[PATCH] pivot_root() circular reference fix

Fix http://bugzilla.kernel.org/show_bug.cgi?id=4857

When pivot_root is called from an init script in an initramfs environment,
it causes a circular reference in the mount tree.

The cause of this is that pivot_root() is not prepared to handle pivoting
an unattached mount.  In an initramfs environment, rootfs is the root of
the namespace, and so it is not attached.

This patch fixes this and related problems, by returning -EINVAL if either
the current root or the new root is detached.
Signed-off-by: Miklos Szeredi <miklos@szeredi.hu>
Acked-by: Al Viro <viro@parcelfarce.linux.theplanet.co.uk>
Cc: <bigfish@asmallpond.org>
Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

fs/namespace.c

@@ -1334,8 +1334,12 @@ asmlinkage long sys_pivot_root(const char __user *new_root, const char __user *p
 	error = -EINVAL;
 	if (user_nd.mnt->mnt_root != user_nd.dentry)
 		goto out2; /* not a mountpoint */
+	if (user_nd.mnt->mnt_parent == user_nd.mnt)
+		goto out2; /* not attached */
 	if (new_nd.mnt->mnt_root != new_nd.dentry)
 		goto out2; /* not a mountpoint */
+	if (new_nd.mnt->mnt_parent == new_nd.mnt)
+		goto out2; /* not attached */
 	tmp = old_nd.mnt; /* make sure we can reach put_old from new_root */
 	spin_lock(&vfsmount_lock);
 	if (tmp != new_nd.mnt) {