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 <sys/cdefs.h> 39 #ifndef lint 40 __RCSID("$NetBSD: in_cksum.c,v 1.2 2017/01/24 23:29:13 christos Exp $"); 41 #endif 42 43 #ifdef HAVE_CONFIG_H 44 # include "config.h" 45 #endif 46 47 #include <netdissect-stdinc.h> 48 49 #include "netdissect.h" 50 51 /* 52 * Checksum routine for Internet Protocol family headers (Portable Version). 53 * 54 * This routine is very heavily used in the network 55 * code and should be modified for each CPU to be as fast as possible. 56 */ 57 58 #define ADDCARRY(x) {if ((x) > 65535) (x) -= 65535;} 59 #define REDUCE {l_util.l = sum; sum = l_util.s[0] + l_util.s[1]; ADDCARRY(sum);} 60 61 uint16_t 62 in_cksum(const struct cksum_vec *vec, int veclen) 63 { 64 register const uint16_t *w; 65 register int sum = 0; 66 register int mlen = 0; 67 int byte_swapped = 0; 68 69 union { 70 uint8_t c[2]; 71 uint16_t s; 72 } s_util; 73 union { 74 uint16_t s[2]; 75 uint32_t l; 76 } l_util; 77 78 for (; veclen != 0; vec++, veclen--) { 79 if (vec->len == 0) 80 continue; 81 w = (const uint16_t *)(const void *)vec->ptr; 82 if (mlen == -1) { 83 /* 84 * The first byte of this chunk is the continuation 85 * of a word spanning between this chunk and the 86 * last chunk. 87 * 88 * s_util.c[0] is already saved when scanning previous 89 * chunk. 90 */ 91 s_util.c[1] = *(const uint8_t *)w; 92 sum += s_util.s; 93 w = (const uint16_t *)(const void *)((const uint8_t *)w + 1); 94 mlen = vec->len - 1; 95 } else 96 mlen = vec->len; 97 /* 98 * Force to even boundary. 99 */ 100 if ((1 & (uintptr_t) w) && (mlen > 0)) { 101 REDUCE; 102 sum <<= 8; 103 s_util.c[0] = *(const uint8_t *)w; 104 w = (const uint16_t *)(const void *)((const uint8_t *)w + 1); 105 mlen--; 106 byte_swapped = 1; 107 } 108 /* 109 * Unroll the loop to make overhead from 110 * branches &c small. 111 */ 112 while ((mlen -= 32) >= 0) { 113 sum += w[0]; sum += w[1]; sum += w[2]; sum += w[3]; 114 sum += w[4]; sum += w[5]; sum += w[6]; sum += w[7]; 115 sum += w[8]; sum += w[9]; sum += w[10]; sum += w[11]; 116 sum += w[12]; sum += w[13]; sum += w[14]; sum += w[15]; 117 w += 16; 118 } 119 mlen += 32; 120 while ((mlen -= 8) >= 0) { 121 sum += w[0]; sum += w[1]; sum += w[2]; sum += w[3]; 122 w += 4; 123 } 124 mlen += 8; 125 if (mlen == 0 && byte_swapped == 0) 126 continue; 127 REDUCE; 128 while ((mlen -= 2) >= 0) { 129 sum += *w++; 130 } 131 if (byte_swapped) { 132 REDUCE; 133 sum <<= 8; 134 byte_swapped = 0; 135 if (mlen == -1) { 136 s_util.c[1] = *(const uint8_t *)w; 137 sum += s_util.s; 138 mlen = 0; 139 } else 140 mlen = -1; 141 } else if (mlen == -1) 142 s_util.c[0] = *(const uint8_t *)w; 143 } 144 if (mlen == -1) { 145 /* The last mbuf has odd # of bytes. Follow the 146 standard (the odd byte may be shifted left by 8 bits 147 or not as determined by endian-ness of the machine) */ 148 s_util.c[1] = 0; 149 sum += s_util.s; 150 } 151 REDUCE; 152 return (~sum & 0xffff); 153 } 154 155 /* 156 * Given the host-byte-order value of the checksum field in a packet 157 * header, and the network-byte-order computed checksum of the data 158 * that the checksum covers (including the checksum itself), compute 159 * what the checksum field *should* have been. 160 */ 161 uint16_t 162 in_cksum_shouldbe(uint16_t sum, uint16_t computed_sum) 163 { 164 uint32_t shouldbe; 165 166 /* 167 * The value that should have gone into the checksum field 168 * is the negative of the value gotten by summing up everything 169 * *but* the checksum field. 170 * 171 * We can compute that by subtracting the value of the checksum 172 * field from the sum of all the data in the packet, and then 173 * computing the negative of that value. 174 * 175 * "sum" is the value of the checksum field, and "computed_sum" 176 * is the negative of the sum of all the data in the packets, 177 * so that's -(-computed_sum - sum), or (sum + computed_sum). 178 * 179 * All the arithmetic in question is one's complement, so the 180 * addition must include an end-around carry; we do this by 181 * doing the arithmetic in 32 bits (with no sign-extension), 182 * and then adding the upper 16 bits of the sum, which contain 183 * the carry, to the lower 16 bits of the sum, and then do it 184 * again in case *that* sum produced a carry. 185 * 186 * As RFC 1071 notes, the checksum can be computed without 187 * byte-swapping the 16-bit words; summing 16-bit words 188 * on a big-endian machine gives a big-endian checksum, which 189 * can be directly stuffed into the big-endian checksum fields 190 * in protocol headers, and summing words on a little-endian 191 * machine gives a little-endian checksum, which must be 192 * byte-swapped before being stuffed into a big-endian checksum 193 * field. 194 * 195 * "computed_sum" is a network-byte-order value, so we must put 196 * it in host byte order before subtracting it from the 197 * host-byte-order value from the header; the adjusted checksum 198 * will be in host byte order, which is what we'll return. 199 */ 200 shouldbe = sum; 201 shouldbe += ntohs(computed_sum); 202 shouldbe = (shouldbe & 0xFFFF) + (shouldbe >> 16); 203 shouldbe = (shouldbe & 0xFFFF) + (shouldbe >> 16); 204 return shouldbe; 205 } 206