busybox/util-linux/switch_root.c

/* vi: set sw=4 ts=4: */
/* Copyright 2005 Rob Landley <rob@landley.net>
 *
 * Switch from rootfs to another filesystem as the root of the mount tree.
 *
 * Licensed under GPL version 2, see file LICENSE in this tarball for details.
 */

#include "busybox.h"
#include <sys/vfs.h>


// Make up for header deficiencies.

#ifndef RAMFS_MAGIC
#define RAMFS_MAGIC		0x858458f6
#endif

#ifndef TMPFS_MAGIC
#define TMPFS_MAGIC		0x01021994
#endif

#ifndef MS_MOVE
#define MS_MOVE			8192
#endif

static dev_t rootdev;

// Recursively delete contents of rootfs.

static void delete_contents(const char *directory)
{
	DIR *dir;
	struct dirent *d;
	struct stat st;

	// Don't descend into other filesystems
	if (lstat(directory, &st) || st.st_dev != rootdev) return;

	// Recursively delete the contents of directories.
	if (S_ISDIR(st.st_mode)) {
		dir = opendir(directory);
		if (dir) {
			while ((d = readdir(dir))) {
				char *newdir = d->d_name;

				// Skip . and ..
				if (*newdir=='.' && (!newdir[1] || (newdir[1]=='.' && !newdir[2])))
					continue;

				// Recurse to delete contents
				newdir = alloca(strlen(directory) + strlen(d->d_name) + 2);
				sprintf(newdir, "%s/%s", directory, d->d_name);
				delete_contents(newdir);
			}
			closedir(dir);

			// Directory should now be empty.  Zap it.
			rmdir(directory);
		}

	// It wasn't a directory.  Zap it.

	} else unlink(directory);
}

int switch_root_main(int argc, char **argv);
int switch_root_main(int argc, char **argv)
{
	char *newroot, *console = NULL;
	struct stat st1, st2;
	struct statfs stfs;

	// Parse args (-c console)

	opt_complementary = "-2";
	getopt32(argc, argv, "c:", &console);

	// Change to new root directory and verify it's a different fs.

	newroot = argv[optind++];

	if (chdir(newroot) || lstat(".", &st1) || lstat("/", &st2) ||
		st1.st_dev == st2.st_dev)
	{
		bb_error_msg_and_die("bad newroot %s", newroot);
	}
	rootdev = st2.st_dev;

	// Additional sanity checks: we're about to rm -rf /,  so be REALLY SURE
	// we mean it.  (I could make this a CONFIG option, but I would get email
	// from all the people who WILL eat their filesystems.)

	if (lstat("/init", &st1) || !S_ISREG(st1.st_mode) || statfs("/", &stfs) ||
		(stfs.f_type != RAMFS_MAGIC && stfs.f_type != TMPFS_MAGIC) ||
		getpid() != 1)
	{
		bb_error_msg_and_die("not rootfs");
	}

	// Zap everything out of rootdev

	delete_contents("/");

	// Overmount / with newdir and chroot into it.  The chdir is needed to
	// recalculate "." and ".." links.

	if (mount(".", "/", NULL, MS_MOVE, NULL) || chroot(".") || chdir("/"))
		bb_error_msg_and_die("error moving root");

	// If a new console specified, redirect stdin/stdout/stderr to that.

	if (console) {
		close(0);
		xopen(console, O_RDWR);
		dup2(0, 1);
		dup2(0, 2);
	}

	// Exec real init.  (This is why we must be pid 1.)
	execv(argv[optind], argv+optind);
	bb_error_msg_and_die("bad init '%s'", argv[optind]);
}