1 /* vi: set sw=4 ts=4: */
2 /* Copyright 2005 Rob Landley <rob@landley.net>
4 * Switch from rootfs to another filesystem as the root of the mount tree.
6 * Licensed under GPLv2, see file LICENSE in this source tree.
8 //config:config SWITCH_ROOT
9 //config: bool "switch_root (5.2 kb)"
11 //config: select PLATFORM_LINUX
13 //config: The switch_root utility is used from initramfs to select a new
14 //config: root device. Under initramfs, you have to use this instead of
15 //config: pivot_root. (Stop reading here if you don't care why.)
17 //config: Booting with initramfs extracts a gzipped cpio archive into rootfs
18 //config: (which is a variant of ramfs/tmpfs). Because rootfs can't be moved
19 //config: or unmounted*, pivot_root will not work from initramfs. Instead,
20 //config: switch_root deletes everything out of rootfs (including itself),
21 //config: does a mount --move that overmounts rootfs with the new root, and
22 //config: then execs the specified init program.
24 //config: * Because the Linux kernel uses rootfs internally as the starting
25 //config: and ending point for searching through the kernel's doubly linked
26 //config: list of active mount points. That's why.
28 // RUN_INIT config item is in klibc-utils
30 //applet:IF_SWITCH_ROOT(APPLET(switch_root, BB_DIR_SBIN, BB_SUID_DROP))
31 // APPLET_ODDNAME:name main location suid_type help
32 //applet:IF_RUN_INIT( APPLET_ODDNAME(run-init, switch_root, BB_DIR_SBIN, BB_SUID_DROP, run_init))
34 //kbuild:lib-$(CONFIG_SWITCH_ROOT) += switch_root.o
35 //kbuild:lib-$(CONFIG_RUN_INIT) += switch_root.o
38 #include <sys/mount.h>
40 # include <sys/prctl.h>
41 # include <linux/capability.h>
42 // #include <sys/capability.h>
43 // This header is in libcap, but the functions are in libc.
44 // Comment in the header says this above capset/capget:
45 /* system calls - look to libc for function to system call mapping */
46 extern int capset(cap_user_header_t header, cap_user_data_t data);
47 extern int capget(cap_user_header_t header, const cap_user_data_t data);
48 // so for bbox, let's just repeat the declarations.
49 // This way, libcap needs not be installed in build environment.
54 // Make up for header deficiencies
56 # define RAMFS_MAGIC ((unsigned)0x858458f6)
59 # define TMPFS_MAGIC ((unsigned)0x01021994)
65 // Recursively delete contents of rootfs
66 static void delete_contents(const char *directory, dev_t rootdev)
72 // Don't descend into other filesystems
73 if (lstat(directory, &st) || st.st_dev != rootdev)
76 // Recursively delete the contents of directories
77 if (S_ISDIR(st.st_mode)) {
78 dir = opendir(directory);
80 while ((d = readdir(dir))) {
81 char *newdir = d->d_name;
84 if (DOT_OR_DOTDOT(newdir))
87 // Recurse to delete contents
88 newdir = concat_path_file(directory, newdir);
89 delete_contents(newdir, rootdev);
94 // Directory should now be empty, zap it
98 // It wasn't a directory, zap it
106 static void drop_capset(int cap_idx)
111 caps.data[CAP_TO_INDEX(cap_idx)].inheritable &= ~CAP_TO_MASK(cap_idx);
112 if (capset(&caps.header, caps.data) != 0)
113 bb_perror_msg_and_die("capset");
116 static void drop_bounding_set(int cap_idx)
120 ret = prctl(PR_CAPBSET_READ, cap_idx, 0, 0, 0);
122 bb_perror_msg_and_die("prctl: %s", "PR_CAPBSET_READ");
125 ret = prctl(PR_CAPBSET_DROP, cap_idx, 0, 0, 0);
127 bb_perror_msg_and_die("prctl: %s", "PR_CAPBSET_DROP");
131 static void drop_usermodehelper(const char *filename, int cap_idx)
134 char buf[sizeof(int)*3 * 2 + 8];
138 ret = open_read_close(filename, buf, sizeof(buf) - 1);
140 return; /* assuming files do not exist */
143 ret = sscanf(buf, "%u %u", &lo, &hi);
145 bb_perror_msg_and_die("can't parse file '%s'", filename);
148 lo &= ~(1 << cap_idx);
150 hi &= ~(1 << (cap_idx - 32));
152 fd = xopen(filename, O_WRONLY);
153 fdprintf(fd, "%u %u", lo, hi);
157 static void drop_capabilities(char *string)
161 cap = strtok(string, ",");
165 cap_idx = cap_name_to_number(cap);
166 drop_usermodehelper("/proc/sys/kernel/usermodehelper/bset", cap_idx);
167 drop_usermodehelper("/proc/sys/kernel/usermodehelper/inheritable", cap_idx);
168 drop_bounding_set(cap_idx);
169 drop_capset(cap_idx);
170 bb_error_msg("dropped capability: %s", cap);
171 cap = strtok(NULL, ",");
176 int switch_root_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
177 int switch_root_main(int argc UNUSED_PARAM, char **argv)
179 char *newroot, *console = NULL;
182 unsigned dry_run = 0;
185 // Parse args. '+': stop at first non-option
186 if (ENABLE_SWITCH_ROOT && (!ENABLE_RUN_INIT || applet_name[0] == 's')) {
187 //usage:#define switch_root_trivial_usage
188 //usage: "[-c CONSOLE_DEV] NEW_ROOT NEW_INIT [ARGS]"
189 //usage:#define switch_root_full_usage "\n\n"
190 //usage: "Free initramfs and switch to another root fs:\n"
191 //usage: "chroot to NEW_ROOT, delete all in /, move NEW_ROOT to /,\n"
192 //usage: "execute NEW_INIT. PID must be 1. NEW_ROOT must be a mountpoint.\n"
193 //usage: "\n -c DEV Reopen stdio to DEV after switch"
196 "\0" "-2" /* minimum 2 args */,
201 //usage:#define run_init_trivial_usage
202 //usage: "[-d CAP,CAP...] [-n] [-c CONSOLE_DEV] NEW_ROOT NEW_INIT [ARGS]"
203 //usage:#define run_init_full_usage "\n\n"
204 //usage: "Free initramfs and switch to another root fs:\n"
205 //usage: "chroot to NEW_ROOT, delete all in /, move NEW_ROOT to /,\n"
206 //usage: "execute NEW_INIT. PID must be 1. NEW_ROOT must be a mountpoint.\n"
207 //usage: "\n -c DEV Reopen stdio to DEV after switch"
208 //usage: "\n -d CAPS Drop capabilities"
209 //usage: "\n -n Dry run"
210 char *cap_list = NULL;
211 dry_run = getopt32(argv, "^+"
213 "\0" "-2" /* minimum 2 args */,
219 drop_capabilities(cap_list);
225 // Change to new root directory and verify it's a different fs
230 if (st.st_dev == rootdev) {
231 // Show usage, it says new root must be a mountpoint
234 if (!dry_run && getpid() != 1) {
235 // Show usage, it says we must be PID 1
239 // Additional sanity checks: we're about to rm -rf /, so be REALLY SURE
240 // we mean it. I could make this a CONFIG option, but I would get email
241 // from all the people who WILL destroy their filesystems.
242 if (stat("/init", &st) != 0 || !S_ISREG(st.st_mode)) {
243 bb_error_msg_and_die("'%s' is not a regular file", "/init");
245 statfs("/", &stfs); // this never fails
246 if ((unsigned)stfs.f_type != RAMFS_MAGIC
247 && (unsigned)stfs.f_type != TMPFS_MAGIC
249 bb_error_msg_and_die("root filesystem is not ramfs/tmpfs");
253 // Zap everything out of rootdev
254 delete_contents("/", rootdev);
256 // Overmount / with newdir and chroot into it
257 if (mount(".", "/", NULL, MS_MOVE, NULL)) {
258 // For example, fails when newroot is not a mountpoint
259 bb_perror_msg_and_die("error moving root");
263 // The chdir is needed to recalculate "." and ".." links
264 /*xchdir("/"); - done in xchroot */
266 // If a new console specified, redirect stdin/stdout/stderr to it
268 int fd = open_or_warn(console, O_RDWR);
277 // Does NEW_INIT look like it can be executed?
278 //xstat(argv[0], &st);
279 //if (!S_ISREG(st.st_mode))
280 // bb_perror_msg_and_die("'%s' is not a regular file", argv[0]);
281 if (access(argv[0], X_OK) == 0)
285 execv(argv[0], argv);
287 bb_perror_msg_and_die("can't execute '%s'", argv[0]);
291 From: Rob Landley <rob@landley.net>
292 Date: Tue, Jun 16, 2009 at 7:47 PM
293 Subject: Re: switch_root...
299 If you're _not_ running out of init_ramfs (if for example you're using initrd
300 instead), you probably shouldn't use switch_root because it's the wrong tool.
302 Basically what the sucker does is something like the following shell script:
304 find / -xdev | xargs rm -rf
310 There are a couple reasons that won't work as a shell script:
312 1) If you delete the commands out of your $PATH, your shell scripts can't run
313 more commands, but you can't start using dynamically linked _new_ commands
314 until after you do the chroot because the path to the dynamic linker is wrong.
315 So there's a step that needs to be sort of atomic but can't be as a shell
316 script. (You can work around this with static linking or very carefully laid
317 out paths and sequencing, but it's brittle, ugly, and non-obvious.)
319 2) The "find | rm" bit will actually delete everything because the mount points
320 still show up (even if their contents don't), and rm -rf will then happily zap
321 that. So the first line is an oversimplification of what you need to do _not_
322 to descend into other filesystems and delete their contents.
324 The reason we do this is to free up memory, by the way. Since initramfs is a
325 ramfs, deleting its contents frees up the memory it uses. (We leave it with
326 one remaining dentry for the new mount point, but that's ok.)
328 Note that you cannot ever umount rootfs, for approximately the same reason you
329 can't kill PID 1. The kernel tracks mount points as a doubly linked list, and
330 the pointer to the start/end of that list always points to an entry that's
331 known to be there (rootfs), so it never has to worry about moving that pointer
332 and it never has to worry about the list being empty. (Back around 2.6.13
333 there _was_ a bug that let you umount rootfs, and the system locked hard the
334 instant you did so endlessly looping to find the end of the mount list and
335 never stopping. They fixed it.)
337 Oh, and the reason we mount --move _and_ do the chroot is due to the way "/"
338 works. Each process has two special symlinks, ".", and "/". Each of them
339 points to the dentry of a directory, and give you a location paths can start
340 from. (Historically ".." was also special, because you could enter a
341 directory via a symlink so backing out to the directory you came from doesn't
342 necessarily mean the one physically above where "." points to. These days I
343 think it's just handed off to the filesystem.)
345 Anyway, path resolution starts with "." or "/" (although the "./" at the start
346 of the path may be implicit), meaning it's relative to one of those two
347 directories. Your current directory, and your current root directory. The
348 chdir() syscall changes where "." points to, and the chroot() syscall changes
349 where "/" points to. (Again, both are per-process which is why chroot only
350 affects your current process and its child processes.)
352 Note that chroot() does _not_ change where "." points to, and back before they
353 put crazy security checks into the kernel your current directory could be
354 somewhere you could no longer access after the chroot. (The command line
355 chroot does a cd as well, the chroot _syscall_ is what I'm talking about.)
357 The reason mounting something new over / has no obvious effect is the same
358 reason mounting something over your current directory has no obvious effect:
359 the . and / links aren't recalculated after a mount, so they still point to
360 the same dentry they did before, even if that dentry is no longer accessible
361 by other means. Note that "cd ." is a NOP, and "chroot /" is a nop; both look
362 up the cached dentry and set it right back. They don't re-parse any paths,
363 because they're what all paths your process uses would be relative to.
365 That's why the careful sequencing above: we cd into the new mount point before
366 we do the mount --move. Moving the mount point would otherwise make it
367 totally inaccessible to us because cd-ing to the old path wouldn't give it to
368 us anymore, and cd "/" just gives us the cached dentry from when the process
369 was created (in this case the old initramfs one). But the "." symlink gives
370 us the dentry of the filesystem we just moved, so we can then "chroot ." to
371 copy that dentry to "/" and get the new filesystem. If we _didn't_ save that
372 dentry in "." we couldn't get it back after the mount --move.
374 (Yes, this is all screwy and I had to email questions to Linus Torvalds to get
375 it straight myself. I keep meaning to write up a "how mount actually works"