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.)

include/applets.h

@@ -625,6 +625,9 @@
 #ifdef CONFIG_SWAPONOFF
 	APPLET(swapon, swap_on_off_main, _BB_DIR_SBIN, _BB_SUID_NEVER)
 #endif
+#ifdef CONFIG_SWITCH_ROOT
+	APPLET(switch_root, switch_root_main, _BB_DIR_SBIN, _BB_SUID_NEVER)
+#endif
 #ifdef CONFIG_SYNC
 	APPLET(sync, sync_main, _BB_DIR_BIN, _BB_SUID_NEVER)
 #endif

include/usage.h

@@ -2735,6 +2735,12 @@
 	"Options:\n" \
 	"\t-a\tStart swapping on all swap devices"
 
+#define switch_root_trivial_usage \
+	"NEW_ROOT NEW_INIT [ARGUMENTS_TO_INIT]"
+#define switch_root_full_usage \
+	"Use from PID 1 under initramfs to free initramfs, chroot to NEW_ROOT,\n" \
+	"and exec NEW_INIT.\n"
+
 #define sync_trivial_usage \
 	""
 #define sync_full_usage \

util-linux/Config.in

@@ -288,6 +288,28 @@ config CONFIG_PIVOT_ROOT
 	  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

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]);
+}