checksum.c

linux 内核源代码

/*
 * 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, <[e-mail]>
 *		Arnt Gulbrandsen, <[e-mail]>
 *		Tom May, <[e-mail]>
 *		Andreas Schwab, <[e-mail]>
 *		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;
}

/*
 * 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);
}

EXPORT_SYMBOL(ip_compute_csum);

/*
 * 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)
{
	int rem;

	if (csum_err)
		*csum_err = 0;

	rem = copy_from_user(dst, src, len);
	if (rem != 0) {
		if (csum_err)
			*csum_err = -EFAULT;
		memset(dst + len - rem, 0, rem);
		len = rem;
	}

	return csum_partial(dst, len, sum);
}

EXPORT_SYMBOL(csum_partial_copy_from_user);

/*
 * 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);
}

EXPORT_SYMBOL(csum_partial_copy_nocheck);