checksum.c

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/*
 * 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, <[email protected]>
 *      Arnt Gulbrandsen, <[email protected]>
 *      Tom May, <[email protected]>
 *      Andreas Schwab, <[email protected]>
 *      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 <net/checksum.h>
#include <linux/module.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
 */
/*
 * Egads...  That thing apparently assumes that *all* checksums it ever sees will
 * be folded.  Very likely a bug.
 */
__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;
    /* 16+c bits -> 16 bits */
    result = (result & 0xffff) + (result >> 16);
    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);
}