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 <tcpdump-stdinc.h> 43 44 #include "interface.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 u_int16_t 57 in_cksum(const struct cksum_vec *vec, int veclen) 58 { 59 register const u_int16_t *w; 60 register int sum = 0; 61 register int mlen = 0; 62 int byte_swapped = 0; 63 64 union { 65 u_int8_t c[2]; 66 u_int16_t s; 67 } s_util; 68 union { 69 u_int16_t s[2]; 70 u_int32_t l; 71 } l_util; 72 73 for (; veclen != 0; vec++, veclen--) { 74 if (vec->len == 0) 75 continue; 76 w = (const u_int16_t *)(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 u_int8_t *)w; 87 sum += s_util.s; 88 w = (const u_int16_t *)(void *)((const u_int8_t *)w + 1); 89 mlen = vec->len - 1; 90 } else 91 mlen = vec->len; 92 /* 93 * Force to even boundary. 94 */ 95 if ((1 & (unsigned long) w) && (mlen > 0)) { 96 REDUCE; 97 sum <<= 8; 98 s_util.c[0] = *(const u_int8_t *)w; 99 w = (const u_int16_t *)(void *)((const u_int8_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 u_int8_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 u_int8_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 u_int16_t 157 in_cksum_shouldbe(u_int16_t sum, u_int16_t computed_sum) 158 { 159 u_int32_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 } 201