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