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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 | /* 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. */ //config:config SWITCH_ROOT //config: bool "switch_root (5.7 kb)" //config: default y //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. //config: // RUN_INIT config item is in klibc-utils //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 #include <sys/vfs.h> #include <sys/mount.h> #if ENABLE_RUN_INIT # include <sys/prctl.h> # ifndef PR_CAPBSET_READ # define PR_CAPBSET_READ 23 # endif # ifndef PR_CAPBSET_DROP # define PR_CAPBSET_DROP 24 # endif # include <linux/capability.h> // #include <sys/capability.h> // 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 static void delete_contents(const char *directory, dev_t rootdev); static int FAST_FUNC rmrf(const char *directory, struct dirent *d, void *rootdevp) { char *newdir = concat_subpath_file(directory, d->d_name); if (newdir) { // not . or .. // Recurse to delete contents delete_contents(newdir, *(dev_t*)rootdevp); free(newdir); } return 0; } // Recursively delete contents of rootfs static void delete_contents(const char *directory, dev_t rootdev) { 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)) { iterate_on_dir(directory, rmrf, &rootdev); } 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; getcaps(&caps); caps.data[CAP_TO_INDEX(cap_idx)].inheritable &= ~CAP_TO_MASK(cap_idx); if (capset(&caps.header, caps.data) != 0) bb_simple_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_r(string, ",", &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_r(NULL, ",", &string); } } #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; unsigned dry_run = 0; dev_t rootdev; // Parse args. '+': stop at first non-option if (ENABLE_SWITCH_ROOT && (!ENABLE_RUN_INIT || applet_name[0] == 's')) { //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" getopt32(argv, "^+" "c:" "\0" "-2" /* minimum 2 args */, &console ); } else { #if ENABLE_RUN_INIT //usage:#define run_init_trivial_usage //usage: "[-d CAP,CAP...] [-n] [-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" //usage: "\n -n Dry run" char *cap_list = NULL; dry_run = getopt32(argv, "^+" "c:d:n" "\0" "-2" /* minimum 2 args */, &console, &cap_list ); dry_run >>= 2; // -n 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) { // Show usage, it says new root must be a mountpoint bb_show_usage(); } if (!dry_run && getpid() != 1) { // Show usage, it says 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("'%s' is not a regular file", "/init"); } statfs("/", &stfs); // this never fails if ((unsigned)stfs.f_type != RAMFS_MAGIC && (unsigned)stfs.f_type != TMPFS_MAGIC ) { bb_simple_error_msg_and_die("root filesystem is not ramfs/tmpfs"); } if (!dry_run) { // 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_simple_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); } } if (dry_run) { // Does NEW_INIT look like it can be executed? //xstat(argv[0], &st); //if (!S_ISREG(st.st_mode)) // bb_perror_msg_and_die("'%s' is not a regular file", argv[0]); if (access(argv[0], X_OK) == 0) return 0; } else { // Exec NEW_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 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...) */ |