The Tcpdump Group git mirrors - tcpdump/blob - 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 # include <config.h>
  39
  40 #include "netdissect-stdinc.h"
  41
  42 #include "netdissect.h"
  43
  44 /*
  45  * Checksum routine for Internet Protocol family headers (Portable Version).
  46  *
  47  * This routine is very heavily used in the network
  48  * code and should be modified for each CPU to be as fast as possible.
  49  */
  50
  51 #define ADDCARRY(x)  {if ((x) > 65535) (x) -= 65535;}
  52 #define REDUCE {l_util.l = sum; sum = l_util.s[0] + l_util.s[1]; ADDCARRY(sum);}
  53
  54 uint16_t
  55 in_cksum(const struct cksum_vec *vec, int veclen)
  56 {
  57         const uint16_t *w;
  58         int sum = 0;
  59         int mlen = 0;
  60         int byte_swapped = 0;
  61
  62         union {
  63                 uint8_t         c[2];
  64                 uint16_t        s;
  65         } s_util;
  66         union {
  67                 uint16_t        s[2];
  68                 uint32_t        l;
  69         } l_util;
  70
  71         for (; veclen != 0; vec++, veclen--) {
  72                 if (vec->len == 0)
  73                         continue;
  74                 w = (const uint16_t *)(const void *)vec->ptr;
  75                 if (mlen == -1) {
  76                         /*
  77                          * The first byte of this chunk is the continuation
  78                          * of a word spanning between this chunk and the
  79                          * last chunk.
  80                          *
  81                          * s_util.c[0] is already saved when scanning previous
  82                          * chunk.
  83                          */
  84                         s_util.c[1] = *(const uint8_t *)w;
  85                         sum += s_util.s;
  86                         w = (const uint16_t *)(const void *)((const uint8_t *)w + 1);
  87                         mlen = vec->len - 1;
  88                 } else
  89                         mlen = vec->len;
  90                 /*
  91                  * Force to even boundary.
  92                  */
  93                 if ((1 & (uintptr_t) w) && (mlen > 0)) {
  94                         REDUCE;
  95                         sum <<= 8;
  96                         s_util.c[0] = *(const uint8_t *)w;
  97                         w = (const uint16_t *)(const void *)((const uint8_t *)w + 1);
  98                         mlen--;
  99                         byte_swapped = 1;
 100                 }
 101                 /*
 102                  * Unroll the loop to make overhead from
 103                  * branches &c small.
 104                  */
 105                 while ((mlen -= 32) >= 0) {
 106                         sum += w[0]; sum += w[1]; sum += w[2]; sum += w[3];
 107                         sum += w[4]; sum += w[5]; sum += w[6]; sum += w[7];
 108                         sum += w[8]; sum += w[9]; sum += w[10]; sum += w[11];
 109                         sum += w[12]; sum += w[13]; sum += w[14]; sum += w[15];
 110                         w += 16;
 111                 }
 112                 mlen += 32;
 113                 while ((mlen -= 8) >= 0) {
 114                         sum += w[0]; sum += w[1]; sum += w[2]; sum += w[3];
 115                         w += 4;
 116                 }
 117                 mlen += 8;
 118                 if (mlen == 0 && byte_swapped == 0)
 119                         continue;
 120                 REDUCE;
 121                 while ((mlen -= 2) >= 0) {
 122                         sum += *w++;
 123                 }
 124                 if (byte_swapped) {
 125                         REDUCE;
 126                         sum <<= 8;
 127                         byte_swapped = 0;
 128                         if (mlen == -1) {
 129                                 s_util.c[1] = *(const uint8_t *)w;
 130                                 sum += s_util.s;
 131                                 mlen = 0;
 132                         } else
 133                                 mlen = -1;
 134                 } else if (mlen == -1)
 135                         s_util.c[0] = *(const uint8_t *)w;
 136         }
 137         if (mlen == -1) {
 138                 /* The last mbuf has odd # of bytes. Follow the
 139                    standard (the odd byte may be shifted left by 8 bits
 140                    or not as determined by endian-ness of the machine) */
 141                 s_util.c[1] = 0;
 142                 sum += s_util.s;
 143         }
 144         REDUCE;
 145         return (~sum & 0xffff);
 146 }
 147
 148 /*
 149  * Given the host-byte-order value of the checksum field in a packet
 150  * header, and the network-byte-order computed checksum of the data
 151  * that the checksum covers (including the checksum itself), compute
 152  * what the checksum field *should* have been.
 153  */
 154 uint16_t
 155 in_cksum_shouldbe(uint16_t sum, uint16_t computed_sum)
 156 {
 157         uint32_t shouldbe;
 158
 159         /*
 160          * The value that should have gone into the checksum field
 161          * is the negative of the value gotten by summing up everything
 162          * *but* the checksum field.
 163          *
 164          * We can compute that by subtracting the value of the checksum
 165          * field from the sum of all the data in the packet, and then
 166          * computing the negative of that value.
 167          *
 168          * "sum" is the value of the checksum field, and "computed_sum"
 169          * is the negative of the sum of all the data in the packets,
 170          * so that's -(-computed_sum - sum), or (sum + computed_sum).
 171          *
 172          * All the arithmetic in question is one's complement, so the
 173          * addition must include an end-around carry; we do this by
 174          * doing the arithmetic in 32 bits (with no sign-extension),
 175          * and then adding the upper 16 bits of the sum, which contain
 176          * the carry, to the lower 16 bits of the sum, and then do it
 177          * again in case *that* sum produced a carry.
 178          *
 179          * As RFC 1071 notes, the checksum can be computed without
 180          * byte-swapping the 16-bit words; summing 16-bit words
 181          * on a big-endian machine gives a big-endian checksum, which
 182          * can be directly stuffed into the big-endian checksum fields
 183          * in protocol headers, and summing words on a little-endian
 184          * machine gives a little-endian checksum, which must be
 185          * byte-swapped before being stuffed into a big-endian checksum
 186          * field.
 187          *
 188          * "computed_sum" is a network-byte-order value, so we must put
 189          * it in host byte order before subtracting it from the
 190          * host-byte-order value from the header; the adjusted checksum
 191          * will be in host byte order, which is what we'll return.
 192          */
 193         shouldbe = sum;
 194         shouldbe += ntohs(computed_sum);
 195         shouldbe = (shouldbe & 0xFFFF) + (shouldbe >> 16);
 196         shouldbe = (shouldbe & 0xFFFF) + (shouldbe >> 16);
 197         return (uint16_t)shouldbe;
 198 }