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.)

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]);
+}