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
in_cksum(const struct cksum_vec * vec,int veclen)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
in_cksum_shouldbe(uint16_t sum,uint16_t computed_sum)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