   /* now that's what I call *compression*. */
                   m_freem(mret);
                   return DECOMP_ERROR;
               }
               dmp = m;
               wptr = mtod(dmp, u_char *);
           }
   
           /*
            * Decode this code and install it in the decompressed buffer.
            */
           p = (wptr += codelen);
           while (finchar > LAST) {
               dictp = &db->dict[db->dict[finchar].cptr];
   #ifdef DEBUG
               if (--codelen <= 0 || dictp->codem1 != finchar-1)
                   goto bad;
   #endif
               *--p = dictp->f.hs.suffix;
               finchar = dictp->f.hs.prefix;
           }
           *--p = finchar;
   
   #ifdef DEBUG
           if (--codelen != 0)
               printf("bsd_decomp%d: short by %d after code 0x%x, max_ent=0x%x\n",
                      db->unit, codelen, incode, max_ent);
  #endif
  
          if (extra)              /* the KwKwK case again */
              *wptr++ = finchar;
  
          /*
           * If not first code in a packet, and
           * if not out of code space, then allocate a new code.
           *
           * Keep the hash table correct so it can be used
           * with uncompressed packets.
           */
          if (oldcode != CLEAR && max_ent < db->maxmaxcode) {
              struct bsd_dict *dictp2;
              u_int32_t fcode;
              u_int32_t hval, disp;
  
              fcode = BSD_KEY(oldcode,finchar);
              hval = BSD_HASH(oldcode,finchar,db->hshift);
              dictp = &db->dict[hval];
  
              /* look for a free hash table entry */
              if (dictp->codem1 < max_ent) {
                  disp = (hval == 0) ? 1 : hval;
                  do {
                      hval += disp;
                      if (hval >= db->hsize)
                          hval -= db->hsize;
                      dictp = &db->dict[hval];
                  } while (dictp->codem1 < max_ent);
              }
  
              /*
               * Invalidate previous hash table entry
               * assigned this code, and then take it over
               */
              dictp2 = &db->dict[max_ent+1];
              if (db->dict[dictp2->cptr].codem1 == max_ent) {
                  db->dict[dictp2->cptr].codem1 = BADCODEM1;
              }
              dictp2->cptr = hval;
              dictp->codem1 = max_ent;
              dictp->f.fcode = fcode;
  
              db->max_ent = ++max_ent;
              db->lens[max_ent] = db->lens[oldcode]+1;
  
              /* Expand code size if needed. */
              if (max_ent >= MAXCODE(n_bits) && max_ent < db->maxmaxcode) {
                  db->n_bits = ++n_bits;
                  tgtbitno = 32-n_bits;
              }
          }
          oldcode = incode;
      }
      dmp->m_len = wptr - mtod(dmp, u_char *);
  
      /*
       * Keep the checkpoint right so that incompressible packets
       * clear the dictionary at the right times.
       */
      db->bytes_out += ilen;
      db->in_count += explen;
      if (bsd_check(db) && db->debug) {
          printf("bsd_decomp%d: peer should have cleared dictionary\n",
                 db->unit);
      }
  
      ++db->comp_count;
      db->comp_bytes += ilen + BSD_OVHD;
      ++db->uncomp_count;
      db->uncomp_bytes += explen;
  
      *dmpp = mret;
      return DECOMP_OK;
  
  #ifdef DEBUG
   bad:
      if (codelen <= 0) {
          printf("bsd_decomp%d: fell off end of chain ", db->unit);
          printf("0x%x at 0x%x by 0x%x, max_ent=0x%x\n",
                 incode, finchar, db->dict[finchar].cptr, max_ent);
      } else if (dictp->codem1 != finchar-1) {
          printf("bsd_decomp%d: bad code chain 0x%x finchar=0x%x ",
                 db->unit, incode, finchar);
          printf("oldcode=0x%x cptr=0x%x codem1=0x%x\n", oldcode,
                 db->dict[finchar].cptr, dictp->codem1);
      }
      m_freem(mret);
      return DECOMP_FATALERROR;
  #endif /* DEBUG */
  }
  #endif /* DO_BSD_COMPRESS */

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