Linux Kernel  3.7.1
 All Data Structures Namespaces Files Functions Variables Typedefs Enumerations Enumerator Macros Groups Pages
checksum.c
Go to the documentation of this file.
1 /*
2  * INET An implementation of the TCP/IP protocol suite for the LINUX
3  * operating system. INET is implemented using the BSD Socket
4  * interface as the means of communication with the user level.
5  *
6  * IP/TCP/UDP checksumming routines
7  *
8  * Authors: Jorge Cwik, <[email protected]>
9  * Arnt Gulbrandsen, <[email protected]>
10  * Tom May, <[email protected]>
11  * Andreas Schwab, <[email protected]>
12  * Lots of code moved from tcp.c and ip.c; see those files
13  * for more names.
14  *
15  * 03/02/96 Jes Sorensen, Andreas Schwab, Roman Hodek:
16  * Fixed some nasty bugs, causing some horrible crashes.
17  * A: At some points, the sum (%0) was used as
18  * length-counter instead of the length counter
19  * (%1). Thanks to Roman Hodek for pointing this out.
20  * B: GCC seems to mess up if one uses too many
21  * data-registers to hold input values and one tries to
22  * specify d0 and d1 as scratch registers. Letting gcc choose these
23  * registers itself solves the problem.
24  *
25  * This program is free software; you can redistribute it and/or
26  * modify it under the terms of the GNU General Public License
27  * as published by the Free Software Foundation; either version
28  * 2 of the License, or (at your option) any later version.
29  */
30 
31 /* Revised by Kenneth Albanowski for m68knommu. Basic problem: unaligned access kills, so most
32  of the assembly has to go. */
33 
34 #include <net/checksum.h>
35 #include <linux/module.h>
36 
37 static inline unsigned short from32to16(unsigned long x)
38 {
39  /* add up 16-bit and 16-bit for 16+c bit */
40  x = (x & 0xffff) + (x >> 16);
41  /* add up carry.. */
42  x = (x & 0xffff) + (x >> 16);
43  return x;
44 }
45 
46 static unsigned long do_csum(const unsigned char * buff, int len)
47 {
48  int odd, count;
49  unsigned long result = 0;
50 
51  if (len <= 0)
52  goto out;
53  odd = 1 & (unsigned long) buff;
54  if (odd) {
55  result = *buff;
56  len--;
57  buff++;
58  }
59  count = len >> 1; /* nr of 16-bit words.. */
60  if (count) {
61  if (2 & (unsigned long) buff) {
62  result += *(unsigned short *) buff;
63  count--;
64  len -= 2;
65  buff += 2;
66  }
67  count >>= 1; /* nr of 32-bit words.. */
68  if (count) {
69  unsigned long carry = 0;
70  do {
71  unsigned long w = *(unsigned long *) buff;
72  count--;
73  buff += 4;
74  result += carry;
75  result += w;
76  carry = (w > result);
77  } while (count);
78  result += carry;
79  result = (result & 0xffff) + (result >> 16);
80  }
81  if (len & 2) {
82  result += *(unsigned short *) buff;
83  buff += 2;
84  }
85  }
86  if (len & 1)
87  result += (*buff << 8);
88  result = from32to16(result);
89  if (odd)
90  result = ((result >> 8) & 0xff) | ((result & 0xff) << 8);
91 out:
92  return result;
93 }
94 
95 /*
96  * This is a version of ip_compute_csum() optimized for IP headers,
97  * which always checksum on 4 octet boundaries.
98  */
99 __sum16 ip_fast_csum(const void *iph, unsigned int ihl)
100 {
101  return (__force __sum16)~do_csum(iph,ihl*4);
102 }
103 
104 /*
105  * computes the checksum of a memory block at buff, length len,
106  * and adds in "sum" (32-bit)
107  *
108  * returns a 32-bit number suitable for feeding into itself
109  * or csum_tcpudp_magic
110  *
111  * this function must be called with even lengths, except
112  * for the last fragment, which may be odd
113  *
114  * it's best to have buff aligned on a 32-bit boundary
115  */
116 /*
117  * Egads... That thing apparently assumes that *all* checksums it ever sees will
118  * be folded. Very likely a bug.
119  */
120 __wsum csum_partial(const void *buff, int len, __wsum sum)
121 {
122  unsigned int result = do_csum(buff, len);
123 
124  /* add in old sum, and carry.. */
125  result += (__force u32)sum;
126  /* 16+c bits -> 16 bits */
127  result = (result & 0xffff) + (result >> 16);
128  return (__force __wsum)result;
129 }
130 
132 
133 /*
134  * this routine is used for miscellaneous IP-like checksums, mainly
135  * in icmp.c
136  */
137 __sum16 ip_compute_csum(const void *buff, int len)
138 {
139  return (__force __sum16)~do_csum(buff,len);
140 }
141 
142 /*
143  * copy from fs while checksumming, otherwise like csum_partial
144  */
145 
146 __wsum
147 csum_partial_copy_from_user(const void __user *src, void *dst, int len,
148  __wsum sum, int *csum_err)
149 {
150  if (csum_err) *csum_err = 0;
151  memcpy(dst, (__force const void *)src, len);
152  return csum_partial(dst, len, sum);
153 }
154 
155 /*
156  * copy from ds while checksumming, otherwise like csum_partial
157  */
158 
159 __wsum
160 csum_partial_copy_nocheck(const void *src, void *dst, int len, __wsum sum)
161 {
162  memcpy(dst, src, len);
163  return csum_partial(dst, len, sum);
164 }