*
* 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.
+ * Licensed under GPLv2, see file LICENSE in this source tree.
*/
-
-#include "busybox.h"
#include <sys/vfs.h>
-
-
-// Make up for header deficiencies.
-
+#include <sys/mount.h>
+#include "libbb.h"
+// Make up for header deficiencies
#ifndef RAMFS_MAGIC
-#define RAMFS_MAGIC 0x858458f6
+# define RAMFS_MAGIC ((unsigned)0x858458f6)
#endif
-
#ifndef TMPFS_MAGIC
-#define TMPFS_MAGIC 0x01021994
+# define TMPFS_MAGIC ((unsigned)0x01021994)
#endif
-
#ifndef MS_MOVE
-#define MS_MOVE 8192
+# define MS_MOVE 8192
#endif
-dev_t rootdev;
-
-// Recursively delete contents of rootfs.
-
-static void delete_contents(const char *directory)
+// 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;
+ if (lstat(directory, &st) || st.st_dev != rootdev)
+ return;
- // Recursively delete the contents of directories.
+ // Recursively delete the contents of directories
if (S_ISDIR(st.st_mode)) {
- if((dir = opendir(directory))) {
+ dir = opendir(directory);
+ if (dir) {
while ((d = readdir(dir))) {
- char *newdir=d->d_name;
+ char *newdir = d->d_name;
// Skip . and ..
- if(*newdir=='.' && (!newdir[1] || (newdir[1]=='.' && !newdir[2])))
+ if (DOT_OR_DOTDOT(newdir))
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);
+ newdir = concat_path_file(directory, newdir);
+ delete_contents(newdir, rootdev);
+ free(newdir);
}
closedir(dir);
- // Directory should now be empty. Zap it.
+ // Directory should now be empty, zap it
rmdir(directory);
}
-
- // It wasn't a directory. Zap it.
-
- } else unlink(directory);
+ } else {
+ // It wasn't a directory, zap it
+ unlink(directory);
+ }
}
-int switch_root_main(int argc, char **argv);
-int switch_root_main(int argc, char **argv)
+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 st1, st2;
+ char *newroot, *console = NULL;
+ struct stat st;
struct statfs stfs;
+ dev_t rootdev;
// 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);
+ 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();
}
- 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 filesystemss.)
- 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");
+ // 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);
- 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("moving root");
-
- // If a new console specified, redirect stdin/stdout/stderr to that.
+ // 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);
- if(open(console, O_RDWR) < 0)
- bb_error_msg_and_die("bad console '%s'", console);
- dup2(0, 1);
- dup2(0, 2);
+ xopen(console, O_RDWR);
+ xdup2(0, 1);
+ xdup2(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]);
+ // 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...)
+*/
projects / oweals / busybox.git / blobdiff