/* vi: set sw=4 ts=4: */ /* Copyright 2005 Rob Landley * * Switch from rootfs to another filesystem as the root of the mount tree. * * Licensed under GPLv2, see file LICENSE in this source tree. */ //config:config SWITCH_ROOT //config: bool "switch_root (5.2 kb)" //config: default y //config: select PLATFORM_LINUX //config: help //config: The switch_root utility is used from initramfs to select a new //config: root device. Under initramfs, you have to use this instead of //config: pivot_root. (Stop reading here if you don't care why.) //config: //config: Booting with initramfs extracts a gzipped cpio archive into rootfs //config: (which is a variant of ramfs/tmpfs). Because rootfs can't be moved //config: or unmounted*, pivot_root will not work from initramfs. Instead, //config: switch_root deletes everything out of rootfs (including itself), //config: does a mount --move that overmounts rootfs with the new root, and //config: then execs the specified init program. //config: //config: * Because the Linux kernel uses rootfs internally as the starting //config: and ending point for searching through the kernel's doubly linked //config: list of active mount points. That's why. //applet:IF_SWITCH_ROOT(APPLET(switch_root, BB_DIR_SBIN, BB_SUID_DROP)) // APPLET_ODDNAME:name main location suid_type help //applet:IF_RUN_INIT( APPLET_ODDNAME(run-init, switch_root, BB_DIR_SBIN, BB_SUID_DROP, run_init)) //kbuild:lib-$(CONFIG_SWITCH_ROOT) += switch_root.o //kbuild:lib-$(CONFIG_RUN_INIT) += switch_root.o //usage:#define switch_root_trivial_usage //usage: "[-c CONSOLE_DEV] 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" //usage:#define run_init_trivial_usage //usage: "[-d CAP,CAP...] [-c CONSOLE_DEV] NEW_ROOT NEW_INIT [ARGS]" //usage:#define run_init_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" //usage: "\n -d CAPS Drop capabilities" #include #include #if ENABLE_RUN_INIT # include # include // #include // This header is in libcap, but the functions are in libc. // Comment in the header says this above capset/capget: /* system calls - look to libc for function to system call mapping */ extern int capset(cap_user_header_t header, cap_user_data_t data); extern int capget(cap_user_header_t header, const cap_user_data_t data); // so for bbox, let's just repeat the declarations. // This way, libcap needs not be installed in build environment. #endif #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); } } #if ENABLE_RUN_INIT DEFINE_STRUCT_CAPS; static void drop_capset(int cap_idx) { struct caps caps; /* Get the current capability mask */ getcaps(&caps); /* Drop the bit */ caps.data[CAP_TO_INDEX(cap_idx)].inheritable &= ~CAP_TO_MASK(cap_idx); /* And drop the capability. */ if (capset(&caps.header, caps.data) != 0) bb_perror_msg_and_die("capset"); } static void drop_bounding_set(int cap_idx) { int ret; ret = prctl(PR_CAPBSET_READ, cap_idx, 0, 0, 0); if (ret < 0) bb_perror_msg_and_die("prctl: %s", "PR_CAPBSET_READ"); if (ret == 1) { ret = prctl(PR_CAPBSET_DROP, cap_idx, 0, 0, 0); if (ret != 0) bb_perror_msg_and_die("prctl: %s", "PR_CAPBSET_DROP"); } } static void drop_usermodehelper(const char *filename, int cap_idx) { unsigned lo, hi; char buf[sizeof(int)*3 * 2 + 8]; int fd; int ret; ret = open_read_close(filename, buf, sizeof(buf) - 1); if (ret < 0) return; /* assuming files do not exist */ buf[ret] = '\0'; ret = sscanf(buf, "%u %u", &lo, &hi); if (ret != 2) bb_perror_msg_and_die("can't parse file '%s'", filename); if (cap_idx < 32) lo &= ~(1 << cap_idx); else hi &= ~(1 << (cap_idx - 32)); fd = xopen(filename, O_WRONLY); fdprintf(fd, "%u %u", lo, hi); close(fd); } static void drop_capabilities(char *string) { char *cap; cap = strtok(string, ","); while (cap) { unsigned cap_idx; cap_idx = cap_name_to_number(cap); drop_usermodehelper("/proc/sys/kernel/usermodehelper/bset", cap_idx); drop_usermodehelper("/proc/sys/kernel/usermodehelper/inheritable", cap_idx); drop_bounding_set(cap_idx); drop_capset(cap_idx); bb_error_msg("dropped capability: %s", cap); cap = strtok(NULL, ","); } } #endif 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). '+': stop at first non-option if (ENABLE_SWITCH_ROOT && (!ENABLE_RUN_INIT || applet_name[0] == 's')) { getopt32(argv, "^+" "c:" "\0" "-2" /* minimum 2 args */, &console ); } else { #if ENABLE_RUN_INIT char *cap_list = NULL; getopt32(argv, "^+" "c:d:" "\0" "-2" /* minimum 2 args */, &console, &cap_list ); if (cap_list) drop_capabilities(cap_list); #endif } 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("/"); - done in xchroot */ // If a new console specified, redirect stdin/stdout/stderr to it if (console) { int fd = open_or_warn(console, O_RDWR); if (fd >= 0) { xmove_fd(fd, 0); 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 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 actually 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 us 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...) */