243 lines
9.1 KiB
C
243 lines
9.1 KiB
C
/* vi: set sw=4 ts=4: */
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/* Copyright 2005 Rob Landley <rob@landley.net>
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*
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* Switch from rootfs to another filesystem as the root of the mount tree.
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*
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* Licensed under GPLv2, see file LICENSE in this source tree.
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*/
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//config:config SWITCH_ROOT
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//config: bool "switch_root"
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//config: default y
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//config: select PLATFORM_LINUX
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//config: help
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//config: The switch_root utility is used from initramfs to select a new
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//config: root device. Under initramfs, you have to use this instead of
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//config: pivot_root. (Stop reading here if you don't care why.)
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//config:
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//config: Booting with initramfs extracts a gzipped cpio archive into rootfs
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//config: (which is a variant of ramfs/tmpfs). Because rootfs can't be moved
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//config: or unmounted*, pivot_root will not work from initramfs. Instead,
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//config: switch_root deletes everything out of rootfs (including itself),
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//config: does a mount --move that overmounts rootfs with the new root, and
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//config: then execs the specified init program.
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//config:
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//config: * Because the Linux kernel uses rootfs internally as the starting
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//config: and ending point for searching through the kernel's doubly linked
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//config: list of active mount points. That's why.
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//applet:IF_SWITCH_ROOT(APPLET(switch_root, BB_DIR_SBIN, BB_SUID_DROP))
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//kbuild:lib-$(CONFIG_SWITCH_ROOT) += switch_root.o
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//usage:#define switch_root_trivial_usage
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//usage: "[-c /dev/console] NEW_ROOT NEW_INIT [ARGS]"
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//usage:#define switch_root_full_usage "\n\n"
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//usage: "Free initramfs and switch to another root fs:\n"
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//usage: "chroot to NEW_ROOT, delete all in /, move NEW_ROOT to /,\n"
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//usage: "execute NEW_INIT. PID must be 1. NEW_ROOT must be a mountpoint.\n"
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//usage: "\n -c DEV Reopen stdio to DEV after switch"
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#include <sys/vfs.h>
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#include <sys/mount.h>
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#include "libbb.h"
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// Make up for header deficiencies
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#ifndef RAMFS_MAGIC
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# define RAMFS_MAGIC ((unsigned)0x858458f6)
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#endif
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#ifndef TMPFS_MAGIC
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# define TMPFS_MAGIC ((unsigned)0x01021994)
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#endif
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#ifndef MS_MOVE
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# define MS_MOVE 8192
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#endif
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// Recursively delete contents of rootfs
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static void delete_contents(const char *directory, dev_t rootdev)
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{
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DIR *dir;
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struct dirent *d;
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struct stat st;
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// Don't descend into other filesystems
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if (lstat(directory, &st) || st.st_dev != rootdev)
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return;
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// Recursively delete the contents of directories
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if (S_ISDIR(st.st_mode)) {
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dir = opendir(directory);
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if (dir) {
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while ((d = readdir(dir))) {
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char *newdir = d->d_name;
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// Skip . and ..
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if (DOT_OR_DOTDOT(newdir))
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continue;
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// Recurse to delete contents
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newdir = concat_path_file(directory, newdir);
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delete_contents(newdir, rootdev);
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free(newdir);
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}
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closedir(dir);
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// Directory should now be empty, zap it
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rmdir(directory);
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}
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} else {
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// It wasn't a directory, zap it
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unlink(directory);
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}
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}
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int switch_root_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
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int switch_root_main(int argc UNUSED_PARAM, char **argv)
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{
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char *newroot, *console = NULL;
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struct stat st;
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struct statfs stfs;
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dev_t rootdev;
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// Parse args (-c console)
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opt_complementary = "-2"; // minimum 2 params
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getopt32(argv, "+c:", &console); // '+': stop at first non-option
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argv += optind;
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newroot = *argv++;
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// Change to new root directory and verify it's a different fs
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xchdir(newroot);
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xstat("/", &st);
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rootdev = st.st_dev;
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xstat(".", &st);
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if (st.st_dev == rootdev || getpid() != 1) {
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// Show usage, it says new root must be a mountpoint
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// and we must be PID 1
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bb_show_usage();
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}
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// Additional sanity checks: we're about to rm -rf /, so be REALLY SURE
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// we mean it. I could make this a CONFIG option, but I would get email
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// from all the people who WILL destroy their filesystems.
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if (stat("/init", &st) != 0 || !S_ISREG(st.st_mode)) {
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bb_error_msg_and_die("/init is not a regular file");
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}
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statfs("/", &stfs); // this never fails
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if ((unsigned)stfs.f_type != RAMFS_MAGIC
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&& (unsigned)stfs.f_type != TMPFS_MAGIC
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) {
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bb_error_msg_and_die("root filesystem is not ramfs/tmpfs");
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}
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// Zap everything out of rootdev
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delete_contents("/", rootdev);
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// Overmount / with newdir and chroot into it
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if (mount(".", "/", NULL, MS_MOVE, NULL)) {
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// For example, fails when newroot is not a mountpoint
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bb_perror_msg_and_die("error moving root");
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}
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xchroot(".");
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// The chdir is needed to recalculate "." and ".." links
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/*xchdir("/"); - done in xchroot */
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// If a new console specified, redirect stdin/stdout/stderr to it
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if (console) {
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close(0);
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xopen(console, O_RDWR);
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xdup2(0, 1);
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xdup2(0, 2);
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}
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// Exec real init
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execv(argv[0], argv);
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bb_perror_msg_and_die("can't execute '%s'", argv[0]);
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}
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/*
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From: Rob Landley <rob@landley.net>
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Date: Tue, Jun 16, 2009 at 7:47 PM
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Subject: Re: switch_root...
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...
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...
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...
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If you're _not_ running out of init_ramfs (if for example you're using initrd
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instead), you probably shouldn't use switch_root because it's the wrong tool.
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Basically what the sucker does is something like the following shell script:
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find / -xdev | xargs rm -rf
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cd "$1"
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shift
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mount --move . /
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exec chroot . "$@"
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There are a couple reasons that won't work as a shell script:
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1) If you delete the commands out of your $PATH, your shell scripts can't run
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more commands, but you can't start using dynamically linked _new_ commands
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until after you do the chroot because the path to the dynamic linker is wrong.
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So there's a step that needs to be sort of atomic but can't be as a shell
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script. (You can work around this with static linking or very carefully laid
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out paths and sequencing, but it's brittle, ugly, and non-obvious.)
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2) The "find | rm" bit will acually delete everything because the mount points
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still show up (even if their contents don't), and rm -rf will then happily zap
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that. So the first line is an oversimplification of what you need to do _not_
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to descend into other filesystems and delete their contents.
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The reason we do this is to free up memory, by the way. Since initramfs is a
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ramfs, deleting its contents frees up the memory it uses. (We leave it with
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one remaining dentry for the new mount point, but that's ok.)
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Note that you cannot ever umount rootfs, for approximately the same reason you
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can't kill PID 1. The kernel tracks mount points as a doubly linked list, and
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the pointer to the start/end of that list always points to an entry that's
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known to be there (rootfs), so it never has to worry about moving that pointer
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and it never has to worry about the list being empty. (Back around 2.6.13
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there _was_ a bug that let you umount rootfs, and the system locked hard the
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instant you did so endlessly looping to find the end of the mount list and
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never stopping. They fixed it.)
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Oh, and the reason we mount --move _and_ do the chroot is due to the way "/"
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works. Each process has two special symlinks, ".", and "/". Each of them
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points to the dentry of a directory, and give you a location paths can start
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from. (Historically ".." was also special, because you could enter a
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directory via a symlink so backing out to the directory you came from doesn't
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necessarily mean the one physically above where "." points to. These days I
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think it's just handed off to the filesystem.)
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Anyway, path resolution starts with "." or "/" (although the "./" at the start
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of the path may be implicit), meaning it's relative to one of those two
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directories. Your current directory, and your current root directory. The
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chdir() syscall changes where "." points to, and the chroot() syscall changes
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where "/" points to. (Again, both are per-process which is why chroot only
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affects your current process and its child processes.)
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Note that chroot() does _not_ change where "." points to, and back before they
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put crazy security checks into the kernel your current directory could be
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somewhere you could no longer access after the chroot. (The command line
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chroot does a cd as well, the chroot _syscall_ is what I'm talking about.)
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The reason mounting something new over / has no obvious effect is the same
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reason mounting something over your current directory has no obvious effect:
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the . and / links aren't recalculated after a mount, so they still point to
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the same dentry they did before, even if that dentry is no longer accessible
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by other means. Note that "cd ." is a NOP, and "chroot /" is a nop; both look
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up the cached dentry and set it right back. They don't re-parse any paths,
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because they're what all paths your process uses would be relative to.
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That's why the careful sequencing above: we cd into the new mount point before
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we do the mount --move. Moving the mount point would otherwise make it
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totally inaccessible to us because cd-ing to the old path wouldn't give it to
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us anymore, and cd "/" just gives us the cached dentry from when the process
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was created (in this case the old initramfs one). But the "." symlink gives
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us the dentry of the filesystem we just moved, so we can then "chroot ." to
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copy that dentry to "/" and get the new filesystem. If we _didn't_ save that
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dentry in "." we couldn't get it back after the mount --move.
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(Yes, this is all screwy and I had to email questions to Linus Torvalds to get
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it straight myself. I keep meaning to write up a "how mount actually works"
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document someday...)
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*/
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