source: MondoRescue/branches/2.2.5/mindi-busybox/util-linux/switch_root.c@ 1765

/* 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 "libbb.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(argv, "c:", &console);
    argv += optind;

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

    newroot = *argv++;

    xchdir(newroot);
    if (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("."))
        bb_error_msg_and_die("error moving root");
    xchdir("/");

    // 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[0], argv);
    bb_perror_msg_and_die("bad init %s", argv[0]);
}