contrib/tcpdump/in_cksum.c

   1 /* in_cksum.c
   2  * 4.4-Lite-2 Internet checksum routine, modified to take a vector of
   3  * pointers/lengths giving the pieces to be checksummed.  Also using
   4  * Tahoe/CGI version of ADDCARRY(x) macro instead of from portable version.
   5  */
   6
   7 /*
   8  * Copyright (c) 1988, 1992, 1993
   9  *      The Regents of the University of California.  All rights reserved.
  10  *
  11  * Redistribution and use in source and binary forms, with or without
  12  * modification, are permitted provided that the following conditions
  13  * are met:
  14  * 1. Redistributions of source code must retain the above copyright
  15  *    notice, this list of conditions and the following disclaimer.
  16  * 2. Redistributions in binary form must reproduce the above copyright
  17  *    notice, this list of conditions and the following disclaimer in the
  18  *    documentation and/or other materials provided with the distribution.
  19  * 3. Neither the name of the University nor the names of its contributors
  20  *    may be used to endorse or promote products derived from this software
  21  *    without specific prior written permission.
  22  *
  23  * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
  24  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  25  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
  26  * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
  27  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
  28  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
  29  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
  30  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
  31  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
  32  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  33  * SUCH DAMAGE.
  34  *
  35  *      @(#)in_cksum.c  8.1 (Berkeley) 6/10/93
  36  */
  37
  38 #ifdef HAVE_CONFIG_H
  39 # include "config.h"
  40 #endif
  41
  42 #include <netdissect-stdinc.h>
  43
  44 #include "netdissect.h"
  45
  46 /*
  47  * Checksum routine for Internet Protocol family headers (Portable Version).
  48  *
  49  * This routine is very heavily used in the network
  50  * code and should be modified for each CPU to be as fast as possible.
  51  */
  52
  53 #define ADDCARRY(x)  {if ((x) > 65535) (x) -= 65535;}
  54 #define REDUCE {l_util.l = sum; sum = l_util.s[0] + l_util.s[1]; ADDCARRY(sum);}
  55
  56 uint16_t
  57 in_cksum(const struct cksum_vec *vec, int veclen)
  58 {
  59         register const uint16_t *w;
  60         register int sum = 0;
  61         register int mlen = 0;
  62         int byte_swapped = 0;
  63
  64         union {
  65                 uint8_t         c[2];
  66                 uint16_t        s;
  67         } s_util;
  68         union {
  69                 uint16_t        s[2];
  70                 uint32_t        l;
  71         } l_util;
  72
  73         for (; veclen != 0; vec++, veclen--) {
  74                 if (vec->len == 0)
  75                         continue;
  76                 w = (const uint16_t *)(const void *)vec->ptr;
  77                 if (mlen == -1) {
  78                         /*
  79                          * The first byte of this chunk is the continuation
  80                          * of a word spanning between this chunk and the
  81                          * last chunk.
  82                          *
  83                          * s_util.c[0] is already saved when scanning previous
  84                          * chunk.
  85                          */
  86                         s_util.c[1] = *(const uint8_t *)w;
  87                         sum += s_util.s;
  88                         w = (const uint16_t *)(const void *)((const uint8_t *)w + 1);
  89                         mlen = vec->len - 1;
  90                 } else
  91                         mlen = vec->len;
  92                 /*
  93                  * Force to even boundary.
  94                  */
  95                 if ((1 & (uintptr_t) w) && (mlen > 0)) {
  96                         REDUCE;
  97                         sum <<= 8;
  98                         s_util.c[0] = *(const uint8_t *)w;
  99                         w = (const uint16_t *)(const void *)((const uint8_t *)w + 1);
 100                         mlen--;
 101                         byte_swapped = 1;
 102                 }
 103                 /*
 104                  * Unroll the loop to make overhead from
 105                  * branches &c small.
 106                  */
 107                 while ((mlen -= 32) >= 0) {
 108                         sum += w[0]; sum += w[1]; sum += w[2]; sum += w[3];
 109                         sum += w[4]; sum += w[5]; sum += w[6]; sum += w[7];
 110                         sum += w[8]; sum += w[9]; sum += w[10]; sum += w[11];
 111                         sum += w[12]; sum += w[13]; sum += w[14]; sum += w[15];
 112                         w += 16;
 113                 }
 114                 mlen += 32;
 115                 while ((mlen -= 8) >= 0) {
 116                         sum += w[0]; sum += w[1]; sum += w[2]; sum += w[3];
 117                         w += 4;
 118                 }
 119                 mlen += 8;
 120                 if (mlen == 0 && byte_swapped == 0)
 121                         continue;
 122                 REDUCE;
 123                 while ((mlen -= 2) >= 0) {
 124                         sum += *w++;
 125                 }
 126                 if (byte_swapped) {
 127                         REDUCE;
 128                         sum <<= 8;
 129                         byte_swapped = 0;
 130                         if (mlen == -1) {
 131                                 s_util.c[1] = *(const uint8_t *)w;
 132                                 sum += s_util.s;
 133                                 mlen = 0;
 134                         } else
 135                                 mlen = -1;
 136                 } else if (mlen == -1)
 137                         s_util.c[0] = *(const uint8_t *)w;
 138         }
 139         if (mlen == -1) {
 140                 /* The last mbuf has odd # of bytes. Follow the
 141                    standard (the odd byte may be shifted left by 8 bits
 142                    or not as determined by endian-ness of the machine) */
 143                 s_util.c[1] = 0;
 144                 sum += s_util.s;
 145         }
 146         REDUCE;
 147         return (~sum & 0xffff);
 148 }
 149
 150 /*
 151  * Given the host-byte-order value of the checksum field in a packet
 152  * header, and the network-byte-order computed checksum of the data
 153  * that the checksum covers (including the checksum itself), compute
 154  * what the checksum field *should* have been.
 155  */
 156 uint16_t
 157 in_cksum_shouldbe(uint16_t sum, uint16_t computed_sum)
 158 {
 159         uint32_t shouldbe;
 160
 161         /*
 162          * The value that should have gone into the checksum field
 163          * is the negative of the value gotten by summing up everything
 164          * *but* the checksum field.
 165          *
 166          * We can compute that by subtracting the value of the checksum
 167          * field from the sum of all the data in the packet, and then
 168          * computing the negative of that value.
 169          *
 170          * "sum" is the value of the checksum field, and "computed_sum"
 171          * is the negative of the sum of all the data in the packets,
 172          * so that's -(-computed_sum - sum), or (sum + computed_sum).
 173          *
 174          * All the arithmetic in question is one's complement, so the
 175          * addition must include an end-around carry; we do this by
 176          * doing the arithmetic in 32 bits (with no sign-extension),
 177          * and then adding the upper 16 bits of the sum, which contain
 178          * the carry, to the lower 16 bits of the sum, and then do it
 179          * again in case *that* sum produced a carry.
 180          *
 181          * As RFC 1071 notes, the checksum can be computed without
 182          * byte-swapping the 16-bit words; summing 16-bit words
 183          * on a big-endian machine gives a big-endian checksum, which
 184          * can be directly stuffed into the big-endian checksum fields
 185          * in protocol headers, and summing words on a little-endian
 186          * machine gives a little-endian checksum, which must be
 187          * byte-swapped before being stuffed into a big-endian checksum
 188          * field.
 189          *
 190          * "computed_sum" is a network-byte-order value, so we must put
 191          * it in host byte order before subtracting it from the
 192          * host-byte-order value from the header; the adjusted checksum
 193          * will be in host byte order, which is what we'll return.
 194          */
 195         shouldbe = sum;
 196         shouldbe += ntohs(computed_sum);
 197         shouldbe = (shouldbe & 0xFFFF) + (shouldbe >> 16);
 198         shouldbe = (shouldbe & 0xFFFF) + (shouldbe >> 16);
 199         return shouldbe;
 200 }