• Home
  • Line#
  • Scopes#
  • Navigate#
  • Raw
  • Download
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 <netdissect-stdinc.h>
43 
44 #include "netdissect.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 
56 uint16_t
in_cksum(const struct cksum_vec * vec,int veclen)57 in_cksum(const struct cksum_vec *vec, int veclen)
58 {
59 	register const uint16_t *w;
60 	register int sum = 0;
61 	register int mlen = 0;
62 	int byte_swapped = 0;
63 
64 	union {
65 		uint8_t		c[2];
66 		uint16_t	s;
67 	} s_util;
68 	union {
69 		uint16_t	s[2];
70 		uint32_t	l;
71 	} l_util;
72 
73 	for (; veclen != 0; vec++, veclen--) {
74 		if (vec->len == 0)
75 			continue;
76 		w = (const uint16_t *)(const 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 uint8_t *)w;
87 			sum += s_util.s;
88 			w = (const uint16_t *)(const void *)((const uint8_t *)w + 1);
89 			mlen = vec->len - 1;
90 		} else
91 			mlen = vec->len;
92 		/*
93 		 * Force to even boundary.
94 		 */
95 		if ((1 & (uintptr_t) w) && (mlen > 0)) {
96 			REDUCE;
97 			sum <<= 8;
98 			s_util.c[0] = *(const uint8_t *)w;
99 			w = (const uint16_t *)(const void *)((const uint8_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 uint8_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 uint8_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  */
156 uint16_t
in_cksum_shouldbe(uint16_t sum,uint16_t computed_sum)157 in_cksum_shouldbe(uint16_t sum, uint16_t computed_sum)
158 {
159 	uint32_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