Add a switch_root utility (like kconfig's utils/run_init.c, although not
actuall using any of that code). This is needed because pivot_root doesn't
work right under initramfs. (See the menuconfig help.)
diff --git a/util-linux/Config.in b/util-linux/Config.in
index dc6d8fd..fe71dac 100644
--- a/util-linux/Config.in
+++ b/util-linux/Config.in
@@ -288,6 +288,28 @@
of wild and crazy things with your Linux system and is far more
powerful than 'chroot'.
+ Note: This is for initrd in linux 2.4. Under initramfs (introduced
+ in linux 2.6) use switch_root instead.
+
+config CONFIG_SWITCH_ROOT
+ bool "switch_root"
+ default n
+ help
+ The switch_root utility is used from initramfs to select a new
+ root device. Under initramfs, you have to use this instead of
+ pivot_root. (Stop reading here if you don't care why.)
+
+ Booting with initramfs extracts a gzipped cpio archive into rootfs
+ (which is a variant of ramfs/tmpfs). Because rootfs can't be moved
+ or unmounted*, pivot_root will not work from initramfs. Instead,
+ switch_root deletes everything out of rootfs (including itself),
+ does a mount --move that overmounts rootfs with the new root, and
+ then execs the specified init program.
+
+ * Because the Linux kernel uses rootfs internally as the starting
+ and ending point for searching through the kernel's doubly linked
+ list of active mount points. That's why.
+
config CONFIG_RDATE
bool "rdate"
default n
diff --git a/util-linux/switch_root.c b/util-linux/switch_root.c
new file mode 100644
index 0000000..2426995
--- /dev/null
+++ b/util-linux/switch_root.c
@@ -0,0 +1,118 @@
+/* vi:set ts=4:*/
+
+#include <dirent.h>
+#include <fcntl.h>
+#include <stdio.h>
+#include <sys/mount.h>
+#include <sys/stat.h>
+#include <sys/types.h>
+#include <sys/vfs.h>
+#include <unistd.h>
+
+#include "busybox.h"
+
+// Make up for header deficiencies.
+
+#ifndef RAMFS_MAGIC
+#define RAMFS_MAGIC 0x858458f6
+#endif
+
+#ifndef TMPFS_MAGIC
+#define TMPFS_MAGIC 0x01021994
+#endif
+
+#ifndef MS_MOVE
+#define MS_MOVE 8192
+#endif
+
+dev_t rootdev;
+
+// Recursively delete contents of rootfs.
+
+static void delete_contents(char *directory)
+{
+ DIR *dir;
+ struct dirent *d;
+ struct stat st;
+
+ // Don't descend into other filesystems
+ if (stat(directory,&st) || st.st_dev != rootdev) return;
+
+ // Recursively delete the contents of directories.
+ if (S_ISDIR(st.st_mode)) {
+ if((dir = opendir(directory))) {
+ while ((d = readdir(dir))) {
+ char *newdir=d->d_name;
+
+ // Skip . and ..
+ if(*newdir=='.' && (!newdir[1] || (newdir[1]=='.' && !newdir[2])))
+ 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);
+ }
+ closedir(dir);
+
+ // Directory should now be empty. Zap it.
+ printf("rmdir %s\n",directory); // rmdir(directory);
+ }
+
+ // It wasn't a directory. Zap it.
+
+ } else printf("unlink %s\n",directory); //unlink(directory);
+}
+
+int switch_root_main(int argc, char *argv[])
+{
+ char *newroot, *console="/dev/console";
+ struct stat st1, st2;
+ struct statfs stfs;
+
+ // Parse args (-c console)
+
+ bb_opt_complementally="-2";
+ bb_getopt_ulflags(argc,argv,"c:",&console);
+
+ // Change to new root directory and verify it's a different fs.
+
+ newroot=argv[optind++];
+
+ if (chdir(newroot) || stat(".", &st1) || stat("/", &st2) ||
+ st1.st_dev == st2.st_dev)
+ {
+ bb_error_msg_and_die("bad newroot %s",newroot);
+ }
+ 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 (stat("/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");
+ }
+
+ // Zap everything out of rootdev
+ delete_contents("/");
+
+ // Overmount / with newdir
+
+ if (mount(".", "/", NULL, MS_MOVE, NULL) || chdir("/"))
+ bb_error_msg_and_die("moving root");
+
+ // Reopen stdin/stdout/stderr to /dev/console
+ close(0);
+ if(open(console, O_RDWR) < 0)
+ bb_error_msg_and_die("Bad console '%s'",console);
+ dup2(0, 1);
+ dup2(0, 2);
+
+ // Exec real init. (This is why we must be pid 1.)
+ execv(argv[optind],argv+optind+1);
+ bb_error_msg_and_die("Bad init '%s'",argv[optind]);
+}
