// SPDX-License-Identifier: GPL-2.0-only

/*
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of version 2 of the GNU General Public License as
 * published by the Free Software Foundation.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
 */

#include <errno.h>
#include <fcntl.h>
#include <limits.h>
#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sysexits.h>
#include <unistd.h>
#include <sys/ioctl.h>

#include <linux/if.h>
#include <linux/if_tun.h>

// Adapted 2019-12-14 from Documentation/networking/tuntap.txt in Linux.
int tap_alloc(char *dev)
{
	struct ifreq ifr;
	int fd, err;

	if ((fd = open("/dev/net/tun", O_RDWR)) < 0)
		return -1;

	memset(&ifr, 0, sizeof(ifr));

	/* Flags: IFF_TUN   - TUN device (no Ethernet headers)
	 *        IFF_TAP   - TAP device
	 *
	 *        IFF_NO_PI - Do not provide packet information
	 */
	ifr.ifr_flags = IFF_TAP;
	if (*dev)
		strncpy(ifr.ifr_name, dev, IFNAMSIZ);

	if ((err = ioctl(fd, TUNSETIFF, (void *) &ifr)) < 0) {
		close(fd);
		return err;
	}
	strcpy(dev, ifr.ifr_name);
	return fd;
}

void ex_usage()
{
	fputs("mktap: usage: mktap [-E] [-i ifr_name] fd prog...\n", stderr);
	exit(EX_USAGE);
}

int main(int argc, char **argv)
{
	char *ifr_name_in = NULL;
	bool name_env = true;

	int c;
	while ((c = getopt(argc, argv, "+Ei:")) != -1) {
		switch (c) {
		case 'E':
			name_env = false;
			break;
		case 'i':
			ifr_name_in = optarg;
			break;
		default:
			ex_usage();
		}
	}

	if (argc - optind < 2)
		ex_usage();

	// ifr_name is a macro, so add a trailing _.
	char *ifr_name_ = calloc(IFNAMSIZ, sizeof(char));
	if (ifr_name_in) {
		if (strlen(ifr_name_in) > IFNAMSIZ - 1) {
			fprintf(stderr, "mktap: ifr_name too long (max %i)\n", IFNAMSIZ - 1);
			return EX_USAGE;
		}
		strcpy(ifr_name_, ifr_name_in);
	}

	errno = 0;
	char *target_fd_end;
	long int target_fd_l = strtol(argv[optind], &target_fd_end, 10);
	if (errno) {
		fprintf(stderr, "mktap: %s\n", strerror(errno));
		return EX_USAGE;
	}
	if (argv[optind][0] == '\0' || target_fd_end[0] != '\0') {
		fprintf(stderr, "mktap: invalid integer: `%s'\n", argv[optind]);
		return EX_USAGE;
	}
	if (target_fd_l < INT_MIN || target_fd_l > INT_MAX) {
		fprintf(stderr, "mktap: out of range for file descriptor: `%li'\n", target_fd_l);
		return EX_USAGE;
	}
	int target_fd = (int)target_fd_l;

	int tap_fd = tap_alloc(ifr_name_);
	if (tap_fd < 0) {
		fprintf(stderr, "mktap: failed to open tap device: %s\n", strerror(errno));
		return EX_IOERR;
	}

	if (dup2(tap_fd, target_fd) == -1) {
		fprintf(stderr, "mktap: %s\n", strerror(errno));
		return EX_IOERR;
	}
	if (close(tap_fd) == -1) {
		fprintf(stderr, "mktap: %s\n", strerror(errno));
		return EX_IOERR;
	}

	if (name_env) {
		if (setenv("TUNTAP_NAME", ifr_name_, 1) == -1) {
			fprintf(stderr, "mktap: %s\n", strerror(errno));
			return EX_OSERR;
		}
	}

	execvp(argv[optind + 1], &argv[optind + 1]);

	fprintf(stderr, "failed to exec: %s\n", strerror(errno));
	return EX_OSERR;
}