Want to explore what's in your rootfs?

rootfs.c

#define _GNU_SOURCE

#include <errno.h>
#include <fcntl.h>
#include <sched.h>
#include <stdio.h>
#include <sys/mount.h>
#include <sys/stat.h>
#include <sys/syscall.h>
#include <sys/types.h>
#include <unistd.h>

static char *default_args[] = {"/bin/busybox", "ash", NULL};

int main(int argc, char **argv, char **envp)
{
	int mntns, exefd;
	char **args;

	if (argc < 2) {
		args = default_args;
	} else {
		args = argv + 1;
	}

	exefd = open(args[0], O_PATH);
	if (exefd == -1) {
		perror(args[0]);
		goto out;
	}

	if (unshare(CLONE_NEWNS) == -1) {
		perror("unshare(CLONE_NEWNS)");
		goto out;
	}

	if (mount("none", "/", NULL, MS_REC | MS_PRIVATE, NULL) == -1) {
		perror("mount(MS_REC | MS_PRIVATE)");
		goto out;
	}

	mntns = open("/proc/self/ns/mnt", O_RDONLY);
	if (mntns == -1) {
		perror("/proc/self/ns/mnt");
		goto out;
	}

	if (umount2("/", MNT_DETACH)) {
		perror("umount2(MNT_DETACH)");
		goto out;
	}

	if (setns(mntns, CLONE_NEWNS)) {
		perror("setns(CLONE_NEWNS)");
		goto out;
	}

	if (access("/proc", F_OK) == -1) {
		fprintf(stderr, "/proc does not exist, creating it\n");

		if (mkdir("/proc", 0755) == -1 && errno != EEXIST) {
			perror("mkdir(/proc)");
			goto out;
		}
	}

	if (mount("proc", "/proc", "proc", MS_NODEV | MS_NOEXEC | MS_NOSUID, NULL) == -1) {
		perror("mount(/proc)");
		goto out;
	}

	syscall(SYS_execveat, exefd, "", args, envp, AT_EMPTY_PATH);

	perror(args[0]);

out:
	return 1;
}

rootfsmount.c

#define _GNU_SOURCE

#include <errno.h>
#include <fcntl.h>
#include <sched.h>
#include <stdio.h>
#include <sys/mount.h>
#include <sys/stat.h>
#include <sys/syscall.h>
#include <sys/types.h>
#include <unistd.h>
#include <stdlib.h>

#include <linux/mount.h>

int main(int argc, char **argv, char **envp)
{
	int oldmntns, tmpmntns, targetfd, rootfs;
	char *path;

	if (argc != 2) {
		fprintf(stderr, "Usage: %s [path]\n", argv[0]);
		exit(1);
	} else {
		path = argv[1];
	}

	targetfd = open(path, O_PATH);
	if (targetfd == -1) {
		perror(path);
		goto out;
	}

	oldmntns = open("/proc/self/ns/mnt", O_RDONLY);
	if (oldmntns == -1) {
		perror("/proc/self/ns/mnt");
		goto out;
	}

	if (unshare(CLONE_NEWNS) == -1) {
		perror("unshare(CLONE_NEWNS)");
		goto out;
	}

	tmpmntns = open("/proc/self/ns/mnt", O_RDONLY);
	if (tmpmntns == -1) {
		perror("/proc/self/ns/mnt");
		goto out;
	}

	if (mount("none", "/", NULL, MS_REC | MS_PRIVATE, NULL) == -1) {
		perror("mount(MS_REC | MS_PRIVATE)");
		goto out;
	}

	if (umount2("/", MNT_DETACH)) {
		perror("umount2(MNT_DETACH)");
		goto out;
	}

	if (setns(tmpmntns, CLONE_NEWNS)) {
		perror("setns(tmpmntns, CLONE_NEWNS)");
		goto out;
	}

	rootfs = syscall(SYS_open_tree, AT_FDCWD, "/",
			 OPEN_TREE_CLONE | AT_RECURSIVE);
	if (rootfs == -1) {
		perror("open_tree");
		goto out;
	}

	if (setns(oldmntns, CLONE_NEWNS)) {
		perror("setns(oldmntns, CLONE_NEWNS)");
		goto out;
	}

	if (syscall(SYS_move_mount, rootfs, "", targetfd, "",
		    MOVE_MOUNT_F_EMPTY_PATH | MOVE_MOUNT_T_EMPTY_PATH) == -1) {
		perror("move_mount");
		goto out;
	}

	return 0;

out:
	return 1;
}