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checksum.c

/*
 * INET           An implementation of the TCP/IP protocol suite for the LINUX
 *          operating system.  INET is implemented using the  BSD Socket
 *          interface as the means of communication with the user level.
 *
 *          IP/TCP/UDP checksumming routines
 *
 * Authors: Jorge Cwik, <jorge@laser.satlink.net>
 *          Arnt Gulbrandsen, <agulbra@nvg.unit.no>
 *          Tom May, <ftom@netcom.com>
 *          Andreas Schwab, <schwab@issan.informatik.uni-dortmund.de>
 *          Lots of code moved from tcp.c and ip.c; see those files
 *          for more names.
 *
 * 03/02/96 Jes Sorensen, Andreas Schwab, Roman Hodek:
 *          Fixed some nasty bugs, causing some horrible crashes.
 *          A: At some points, the sum (%0) was used as
 *          length-counter instead of the length counter
 *          (%1). Thanks to Roman Hodek for pointing this out.
 *          B: GCC seems to mess up if one uses too many
 *          data-registers to hold input values and one tries to
 *          specify d0 and d1 as scratch registers. Letting gcc choose these
 *      registers itself solves the problem.
 *
 *          This program is free software; you can redistribute it and/or
 *          modify it under the terms of the GNU General Public License
 *          as published by the Free Software Foundation; either version
 *          2 of the License, or (at your option) any later version.
 */
 
/* Revised by Kenneth Albanowski for m68knommu. Basic problem: unaligned access kills, so most
   of the assembly has to go. */

#include <linux/module.h>
#include <net/checksum.h>

static inline unsigned short from32to16(unsigned long x)
{
      /* add up 16-bit and 16-bit for 16+c bit */
      x = (x & 0xffff) + (x >> 16);
      /* add up carry.. */
      x = (x & 0xffff) + (x >> 16);
      return x;
}

static unsigned long do_csum(const unsigned char * buff, int len)
{
      int odd, count;
      unsigned long result = 0;

      if (len <= 0)
            goto out;
      odd = 1 & (unsigned long) buff;
      if (odd) {
            result = *buff;
            len--;
            buff++;
      }
      count = len >> 1;       /* nr of 16-bit words.. */
      if (count) {
            if (2 & (unsigned long) buff) {
                  result += *(unsigned short *) buff;
                  count--;
                  len -= 2;
                  buff += 2;
            }
            count >>= 1;            /* nr of 32-bit words.. */
            if (count) {
                    unsigned long carry = 0;
                  do {
                        unsigned long w = *(unsigned long *) buff;
                        count--;
                        buff += 4;
                        result += carry;
                        result += w;
                        carry = (w > result);
                  } while (count);
                  result += carry;
                  result = (result & 0xffff) + (result >> 16);
            }
            if (len & 2) {
                  result += *(unsigned short *) buff;
                  buff += 2;
            }
      }
      if (len & 1)
            result += (*buff << 8);
      result = from32to16(result);
      if (odd)
            result = ((result >> 8) & 0xff) | ((result & 0xff) << 8);
out:
      return result;
}

/*
 *    This is a version of ip_compute_csum() optimized for IP headers,
 *    which always checksum on 4 octet boundaries.
 */
__sum16 ip_fast_csum(const void *iph, unsigned int ihl)
{
      return (__force __sum16)~do_csum(iph,ihl*4);
}

/*
 * computes the checksum of a memory block at buff, length len,
 * and adds in "sum" (32-bit)
 *
 * returns a 32-bit number suitable for feeding into itself
 * or csum_tcpudp_magic
 *
 * this function must be called with even lengths, except
 * for the last fragment, which may be odd
 *
 * it's best to have buff aligned on a 32-bit boundary
 */
__wsum csum_partial(const void *buff, int len, __wsum sum)
{
      unsigned int result = do_csum(buff, len);

      /* add in old sum, and carry.. */
      result += (__force u32)sum;
      if ((__force u32)sum > result)
            result += 1;
      return (__force __wsum)result;
}

EXPORT_SYMBOL(csum_partial);

/*
 * this routine is used for miscellaneous IP-like checksums, mainly
 * in icmp.c
 */
__sum16 ip_compute_csum(const void *buff, int len)
{
      return (__force __sum16)~do_csum(buff,len);
}

/*
 * copy from fs while checksumming, otherwise like csum_partial
 */

__wsum
csum_partial_copy_from_user(const void __user *src, void *dst,
                      int len, __wsum sum, int *csum_err)
{
      if (csum_err) *csum_err = 0;
      memcpy(dst, (__force const void *)src, len);
      return csum_partial(dst, len, sum);
}

/*
 * copy from ds while checksumming, otherwise like csum_partial
 */

__wsum
csum_partial_copy_nocheck(const void *src, void *dst, int len, __wsum sum)
{
      memcpy(dst, src, len);
      return csum_partial(dst, len, sum);
}

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