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in_cksum.c
/* in_cksum.c
 * 4.4-Lite-2 Internet checksum routine, modified to take a vector of
 * pointers/lengths giving the pieces to be checksummed.
 *
 * $Id: in_cksum.c 12117 2004-09-28 00:06:32Z guy $
 */

/*
 * Copyright (c) 1988, 1992, 1993
 *    The Regents of the University of California.  All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. Neither the name of the University nor the names of its contributors
 *    may be used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 *
 *    @(#)in_cksum.c    8.1 (Berkeley) 6/10/93
 */

#ifdef HAVE_CONFIG_H
# include "config.h"
#endif

#include <glib.h>

#include <epan/in_cksum.h>

/*
 * Checksum routine for Internet Protocol family headers (Portable Version).
 *
 * This routine is very heavily used in the network
 * code and should be modified for each CPU to be as fast as possible.
 */

#define ADDCARRY(x)  (x > 65535 ? x -= 65535 : x)
#define REDUCE {l_util.l = sum; sum = l_util.s[0] + l_util.s[1]; ADDCARRY(sum);}

int
in_cksum(const vec_t *vec, int veclen)
{
      register const guint16 *w;
      register int sum = 0;
      register int mlen = 0;
      int byte_swapped = 0;

      union {
            guint8      c[2];
            guint16     s;
      } s_util;
      union {
            guint16 s[2];
            guint32     l;
      } l_util;

      for (; veclen != 0; vec++, veclen--) {
            if (vec->len == 0)
                  continue;
            w = (const guint16 *)vec->ptr;
            if (mlen == -1) {
                  /*
                   * The first byte of this chunk is the continuation
                   * of a word spanning between this chunk and the
                   * last chunk.
                   *
                   * s_util.c[0] is already saved when scanning previous
                   * chunk.
                   */
                  s_util.c[1] = *(const guint8 *)w;
                  sum += s_util.s;
                  w = (const guint16 *)((const guint8 *)w + 1);
                  mlen = vec->len - 1;
            } else
                  mlen = vec->len;
            /*
             * Force to even boundary.
             */
            if ((1 & (unsigned long) w) && (mlen > 0)) {
                  REDUCE;
                  sum <<= 8;
                  s_util.c[0] = *(const guint8 *)w;
                  w = (const guint16 *)((const guint8 *)w + 1);
                  mlen--;
                  byte_swapped = 1;
            }
            /*
             * Unroll the loop to make overhead from
             * branches &c small.
             */
            while ((mlen -= 32) >= 0) {
                  sum += w[0]; sum += w[1]; sum += w[2]; sum += w[3];
                  sum += w[4]; sum += w[5]; sum += w[6]; sum += w[7];
                  sum += w[8]; sum += w[9]; sum += w[10]; sum += w[11];
                  sum += w[12]; sum += w[13]; sum += w[14]; sum += w[15];
                  w += 16;
            }
            mlen += 32;
            while ((mlen -= 8) >= 0) {
                  sum += w[0]; sum += w[1]; sum += w[2]; sum += w[3];
                  w += 4;
            }
            mlen += 8;
            if (mlen == 0 && byte_swapped == 0)
                  continue;
            REDUCE;
            while ((mlen -= 2) >= 0) {
                  sum += *w++;
            }
            if (byte_swapped) {
                  REDUCE;
                  sum <<= 8;
                  byte_swapped = 0;
                  if (mlen == -1) {
                        s_util.c[1] = *(const guint8 *)w;
                        sum += s_util.s;
                        mlen = 0;
                  } else
                        mlen = -1;
            } else if (mlen == -1)
                  s_util.c[0] = *(const guint8 *)w;
      }
      if (mlen == -1) {
            /* The last mbuf has odd # of bytes. Follow the
               standard (the odd byte may be shifted left by 8 bits
               or not as determined by endian-ness of the machine) */
            s_util.c[1] = 0;
            sum += s_util.s;
      }
      REDUCE;
      return (~sum & 0xffff);
}

/*
 * Given the host-byte-order value of the checksum field in a packet
 * header, and the network-byte-order computed checksum of the data
 * that the checksum covers (including the checksum itself), compute
 * what the checksum field *should* have been.
 */
guint16
in_cksum_shouldbe(guint16 sum, guint16 computed_sum)
{
      guint32 shouldbe;

      /*
       * The value that should have gone into the checksum field
       * is the negative of the value gotten by summing up everything
       * *but* the checksum field.
       *
       * We can compute that by subtracting the value of the checksum
       * field from the sum of all the data in the packet, and then
       * computing the negative of that value.
       *
       * "sum" is the value of the checksum field, and "computed_sum"
       * is the negative of the sum of all the data in the packets,
       * so that's -(-computed_sum - sum), or (sum + computed_sum).
       *
       * All the arithmetic in question is one's complement, so the
       * addition must include an end-around carry; we do this by
       * doing the arithmetic in 32 bits (with no sign-extension),
       * and then adding the upper 16 bits of the sum, which contain
       * the carry, to the lower 16 bits of the sum, and then do it
       * again in case *that* sum produced a carry.
       *
       * As RFC 1071 notes, the checksum can be computed without
       * byte-swapping the 16-bit words; summing 16-bit words
       * on a big-endian machine gives a big-endian checksum, which
       * can be directly stuffed into the big-endian checksum fields
       * in protocol headers, and summing words on a little-endian
       * machine gives a little-endian checksum, which must be
       * byte-swapped before being stuffed into a big-endian checksum
       * field.
       *
       * "computed_sum" is a network-byte-order value, so we must put
       * it in host byte order before subtracting it from the
       * host-byte-order value from the header; the adjusted checksum
       * will be in host byte order, which is what we'll return.
       */
      shouldbe = sum;
      shouldbe += g_ntohs(computed_sum);
      shouldbe = (shouldbe & 0xFFFF) + (shouldbe >> 16);
      shouldbe = (shouldbe & 0xFFFF) + (shouldbe >> 16);
      return shouldbe;
}

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