#define _GNU_SOURCE
#include <sched.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <errno.h>
#include <sys/mount.h>
#include <sys/stat.h>
#include <sys/sysinfo.h>

#define DIR1 "/tmp/dir1_%d"
#define DIR2 "/tmp/dir2_%d"

int main(void)
{
	/* unshare -rm: new mount + user namespace */
	if (getuid() != 0 && unshare(CLONE_NEWNS | CLONE_NEWUSER) == -1) {
		perror("unshare");
		return 1;
	}

	int nprocs = get_nprocs();
	int id = 0;

	for (int i = 1; i < nprocs; i++) {
		pid_t pid = fork();
		if (pid == -1) {
			perror("fork");
			return 1;
		}
		if (pid == 0) {
			id = i;
			break;
		}
	}

	char dir1[32], dir2[32];
	snprintf(dir1, sizeof(dir1), DIR1, id);
	snprintf(dir2, sizeof(dir2), DIR2, id);
	mkdir(dir1, 0755);
	mkdir(dir2, 0755);

	while (1) {
		if (mount(dir1, dir2, NULL, MS_BIND, NULL) == -1) {
			fprintf(stderr, "mount --bind %s %s: %s\n", dir1, dir2, strerror(errno));
			return 1;
		}

		if (umount(dir2) == -1) {
			fprintf(stderr, "umount %s: %s\n", dir2, strerror(errno));
			return 1;
		}
	}

	return 0;
}