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 } 199