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#ifdef HAVE_CONFIG_H
39# include "config.h"
40#endif
41
42#include <tcpdump-stdinc.h>
43
44#include "interface.h"
45
46/*
47 * Checksum routine for Internet Protocol family headers (Portable Version).
48 *
49 * This routine is very heavily used in the network
50 * code and should be modified for each CPU to be as fast as possible.
51 */
52
53#define ADDCARRY(x)  {if ((x) > 65535) (x) -= 65535;}
54#define REDUCE {l_util.l = sum; sum = l_util.s[0] + l_util.s[1]; ADDCARRY(sum);}
55
56u_int16_t
57in_cksum(const struct cksum_vec *vec, int veclen)
58{
59	register const u_int16_t *w;
60	register int sum = 0;
61	register int mlen = 0;
62	int byte_swapped = 0;
63
64	union {
65		u_int8_t	c[2];
66		u_int16_t	s;
67	} s_util;
68	union {
69		u_int16_t	s[2];
70		u_int32_t	l;
71	} l_util;
72
73	for (; veclen != 0; vec++, veclen--) {
74		if (vec->len == 0)
75			continue;
76		w = (const u_int16_t *)(void *)vec->ptr;
77		if (mlen == -1) {
78			/*
79			 * The first byte of this chunk is the continuation
80			 * of a word spanning between this chunk and the
81			 * last chunk.
82			 *
83			 * s_util.c[0] is already saved when scanning previous
84			 * chunk.
85			 */
86			s_util.c[1] = *(const u_int8_t *)w;
87			sum += s_util.s;
88			w = (const u_int16_t *)(void *)((const u_int8_t *)w + 1);
89			mlen = vec->len - 1;
90		} else
91			mlen = vec->len;
92		/*
93		 * Force to even boundary.
94		 */
95		if ((1 & (unsigned long) w) && (mlen > 0)) {
96			REDUCE;
97			sum <<= 8;
98			s_util.c[0] = *(const u_int8_t *)w;
99			w = (const u_int16_t *)(void *)((const u_int8_t *)w + 1);
100			mlen--;
101			byte_swapped = 1;
102		}
103		/*
104		 * Unroll the loop to make overhead from
105		 * branches &c small.
106		 */
107		while ((mlen -= 32) >= 0) {
108			sum += w[0]; sum += w[1]; sum += w[2]; sum += w[3];
109			sum += w[4]; sum += w[5]; sum += w[6]; sum += w[7];
110			sum += w[8]; sum += w[9]; sum += w[10]; sum += w[11];
111			sum += w[12]; sum += w[13]; sum += w[14]; sum += w[15];
112			w += 16;
113		}
114		mlen += 32;
115		while ((mlen -= 8) >= 0) {
116			sum += w[0]; sum += w[1]; sum += w[2]; sum += w[3];
117			w += 4;
118		}
119		mlen += 8;
120		if (mlen == 0 && byte_swapped == 0)
121			continue;
122		REDUCE;
123		while ((mlen -= 2) >= 0) {
124			sum += *w++;
125		}
126		if (byte_swapped) {
127			REDUCE;
128			sum <<= 8;
129			byte_swapped = 0;
130			if (mlen == -1) {
131				s_util.c[1] = *(const u_int8_t *)w;
132				sum += s_util.s;
133				mlen = 0;
134			} else
135				mlen = -1;
136		} else if (mlen == -1)
137			s_util.c[0] = *(const u_int8_t *)w;
138	}
139	if (mlen == -1) {
140		/* The last mbuf has odd # of bytes. Follow the
141		   standard (the odd byte may be shifted left by 8 bits
142		   or not as determined by endian-ness of the machine) */
143		s_util.c[1] = 0;
144		sum += s_util.s;
145	}
146	REDUCE;
147	return (~sum & 0xffff);
148}
149
150/*
151 * Given the host-byte-order value of the checksum field in a packet
152 * header, and the network-byte-order computed checksum of the data
153 * that the checksum covers (including the checksum itself), compute
154 * what the checksum field *should* have been.
155 */
156u_int16_t
157in_cksum_shouldbe(u_int16_t sum, u_int16_t computed_sum)
158{
159	u_int32_t shouldbe;
160
161	/*
162	 * The value that should have gone into the checksum field
163	 * is the negative of the value gotten by summing up everything
164	 * *but* the checksum field.
165	 *
166	 * We can compute that by subtracting the value of the checksum
167	 * field from the sum of all the data in the packet, and then
168	 * computing the negative of that value.
169	 *
170	 * "sum" is the value of the checksum field, and "computed_sum"
171	 * is the negative of the sum of all the data in the packets,
172	 * so that's -(-computed_sum - sum), or (sum + computed_sum).
173	 *
174	 * All the arithmetic in question is one's complement, so the
175	 * addition must include an end-around carry; we do this by
176	 * doing the arithmetic in 32 bits (with no sign-extension),
177	 * and then adding the upper 16 bits of the sum, which contain
178	 * the carry, to the lower 16 bits of the sum, and then do it
179	 * again in case *that* sum produced a carry.
180	 *
181	 * As RFC 1071 notes, the checksum can be computed without
182	 * byte-swapping the 16-bit words; summing 16-bit words
183	 * on a big-endian machine gives a big-endian checksum, which
184	 * can be directly stuffed into the big-endian checksum fields
185	 * in protocol headers, and summing words on a little-endian
186	 * machine gives a little-endian checksum, which must be
187	 * byte-swapped before being stuffed into a big-endian checksum
188	 * field.
189	 *
190	 * "computed_sum" is a network-byte-order value, so we must put
191	 * it in host byte order before subtracting it from the
192	 * host-byte-order value from the header; the adjusted checksum
193	 * will be in host byte order, which is what we'll return.
194	 */
195	shouldbe = sum;
196	shouldbe += ntohs(computed_sum);
197	shouldbe = (shouldbe & 0xFFFF) + (shouldbe >> 16);
198	shouldbe = (shouldbe & 0xFFFF) + (shouldbe >> 16);
199	return shouldbe;
200}
201