lib/csum.c

   1 /*
   2  * Copyright (c) 2008, 2009, 2010, 2011 Nicira, Inc.
   3  *
   4  * Licensed under the Apache License, Version 2.0 (the "License");
   5  * you may not use this file except in compliance with the License.
   6  * You may obtain a copy of the License at:
   7  *
   8  *     http://www.apache.org/licenses/LICENSE-2.0
   9  *
  10  * Unless required by applicable law or agreed to in writing, software
  11  * distributed under the License is distributed on an "AS IS" BASIS,
  12  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  13  * See the License for the specific language governing permissions and
  14  * limitations under the License.
  15  */
  16
  17 #include <config.h>
  18 #include "csum.h"
  19 #include "unaligned.h"
  20
  21 #ifndef __CHECKER__
  22
  23 /* Returns the IP checksum of the 'n' bytes in 'data'.
  24  *
  25  * The return value has the same endianness as the data.  That is, if 'data'
  26  * consists of a packet in network byte order, then the return value is a value
  27  * in network byte order, and if 'data' consists of a data structure in host
  28  * byte order, then the return value is in host byte order. */
  29 ovs_be16
  30 csum(const void *data, size_t n)
  31 {
  32     return csum_finish(csum_continue(0, data, n));
  33 }
  34
  35 /* Adds the 16 bits in 'new' to the partial IP checksum 'partial' and returns
  36  * the updated checksum.  (To start a new checksum, pass 0 for 'partial'.  To
  37  * obtain the finished checksum, pass the return value to csum_finish().) */
  38 uint32_t
  39 csum_add16(uint32_t partial, ovs_be16 new)
  40 {
  41     return partial + new;
  42 }
  43
  44 /* Adds the 32 bits in 'new' to the partial IP checksum 'partial' and returns
  45  * the updated checksum.  (To start a new checksum, pass 0 for 'partial'.  To
  46  * obtain the finished checksum, pass the return value to csum_finish().) */
  47 uint32_t
  48 csum_add32(uint32_t partial, ovs_be32 new)
  49 {
  50     return partial + (new >> 16) + (new & 0xffff);
  51 }
  52
  53
  54 /* Adds the 'n' bytes in 'data' to the partial IP checksum 'partial' and
  55  * returns the updated checksum.  (To start a new checksum, pass 0 for
  56  * 'partial'.  To obtain the finished checksum, pass the return value to
  57  * csum_finish().) */
  58 uint32_t
  59 csum_continue(uint32_t partial, const void *data_, size_t n)
  60 {
  61     const ovs_be16 *data = data_;
  62
  63     for (; n > 1; n -= 2, data++) {
  64         partial = csum_add16(partial, get_unaligned_be16(data));
  65     }
  66     if (n) {
  67         partial += *(uint8_t *) data;
  68     }
  69     return partial;
  70 }
  71
  72 /* Returns the IP checksum corresponding to 'partial', which is a value updated
  73  * by some combination of csum_add16(), csum_add32(), and csum_continue().
  74  *
  75  * The return value has the same endianness as the checksummed data.  That is,
  76  * if the data consist of a packet in network byte order, then the return value
  77  * is a value in network byte order, and if the data are a data structure in
  78  * host byte order, then the return value is in host byte order. */
  79 ovs_be16
  80 csum_finish(uint32_t partial)
  81 {
  82     while (partial >> 16) {
  83         partial = (partial & 0xffff) + (partial >> 16);
  84     }
  85     return ~partial;
  86 }
  87
  88 /* Returns the new checksum for a packet in which the checksum field previously
  89  * contained 'old_csum' and in which a field that contained 'old_u16' was
  90  * changed to contain 'new_u16'. */
  91 ovs_be16
  92 recalc_csum16(ovs_be16 old_csum, ovs_be16 old_u16, ovs_be16 new_u16)
  93 {
  94     /* Ones-complement arithmetic is endian-independent, so this code does not
  95      * use htons() or ntohs().
  96      *
  97      * See RFC 1624 for formula and explanation. */
  98     uint16_t hc_complement = ~old_csum;
  99     uint16_t m_complement = ~old_u16;
 100     uint16_t m_prime = new_u16;
 101     uint32_t sum = hc_complement + m_complement + m_prime;
 102     return csum_finish(sum);
 103 }
 104
 105 /* Returns the new checksum for a packet in which the checksum field previously
 106  * contained 'old_csum' and in which a field that contained 'old_u32' was
 107  * changed to contain 'new_u32'. */
 108 ovs_be16
 109 recalc_csum32(ovs_be16 old_csum, ovs_be32 old_u32, ovs_be32 new_u32)
 110 {
 111     return recalc_csum16(recalc_csum16(old_csum, old_u32, new_u32),
 112                          old_u32 >> 16, new_u32 >> 16);
 113 }
 114
 115 #else  /* __CHECKER__ */
 116 /* Making sparse happy with these functions also makes them unreadable, so
 117  * don't bother to show it their implementations. */
 118 #endif