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 1.4 2024/09/02 16:15:30 christos Exp $"); 41 #endif 42 43 # include <config.h> 44 45 #include "netdissect-stdinc.h" 46 47 #include "netdissect.h" 48 49 /* 50 * Checksum routine for Internet Protocol family headers (Portable Version). 51 * 52 * This routine is very heavily used in the network 53 * code and should be modified for each CPU to be as fast as possible. 54 */ 55 56 #define ADDCARRY(x) {if ((x) > 65535) (x) -= 65535;} 57 #define REDUCE {l_util.l = sum; sum = l_util.s[0] + l_util.s[1]; ADDCARRY(sum);} 58 59 uint16_t 60 in_cksum(const struct cksum_vec *vec, int veclen) 61 { 62 const uint16_t *w; 63 int sum = 0; 64 int mlen = 0; 65 int byte_swapped = 0; 66 67 union { 68 uint8_t c[2]; 69 uint16_t s; 70 } s_util; 71 union { 72 uint16_t s[2]; 73 uint32_t l; 74 } l_util; 75 76 for (; veclen != 0; vec++, veclen--) { 77 if (vec->len == 0) 78 continue; 79 w = (const uint16_t *)(const void *)vec->ptr; 80 if (mlen == -1) { 81 /* 82 * The first byte of this chunk is the continuation 83 * of a word spanning between this chunk and the 84 * last chunk. 85 * 86 * s_util.c[0] is already saved when scanning previous 87 * chunk. 88 */ 89 s_util.c[1] = *(const uint8_t *)w; 90 sum += s_util.s; 91 w = (const uint16_t *)(const void *)((const uint8_t *)w + 1); 92 mlen = vec->len - 1; 93 } else 94 mlen = vec->len; 95 /* 96 * Force to even boundary. 97 */ 98 if ((1 & (uintptr_t) w) && (mlen > 0)) { 99 REDUCE; 100 sum <<= 8; 101 s_util.c[0] = *(const uint8_t *)w; 102 w = (const uint16_t *)(const void *)((const uint8_t *)w + 1); 103 mlen--; 104 byte_swapped = 1; 105 } 106 /* 107 * Unroll the loop to make overhead from 108 * branches &c small. 109 */ 110 while ((mlen -= 32) >= 0) { 111 sum += w[0]; sum += w[1]; sum += w[2]; sum += w[3]; 112 sum += w[4]; sum += w[5]; sum += w[6]; sum += w[7]; 113 sum += w[8]; sum += w[9]; sum += w[10]; sum += w[11]; 114 sum += w[12]; sum += w[13]; sum += w[14]; sum += w[15]; 115 w += 16; 116 } 117 mlen += 32; 118 while ((mlen -= 8) >= 0) { 119 sum += w[0]; sum += w[1]; sum += w[2]; sum += w[3]; 120 w += 4; 121 } 122 mlen += 8; 123 if (mlen == 0 && byte_swapped == 0) 124 continue; 125 REDUCE; 126 while ((mlen -= 2) >= 0) { 127 sum += *w++; 128 } 129 if (byte_swapped) { 130 REDUCE; 131 sum <<= 8; 132 byte_swapped = 0; 133 if (mlen == -1) { 134 s_util.c[1] = *(const uint8_t *)w; 135 sum += s_util.s; 136 mlen = 0; 137 } else 138 mlen = -1; 139 } else if (mlen == -1) 140 s_util.c[0] = *(const uint8_t *)w; 141 } 142 if (mlen == -1) { 143 /* The last mbuf has odd # of bytes. Follow the 144 standard (the odd byte may be shifted left by 8 bits 145 or not as determined by endian-ness of the machine) */ 146 s_util.c[1] = 0; 147 sum += s_util.s; 148 } 149 REDUCE; 150 return (~sum & 0xffff); 151 } 152 153 /* 154 * Given the host-byte-order value of the checksum field in a packet 155 * header, and the network-byte-order computed checksum of the data 156 * that the checksum covers (including the checksum itself), compute 157 * what the checksum field *should* have been. 158 */ 159 uint16_t 160 in_cksum_shouldbe(uint16_t sum, uint16_t computed_sum) 161 { 162 uint32_t shouldbe; 163 164 /* 165 * The value that should have gone into the checksum field 166 * is the negative of the value gotten by summing up everything 167 * *but* the checksum field. 168 * 169 * We can compute that by subtracting the value of the checksum 170 * field from the sum of all the data in the packet, and then 171 * computing the negative of that value. 172 * 173 * "sum" is the value of the checksum field, and "computed_sum" 174 * is the negative of the sum of all the data in the packets, 175 * so that's -(-computed_sum - sum), or (sum + computed_sum). 176 * 177 * All the arithmetic in question is one's complement, so the 178 * addition must include an end-around carry; we do this by 179 * doing the arithmetic in 32 bits (with no sign-extension), 180 * and then adding the upper 16 bits of the sum, which contain 181 * the carry, to the lower 16 bits of the sum, and then do it 182 * again in case *that* sum produced a carry. 183 * 184 * As RFC 1071 notes, the checksum can be computed without 185 * byte-swapping the 16-bit words; summing 16-bit words 186 * on a big-endian machine gives a big-endian checksum, which 187 * can be directly stuffed into the big-endian checksum fields 188 * in protocol headers, and summing words on a little-endian 189 * machine gives a little-endian checksum, which must be 190 * byte-swapped before being stuffed into a big-endian checksum 191 * field. 192 * 193 * "computed_sum" is a network-byte-order value, so we must put 194 * it in host byte order before subtracting it from the 195 * host-byte-order value from the header; the adjusted checksum 196 * will be in host byte order, which is what we'll return. 197 */ 198 shouldbe = sum; 199 shouldbe += ntohs(computed_sum); 200 shouldbe = (shouldbe & 0xFFFF) + (shouldbe >> 16); 201 shouldbe = (shouldbe & 0xFFFF) + (shouldbe >> 16); 202 return (uint16_t)shouldbe; 203 } 204
Indexes created Mon Jun 29 00:24:49 UTC 2026