util/nvramtool/compute_ip_checksum.c

   1 /*****************************************************************************\
   2  * compute_ip_checksum.c
   3 \*****************************************************************************/
   4
   5 #include <stdint.h>
   6 #include "ip_checksum.h"
   7
   8 /* Note: The contents of this file were borrowed from the coreboot source
   9  *       code which may be obtained from https://www.coreboot.org.
  10  *       Specifically, this code was obtained from coreboot (LinuxBIOS)
  11  *       version 1.0.0.8.
  12  */
  13
  14 unsigned long compute_ip_checksum(void *addr, unsigned long length)
  15 {
  16         uint8_t *ptr;
  17         volatile union {
  18                 uint8_t byte[2];
  19                 uint16_t word;
  20         } value;
  21         unsigned long sum;
  22         unsigned long i;
  23         /* In the most straight forward way possible,
  24          * compute an ip style checksum.
  25          */
  26         sum = 0;
  27         ptr = addr;
  28         for (i = 0; i < length; i++) {
  29                 unsigned long value;
  30                 value = ptr[i];
  31                 if (i & 1) {
  32                         value <<= 8;
  33                 }
  34                 /* Add the new value */
  35                 sum += value;
  36                 /* Wrap around the carry */
  37                 if (sum > 0xFFFF) {
  38                         sum = (sum + (sum >> 16)) & 0xFFFF;
  39                 }
  40         }
  41         value.byte[0] = sum & 0xff;
  42         value.byte[1] = (sum >> 8) & 0xff;
  43         return (~value.word) & 0xFFFF;
  44 }