/*
 * Minimal reproducer for https://bugs.gentoo.org/697672
 *
 *   $ gcc dhcp.c -o dhcp -O2                      && ./dhcp
 *   packet sum: 0x011f5b
 *   $ gcc dhcp.c -o dhcp -O2 -fno-strict-aliasing && ./dhcp
 *   packet sum: 0x005555
 */

#include <stddef.h>
#include <stdint.h>
#include <stdio.h>

struct foo {
    uint32_t ip_src;
    uint32_t ip_dst; // not really needed to reproduce bug. elides a warning
};

// Trimmed-down version of dhcpcd-8.1.0:dhcp.c:dhcp_readbpf() function.
// Without top-level loop.
uint32_t do_sum(const void * packet) __attribute__((noinline));
uint32_t do_sum(const void * packet)
{
	const struct foo *ip = packet; // interpret network packet as a struct

	// creaft a new packet based on a subset of fields for original packet
	const struct foo pseudo_ip = {
		// -fno-strict-aliasing affects ths line
		.ip_src = ip->ip_src,
	};

	// and compute checksum based on uint16_t
	const void *data = &pseudo_ip;
	size_t len = sizeof (pseudo_ip);

	const uint16_t *word = data;

	uint32_t sum = 0;

	for (; len > 1; len -= sizeof(*word))
		sum += *word++;

	return sum;
}

int main(void) {
	printf("packet sum: %#08x\n", do_sum("\x11\x11\x44\x44\x22\x22\x11\x11"));
}