epan/in_cksum.c

   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  * Copyright (c) 1988, 1992, 1993
   7  *      The Regents of the University of California.  All rights reserved.
   8  *
   9  * SPDX-License-Identifier: BSD-3-Clause
  10  *
  11  *      @(#)in_cksum.c  8.1 (Berkeley) 6/10/93
  12  */
  13
  14 #include "config.h"
  15
  16 #include <glib.h>
  17
  18 #include <epan/tvbuff.h>
  19 #include <epan/in_cksum.h>
  20
  21 /*
  22  * Checksum routine for Internet Protocol family headers (Portable Version).
  23  *
  24  * This routine is very heavily used in the network
  25  * code and should be modified for each CPU to be as fast as possible.
  26  */
  27
  28 #define ADDCARRY(x)  {if ((x) > 65535) (x) -= 65535;}
  29 #define REDUCE {l_util.l = sum; sum = l_util.s[0] + l_util.s[1]; ADDCARRY(sum);}
  30
  31 /*
  32  * Linux and Windows, at least, when performing Local Checksum Offload
  33  * store the one's complement sum (not inverted to its bitwise complement)
  34  * of the pseudo header in the checksum field (instead of intializing
  35  * to zero), allowing the device driver to calculate the real checksum
  36  * later without needing knowledge of the pseudoheader itself.
  37  * (This is presumably why GSO requires equal length buffers - so that the
  38  * pseudo header contribution to the checksum, which includes the payload
  39  * length, is the same.)
  40  *
  41  * We can output this partial checksum as an intermediate result,
  42  * assuming that the pseudo header is all but the last chunk in the vector.
  43  * Note that unlike the final output it is not inverted, and that it
  44  * (like the final computed checksum) is is network byte order.
  45  */
  46 int
  47 in_cksum_ret_partial(const vec_t *vec, int veclen, uint16_t *partial)
  48 {
  49         register const uint16_t *w;
  50         register int sum = 0;
  51         register int mlen = 0;
  52         int byte_swapped = 0;
  53
  54         union {
  55                 uint8_t c[2];
  56                 uint16_t        s;
  57         } s_util;
  58         union {
  59                 uint16_t s[2];
  60                 uint32_t        l;
  61         } l_util;
  62
  63         for (; veclen != 0; vec++, veclen--) {
  64                 if (veclen == 1 && partial) {
  65                         REDUCE;
  66                         *partial = sum;
  67                 }
  68                 if (vec->len == 0)
  69                         continue;
  70                 w = (const uint16_t *)(const void *)vec->ptr;
  71                 if (mlen == -1) {
  72                         /*
  73                          * The first byte of this chunk is the continuation
  74                          * of a word spanning between this chunk and the
  75                          * last chunk.
  76                          *
  77                          * s_util.c[0] is already saved when scanning previous
  78                          * chunk.
  79                          */
  80                         s_util.c[1] = *(const uint8_t *)w;
  81                         sum += s_util.s;
  82                         w = (const uint16_t *)(const void *)((const uint8_t *)w + 1);
  83                         mlen = vec->len - 1;
  84                 } else
  85                         mlen = vec->len;
  86                 /*
  87                  * Force to even boundary.
  88                  */
  89                 if ((1 & (intptr_t)w) && (mlen > 0)) {
  90                         REDUCE;
  91                         sum <<= 8;
  92                         s_util.c[0] = *(const uint8_t *)w;
  93                         w = (const uint16_t *)(const void *)((const uint8_t *)w + 1);
  94                         mlen--;
  95                         byte_swapped = 1;
  96                 }
  97                 /*
  98                  * Unroll the loop to make overhead from
  99                  * branches &c small.
 100                  */
 101                 while ((mlen -= 32) >= 0) {
 102                         sum += w[0]; sum += w[1]; sum += w[2]; sum += w[3];
 103                         sum += w[4]; sum += w[5]; sum += w[6]; sum += w[7];
 104                         sum += w[8]; sum += w[9]; sum += w[10]; sum += w[11];
 105                         sum += w[12]; sum += w[13]; sum += w[14]; sum += w[15];
 106                         w += 16;
 107                 }
 108                 mlen += 32;
 109                 while ((mlen -= 8) >= 0) {
 110                         sum += w[0]; sum += w[1]; sum += w[2]; sum += w[3];
 111                         w += 4;
 112                 }
 113                 mlen += 8;
 114                 if (mlen == 0 && byte_swapped == 0)
 115                         continue;
 116                 REDUCE;
 117                 while ((mlen -= 2) >= 0) {
 118                         sum += *w++;
 119                 }
 120                 if (byte_swapped) {
 121                         REDUCE;
 122                         sum <<= 8;
 123                         byte_swapped = 0;
 124                         if (mlen == -1) {
 125                                 s_util.c[1] = *(const uint8_t *)w;
 126                                 sum += s_util.s;
 127                                 mlen = 0;
 128                         } else
 129                                 mlen = -1;
 130                 } else if (mlen == -1)
 131                         s_util.c[0] = *(const uint8_t *)w;
 132         }
 133         if (mlen == -1) {
 134                 /* The last mbuf has odd # of bytes. Follow the
 135                    standard (the odd byte may be shifted left by 8 bits
 136                    or not as determined by endian-ness of the machine) */
 137                 s_util.c[1] = 0;
 138                 sum += s_util.s;
 139         }
 140         REDUCE;
 141         return (~sum & 0xffff);
 142 }
 143
 144 int
 145 in_cksum(const vec_t *vec, int veclen)
 146 {
 147         return in_cksum_ret_partial(vec, veclen, NULL);
 148 }
 149
 150 uint16_t
 151 ip_checksum(const uint8_t *ptr, int len)
 152 {
 153         vec_t cksum_vec[1];
 154
 155         SET_CKSUM_VEC_PTR(cksum_vec[0], ptr, len);
 156         return in_cksum_ret_partial(&cksum_vec[0], 1, NULL);
 157 }
 158
 159 uint16_t
 160 ip_checksum_tvb(tvbuff_t *tvb, int offset, int len)
 161 {
 162         vec_t cksum_vec[1];
 163
 164         SET_CKSUM_VEC_TVB(cksum_vec[0], tvb, offset, len);
 165         return in_cksum_ret_partial(&cksum_vec[0], 1, NULL);
 166 }
 167
 168 /*
 169  * Given the host-byte-order value of the checksum field in a packet
 170  * header, and the network-byte-order computed checksum of the data
 171  * that the checksum covers (including the checksum itself), compute
 172  * what the checksum field *should* have been.
 173  */
 174 uint16_t
 175 in_cksum_shouldbe(uint16_t sum, uint16_t computed_sum)
 176 {
 177         uint32_t shouldbe;
 178
 179         /*
 180          * The value that should have gone into the checksum field
 181          * is the negative of the value gotten by summing up everything
 182          * *but* the checksum field.
 183          *
 184          * We can compute that by subtracting the value of the checksum
 185          * field from the sum of all the data in the packet, and then
 186          * computing the negative of that value.
 187          *
 188          * "sum" is the value of the checksum field, and "computed_sum"
 189          * is the negative of the sum of all the data in the packets,
 190          * so that's -(-computed_sum - sum), or (sum + computed_sum).
 191          *
 192          * All the arithmetic in question is one's complement, so the
 193          * addition must include an end-around carry; we do this by
 194          * doing the arithmetic in 32 bits (with no sign-extension),
 195          * and then adding the upper 16 bits of the sum, which contain
 196          * the carry, to the lower 16 bits of the sum, and then do it
 197          * again in case *that* sum produced a carry.
 198          *
 199          * As RFC 1071 notes, the checksum can be computed without
 200          * byte-swapping the 16-bit words; summing 16-bit words
 201          * on a big-endian machine gives a big-endian checksum, which
 202          * can be directly stuffed into the big-endian checksum fields
 203          * in protocol headers, and summing words on a little-endian
 204          * machine gives a little-endian checksum, which must be
 205          * byte-swapped before being stuffed into a big-endian checksum
 206          * field.
 207          *
 208          * "computed_sum" is a network-byte-order value, so we must put
 209          * it in host byte order before subtracting it from the
 210          * host-byte-order value from the header; the adjusted checksum
 211          * will be in host byte order, which is what we'll return.
 212          */
 213         shouldbe = sum;
 214         shouldbe += g_ntohs(computed_sum);
 215         shouldbe = (shouldbe & 0xFFFF) + (shouldbe >> 16);
 216         shouldbe = (shouldbe & 0xFFFF) + (shouldbe >> 16);
 217         return shouldbe;
 218 }
 219
 220 /*
 221  * Editor modelines  -  https://www.wireshark.org/tools/modelines.html
 222  *
 223  * Local variables:
 224  * c-basic-offset: 8
 225  * tab-width: 8
 226  * indent-tabs-mode: t
 227  * End:
 228  *
 229  * vi: set shiftwidth=8 tabstop=8 noexpandtab:
 230  * :indentSize=8:tabSize=8:noTabs=false:
 231  */