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 #define NETDISSECT_REWORKED 39 #ifdef HAVE_CONFIG_H 40 # include "config.h" 41 #endif 42 43 #include <tcpdump-stdinc.h> 44 45 #include "interface.h" 46 47 /* 48 * Checksum routine for Internet Protocol family headers (Portable Version). 49 * 50 * This routine is very heavily used in the network 51 * code and should be modified for each CPU to be as fast as possible. 52 */ 53 54 #define ADDCARRY(x) {if ((x) > 65535) (x) -= 65535;} 55 #define REDUCE {l_util.l = sum; sum = l_util.s[0] + l_util.s[1]; ADDCARRY(sum);} 56 57 uint16_t 58 in_cksum(const struct cksum_vec *vec, int veclen) 59 { 60 register const uint16_t *w; 61 register int sum = 0; 62 register int mlen = 0; 63 int byte_swapped = 0; 64 65 union { 66 uint8_t c[2]; 67 uint16_t s; 68 } s_util; 69 union { 70 uint16_t s[2]; 71 uint32_t l; 72 } l_util; 73 74 for (; veclen != 0; vec++, veclen--) { 75 if (vec->len == 0) 76 continue; 77 w = (const uint16_t *)(void *)vec->ptr; 78 if (mlen == -1) { 79 /* 80 * The first byte of this chunk is the continuation 81 * of a word spanning between this chunk and the 82 * last chunk. 83 * 84 * s_util.c[0] is already saved when scanning previous 85 * chunk. 86 */ 87 s_util.c[1] = *(const uint8_t *)w; 88 sum += s_util.s; 89 w = (const uint16_t *)(void *)((const uint8_t *)w + 1); 90 mlen = vec->len - 1; 91 } else 92 mlen = vec->len; 93 /* 94 * Force to even boundary. 95 */ 96 if ((1 & (unsigned long) w) && (mlen > 0)) { 97 REDUCE; 98 sum <<= 8; 99 s_util.c[0] = *(const uint8_t *)w; 100 w = (const uint16_t *)(void *)((const uint8_t *)w + 1); 101 mlen--; 102 byte_swapped = 1; 103 } 104 /* 105 * Unroll the loop to make overhead from 106 * branches &c small. 107 */ 108 while ((mlen -= 32) >= 0) { 109 sum += w[0]; sum += w[1]; sum += w[2]; sum += w[3]; 110 sum += w[4]; sum += w[5]; sum += w[6]; sum += w[7]; 111 sum += w[8]; sum += w[9]; sum += w[10]; sum += w[11]; 112 sum += w[12]; sum += w[13]; sum += w[14]; sum += w[15]; 113 w += 16; 114 } 115 mlen += 32; 116 while ((mlen -= 8) >= 0) { 117 sum += w[0]; sum += w[1]; sum += w[2]; sum += w[3]; 118 w += 4; 119 } 120 mlen += 8; 121 if (mlen == 0 && byte_swapped == 0) 122 continue; 123 REDUCE; 124 while ((mlen -= 2) >= 0) { 125 sum += *w++; 126 } 127 if (byte_swapped) { 128 REDUCE; 129 sum <<= 8; 130 byte_swapped = 0; 131 if (mlen == -1) { 132 s_util.c[1] = *(const uint8_t *)w; 133 sum += s_util.s; 134 mlen = 0; 135 } else 136 mlen = -1; 137 } else if (mlen == -1) 138 s_util.c[0] = *(const uint8_t *)w; 139 } 140 if (mlen == -1) { 141 /* The last mbuf has odd # of bytes. Follow the 142 standard (the odd byte may be shifted left by 8 bits 143 or not as determined by endian-ness of the machine) */ 144 s_util.c[1] = 0; 145 sum += s_util.s; 146 } 147 REDUCE; 148 return (~sum & 0xffff); 149 } 150 151 /* 152 * Given the host-byte-order value of the checksum field in a packet 153 * header, and the network-byte-order computed checksum of the data 154 * that the checksum covers (including the checksum itself), compute 155 * what the checksum field *should* have been. 156 */ 157 uint16_t 158 in_cksum_shouldbe(uint16_t sum, uint16_t computed_sum) 159 { 160 uint32_t shouldbe; 161 162 /* 163 * The value that should have gone into the checksum field 164 * is the negative of the value gotten by summing up everything 165 * *but* the checksum field. 166 * 167 * We can compute that by subtracting the value of the checksum 168 * field from the sum of all the data in the packet, and then 169 * computing the negative of that value. 170 * 171 * "sum" is the value of the checksum field, and "computed_sum" 172 * is the negative of the sum of all the data in the packets, 173 * so that's -(-computed_sum - sum), or (sum + computed_sum). 174 * 175 * All the arithmetic in question is one's complement, so the 176 * addition must include an end-around carry; we do this by 177 * doing the arithmetic in 32 bits (with no sign-extension), 178 * and then adding the upper 16 bits of the sum, which contain 179 * the carry, to the lower 16 bits of the sum, and then do it 180 * again in case *that* sum produced a carry. 181 * 182 * As RFC 1071 notes, the checksum can be computed without 183 * byte-swapping the 16-bit words; summing 16-bit words 184 * on a big-endian machine gives a big-endian checksum, which 185 * can be directly stuffed into the big-endian checksum fields 186 * in protocol headers, and summing words on a little-endian 187 * machine gives a little-endian checksum, which must be 188 * byte-swapped before being stuffed into a big-endian checksum 189 * field. 190 * 191 * "computed_sum" is a network-byte-order value, so we must put 192 * it in host byte order before subtracting it from the 193 * host-byte-order value from the header; the adjusted checksum 194 * will be in host byte order, which is what we'll return. 195 */ 196 shouldbe = sum; 197 shouldbe += ntohs(computed_sum); 198 shouldbe = (shouldbe & 0xFFFF) + (shouldbe >> 16); 199 shouldbe = (shouldbe & 0xFFFF) + (shouldbe >> 16); 200 return shouldbe; 201 } 202