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