1 files changed, 122 insertions, 0 deletions

diff --git a/util-linux/switch_root.c b/util-linux/switch_root.c
new file mode 100644
index 000000000..4c23f69da
--- /dev/null
+++ b/util-linux/switch_root.c
@@ -0,0 +1,122 @@
+/* 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 GPL version 2, see file LICENSE in this tarball for details.
+ */
+
+#include "busybox.h"
+#include <sys/vfs.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 (lstat(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.
+                        rmdir(directory);
+                }
+
+        // It wasn't a directory.  Zap it.
+
+        } else unlink(directory);
+}
+
+int switch_root_main(int argc, char *argv[])
+{
+        char *newroot, *console=NULL;
+        struct stat st1, st2;
+        struct statfs stfs;
+
+        // Parse args (-c console)
+
+        opt_complementary = "-2";
+        getopt32(argc, argv, "c:", &console);
+
+        // Change to new root directory and verify it's a different fs.
+
+        newroot=argv[optind++];
+
+        if (chdir(newroot) || lstat(".", &st1) || lstat("/", &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 (lstat("/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 and chroot into it.  The chdir is needed to
+        // recalculate "." and ".." links.
+
+        if (mount(".", "/", NULL, MS_MOVE, NULL) || chroot(".") || chdir("/"))
+                bb_error_msg_and_die("moving root");
+
+        // If a new console specified, redirect stdin/stdout/stderr to that.
+
+        if (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);
+        bb_error_msg_and_die("bad init '%s'", argv[optind]);
+}