rm_loop.c

/*
 * remove loop device created by losetup
 *
 * SPDX-License-Identifier: LGPL-2.1-or-later
 *
 * Copyright (C) 2025 Ben Song <[email protected]>
 *
 * This library is free software; you can redistribute it and/or modify it
 * under the terms of the GNU Lesser General Public License as published by the
 * Free Software Foundation; either version 2.1 of the License, or (at your
 * option) any later version.
 *
 * This library 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 Lesser General Public License
 * for more details.
 *
 * You should have received a copy of the GNU Lesser General Public License
 * along with this library; if not, write to the Free Software Foundation, Inc.,
 * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
 */

/*
 * Please refer to `man 4 loop`
 *
 * This scripts will delete node created by losetup, not by mknod
 * Case 1
 * 1. losetup /dev/loop0 a.img
 * 2. losetup -d /dev/loop0
 * 3. ./rm_loop 0 # OK
 *
 * Case 2:
 * 1. losetup /dev/loop0 a.img
 * 2. losetup -d /dev/loop0
 * 3. rm /dev/loop0
 * 4. losetup /dev/loop0 a.img # <- failed
 * 5. sudo mknod /dev/loop0 b 7 0
 * 6. losetup /dev/loop0 a.img # <- success
 * 7. losetup -d /dev/loop0
 *
 * after ./rm_loop 0, you need to delete the manual created /dev/loop0
 */

#include <stdio.h>
#include <sys/ioctl.h>
#include <unistd.h>
#include <fcntl.h>
#include <linux/loop.h>
#include <errno.h>
#include <stdlib.h>

int main(int argc, char* argv[]){
	int ret=0;
	if (argc != 2){
		ret=-1;
		printf("Usage:\n");
		printf("%s <loop index to delete>\n", argv[0]);
		goto final;
	}

	long loop_dev_index=atol(argv[1]);

	int fd = open("/dev/loop-control", O_RDWR);
	if (fd < 0){
		perror("Failed to open /dev/loop-control");
		ret=errno;
		goto final;
	}

	if ( ioctl(fd, LOOP_CTL_REMOVE, loop_dev_index) < 0 ){
		printf("/dev/loop");
		fflush(stdout);
		perror(argv[1]);
		ret=errno;
		goto close_fd;
	}

close_fd:
	close(fd);
final:
	return ret;
}