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