http://www.cs.fsu.edu/~baker/devices/lxr/http/source/linux/lib/checksum.c#L49 考虑了字节对齐的校验和源码
http://blog.csdn.net/qy532846454/article/details/7010852 内核分析

 

 /*
*
* 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. */
//字节对齐这块儿,确实还不懂,但是这个源码也说明了校验需要考虑大端小端的情况 
#include <linux/module.h>
#include <net/checksum.h>
#include <asm/byteorder.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 int 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) {
#ifdef __LITTLE_ENDIAN
result = *buff;
#else
result += (*buff << 8);
#endif
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 int *) buff;
count--;
buff += 4;
result += carry;
result += w;
carry = (w > result);
} while (count);
result += carry;
result = (result & 0xffff) + (result >> 16);//加高位进位的1
}
if (len & 2) {
result += *(unsigned short *) buff;
buff += 2;
}
}
if (len & 1)
#ifdef __LITTLE_ENDIAN
result += *buff;
#else
result += (*buff << 8);
#endif
result = from32to16(result);
if (odd)
result = ((result >> 8) & 0xff) | ((result & 0xff) << 8);//从奇数地址开始校验,最后需要高低交换
out:
return result;//这个结果在调用后需要取反
}
/*
*      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);
}
EXPORT_SYMBOL(ip_fast_csum);
/*
* 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 wsum)
{
unsigned int sum = (__force unsigned int)wsum;
unsigned int result = do_csum(buff, len);
/* add in old sum, and carry.. */
result += sum;
if (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 missing;
missing = __copy_from_user(dst, src, len);
if (missing) {
memset(dst + len - missing, 0, missing);
*csum_err = -EFAULT;
} else
*csum_err = 0;
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(const void *src, void *dst, int len, __wsum sum)
{
memcpy(dst, src, len);
return csum_partial(dst, len, sum);
}
EXPORT_SYMBOL(csum_partial_copy);
#ifndef csum_tcpudp_nofold
__wsum csum_tcpudp_nofold(__be32 saddr, __be32 daddr,
unsigned short len,
unsigned short proto,
__wsum sum)
{
unsigned long long s = (__force u32)sum;
s += (__force u32)saddr;
s += (__force u32)daddr;
#ifdef __BIG_ENDIAN
s += proto + len;
#else
s += (proto + len) << 8;
#endif
s += (s >> 32);
return (__force __wsum)s;
}
EXPORT_SYMBOL(csum_tcpudp_nofold);
#endif



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