switch_root.c

/*
 * switchroot.c - switch to new root directory and start init.
 *
 * Copyright 2002-2009 Red Hat, Inc.  All rights reserved.
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * 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, see <http://www.gnu.org/licenses/>.
 *
 * Authors:
 *	Peter Jones <pjones@redhat.com>
 *	Jeremy Katz <katzj@redhat.com>
 */
#include <sys/mount.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/param.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <errno.h>
#include <ctype.h>
#include <dirent.h>
#include <err.h>
#include <libgen.h>

#ifndef MS_MOVE
#define MS_MOVE 8192
#endif

/* find the enclosing mount point of a path, by examining the backing device
 * of parent directories until we reach / or find a parent with a differing
 * device.
 * result must be freed.
 */
static char *get_parent_mount(const char *path)
{
	struct stat sb;
	char dir[PATH_MAX];
	char tmp[PATH_MAX];
	dev_t inner_dev;
	int r;

	r = stat(path, &sb);
	if (r != 0) {
		warn("failed to stat %s", path);
		return NULL;
	}
	inner_dev = sb.st_dev;

	/* dirname has some annoying properties of modifying the input... */
	strncpy(dir, path, PATH_MAX);
	dir[PATH_MAX - 1] = 0; /* for safety */

	while (1) {
		char *parent;

		strncpy(tmp, dir, PATH_MAX);
		tmp[PATH_MAX - 1] = 0;
		parent = dirname(tmp);

		r = stat(parent, &sb);
		if (r != 0) {
			warn("failed to stat %s", parent);
			return NULL;
		}

		/* if the parent directory's device differs then we have found a mount
		 * point */
		if (sb.st_dev != inner_dev)
			return strdup(dir);

		strncpy(dir, parent, PATH_MAX);
		dir[PATH_MAX - 1] = 0;

		/* maybe we've reached / */
		if (strlen(dir) == 1)
			return strdup(dir);
	}
}

/* remove all files/directories below dirName -- don't cross mountpoints */
static int recursiveRemove(char *dirName)
{
	struct stat rb;
	DIR *dir;
	int rc = -1;
	int dfd;

	if (!(dir = opendir(dirName))) {
		warn("failed to open %s", dirName);
		goto done;
	}

	dfd = dirfd(dir);

	if (fstat(dfd, &rb)) {
		warn("failed to stat %s", dirName);
		goto done;
	}

	while(1) {
		struct dirent *d;

		errno = 0;
		if (!(d = readdir(dir))) {
			if (errno) {
				warn("failed to read %s", dirName);
				goto done;
			}
			break;	/* end of directory */
		}

		if (!strcmp(d->d_name, ".") || !strcmp(d->d_name, ".."))
			continue;

		if (d->d_type == DT_DIR) {
			struct stat sb;

			if (fstatat(dfd, d->d_name, &sb, AT_SYMLINK_NOFOLLOW)) {
				warn("failed to stat %s/%s", dirName, d->d_name);
				continue;
			}

			/* remove subdirectories if device is same as dir */
			if (sb.st_dev == rb.st_dev) {
				char subdir[ strlen(dirName) +
					     strlen(d->d_name) + 2 ];

				sprintf(subdir, "%s/%s", dirName, d->d_name);
				recursiveRemove(subdir);
			} else
				continue;
		}

		if (unlinkat(dfd, d->d_name,
			     d->d_type == DT_DIR ? AT_REMOVEDIR : 0))
			warn("failed to unlink %s/%s", dirName, d->d_name);
	}

	rc = 0;	/* success */

done:
	if (dir)
		closedir(dir);
	return rc;
}

static int switchroot(const char *newroot)
{
	/*  Don't try to unmount the old "/", there's no way to do it. */
	const char *umounts[] = { "/dev", "/proc", "/sys", NULL };
	char *newroot_mnt;
	const char *chroot_path = NULL;
	int i;
	int r = -1;

	for (i = 0; umounts[i] != NULL; i++) {
		char newmount[PATH_MAX];

		snprintf(newmount, sizeof(newmount), "%s%s", newroot, umounts[i]);

		if (mount(umounts[i], newmount, NULL, MS_MOVE, NULL) < 0) {
			warn("failed to mount moving %s to %s",
				umounts[i], newmount);
			warnx("forcing unmount of %s", umounts[i]);
			umount2(umounts[i], MNT_FORCE);
		}
	}

	if (chdir(newroot)) {
		warn("failed to change directory to %s", newroot);
		return -1;
	}

	recursiveRemove("/");

	newroot_mnt = get_parent_mount(newroot);
	if (newroot_mnt && strcmp(newroot, newroot_mnt)) {
		/* newroot is not a mount point, so we have to MS_MOVE the parent
		 * mount point and then chroot in to the "subroot" */
		chroot_path = newroot + strlen(newroot_mnt);
		newroot = newroot_mnt;

		if (chdir(newroot)) {
			warn("failed to chdir to newroot mount %s", newroot);
			goto err;
		}
	}

	if (mount(newroot, "/", NULL, MS_MOVE, NULL) < 0) {
		warn("failed to mount moving %s to /", newroot);
		goto err;
	}

	if (chroot(".")) {
		warn("failed to change root");
		goto err;
	}

	if (chroot_path) {
		if (chdir(chroot_path)) {
			warn("failed to chdir to subroot %s", chroot_path);
			goto err;
		}

		if (chroot(".")) {
			warn("failed to change root to subroot");
			goto err;
		}
	}

	r = 0;
err:
	if (newroot_mnt)
		free(newroot_mnt);
	return r;
}

static void usage(FILE *output)
{
	fprintf(output, "usage: %s <newrootdir> <init> <args to init>\n",
			program_invocation_short_name);
	exit(output == stderr ? EXIT_FAILURE : EXIT_SUCCESS);
}

static void version(void)
{
	fprintf(stdout,  "%s from %s\n", program_invocation_short_name,
			PACKAGE_STRING);
	exit(EXIT_SUCCESS);
}

int main(int argc, char *argv[])
{
	char *newroot, *init, **initargs;

	if (argv[1] && (!strcmp(argv[1], "--help") || !strcmp(argv[1], "-h")))
		usage(stdout);
	if (argv[1] && (!strcmp(argv[1], "--version") || !strcmp(argv[1], "-V")))
		version();
	if (argc < 3)
		usage(stderr);

	newroot = argv[1];
	init = argv[2];
	initargs = &argv[2];

	if (!*newroot || !*init)
		usage(stderr);

	if (switchroot(newroot))
		errx(EXIT_FAILURE, "failed. Sorry.");

	if (access(init, X_OK))
		warn("cannot access %s", init);

	/* get session leader */
	setsid();

	/* set controlling terminal */
	if (ioctl (0, TIOCSCTTY, 1))
		warn("failed to TIOCSCTTY");

	execv(init, initargs);
	err(EXIT_FAILURE, "failed to execute %s", init);
}