[T106][ZXW-22]7520V3SCV2.01.01.02P42U09_VEC_V0.8_AP_VEC origin source commit
Change-Id: Ic6e05d89ecd62fc34f82b23dcf306c93764aec4b
diff --git a/ap/app/busybox/src/util-linux/switch_root.c b/ap/app/busybox/src/util-linux/switch_root.c
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+++ b/ap/app/busybox/src/util-linux/switch_root.c
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+/* 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("/"); - done in xchroot */
+
+ // 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...)
+*/