tools/testing/selftests/net/ipv6_fragmentation.c

// SPDX-License-Identifier: GPL-2.0
/*
 * Author: Brett A C Sheffield <bacs@librecast.net>
 *
 * Kernel selftest for the IPv6 fragmentation regression which affected stable
 * kernels:
 *
 *   https://lore.kernel.org/stable/aElivdUXqd1OqgMY@karahi.gladserv.com
 *
 * Commit: a18dfa9925b9 ("ipv6: save dontfrag in cork") was backported to stable
 * without some prerequisite commits.
 *
 * This caused a regression when sending IPv6 UDP packets by preventing
 * fragmentation and instead returning -1 (EMSGSIZE).
 *
 * This selftest demonstrates the issue by sending an IPv6 UDP packet to
 * localhost (::1) on the loopback interface from the autoconfigured link-local
 * address.
 *
 * sendmsg(2) returns bytes sent correctly on a working kernel, and returns -1
 * (EMSGSIZE) when the regression is present.
 *
 * The regression was not present in the mainline kernel, but add this test to
 * catch similar breakage in future.
 */

#define _GNU_SOURCE

#include <error.h>
#include <net/if.h>
#include <netinet/in.h>
#include <sched.h>
#include <stdio.h>
#include <sys/ioctl.h>
#include <sys/socket.h>
#include <unistd.h>
#include "kselftest.h"

#define MTU 1500
#define LARGER_THAN_MTU 8192

static void setup(void)
{
	struct ifreq ifr = {
		.ifr_name = "lo"
	};
	int ctl;

	/* we need to set MTU, so do this in a namespace to play nicely */
	if (unshare(CLONE_NEWNET) == -1)
		error(KSFT_FAIL, errno, "unshare");

	ctl = socket(AF_LOCAL, SOCK_STREAM, 0);
	if (ctl == -1)
		error(KSFT_FAIL, errno, "socket");

	/* ensure MTU is smaller than what we plan to send */
	ifr.ifr_mtu = MTU;
	if (ioctl(ctl, SIOCSIFMTU, &ifr) == -1)
		error(KSFT_FAIL, errno, "ioctl: set MTU");

	/* bring up interface */
	if (ioctl(ctl, SIOCGIFFLAGS, &ifr) == -1)
		error(KSFT_FAIL, errno, "ioctl SIOCGIFFLAGS");
	ifr.ifr_flags = ifr.ifr_flags | IFF_UP;
	if (ioctl(ctl, SIOCSIFFLAGS, &ifr) == -1)
		error(KSFT_FAIL, errno, "ioctl: bring interface up");

	if (close(ctl) == -1)
		error(KSFT_FAIL, errno, "close");
}

int main(void)
{
	struct in6_addr addr = {
		.s6_addr[15] = 0x01,  /* ::1 */
	};
	struct sockaddr_in6 sa = {
		.sin6_family = AF_INET6,
		.sin6_addr = addr,
		.sin6_port = htons(9) /* port 9/udp (DISCARD) */
	};
	static char buf[LARGER_THAN_MTU] = {0};
	struct iovec iov = { .iov_base = buf, .iov_len = sizeof(buf) };
	struct msghdr msg = {
		.msg_iov = &iov,
		.msg_iovlen = 1,
		.msg_name = (struct sockaddr *)&sa,
		.msg_namelen = sizeof(sa),
	};
	ssize_t rc;
	int s;

	printf("Testing IPv6 fragmentation\n");
	setup();
	s = socket(AF_INET6, SOCK_DGRAM, 0);
send_again:
	rc = sendmsg(s, &msg, 0);
	if (rc == -1) {
		/* if interface wasn't ready, try again */
		if (errno == EADDRNOTAVAIL) {
			usleep(1000);
			goto send_again;
		}
		error(KSFT_FAIL, errno, "sendmsg");
	} else if (rc != LARGER_THAN_MTU) {
		error(KSFT_FAIL, errno, "sendmsg returned %zi, expected %i",
				rc, LARGER_THAN_MTU);
	}
	printf("[PASS] sendmsg() returned %zi\n", rc);
	if (close(s) == -1)
		error(KSFT_FAIL, errno, "close");
	return KSFT_PASS;
}