ipcsum_alth.c

// Rawsock library, licensed under GPLv2
// Version 0.3.4
#include "ipcsum_alth.h"

// This is all taken from Linux kernel 4.19.1 (this is not original work)
static inline unsigned short from32to16(unsigned int 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;
}

// This is all taken from Linux kernel 4.19.1 (this is not original work)
static unsigned int do_csum(const unsigned char *buff, int len) {
    int odd;
    unsigned int result = 0;

    if (len <= 0)
        goto out;
    odd = 1 & (unsigned long) buff;
    if (odd) {
#ifdef __LITTLE_ENDIAN
        result += (*buff << 8);
#else
        result = *buff;
#endif
        len--;
        buff++;
    }
    if (len >= 2) {
        if (2 & (unsigned long) buff) {
            result += *(unsigned short *) buff;
            len -= 2;
            buff += 2;
        }
        if (len >= 4) {
            const unsigned char *end = buff + ((unsigned)len & ~3);
            unsigned int carry = 0;
            do {
                unsigned int w = *(unsigned int *) buff;
                buff += 4;
                result += carry;
                result += w;
                carry = (w > result);
            } while (buff < end);
            result += carry;
            result = (result & 0xffff) + (result >> 16);
        }
        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);
out:
    return result;
}

__sum16 ip_fast_csum(const void *iph, unsigned int ihl)
{
    return (__sum16)~do_csum(iph,ihl*4);
}