vfs: rename: check backing inode being equal

If a file is renamed to a hardlink of itself POSIX specifies that rename(2)
should do nothing and return success.

This condition is checked in vfs_rename().  However it won't detect hard
links on overlayfs where these are given separate inodes on the overlayfs
layer.

Overlayfs itself detects this condition and returns success without doing
anything, but then vfs_rename() will proceed as if this was a successful
rename (detach_mounts(), d_move()).

The correct thing to do is to detect this condition before even calling
into overlayfs.  This patch does this by calling vfs_select_inode() to get
the underlying inodes.

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Cc: <stable@vger.kernel.org> # v4.2+
static int walk_component(struct nameidata *nd, int flags)
{
	struct path path;
	struct inode *inode;
	unsigned seq;
	int err;
	/*
	 * "." and ".." are special - ".." especially so because it has
	 * to be able to know about the current root directory and
	 * parent relationships.
	 */
	if (unlikely(nd->last_type != LAST_NORM)) {
		err = handle_dots(nd, nd->last_type);
		if (flags & WALK_PUT)
			put_link(nd);
		return err;
	}
	err = lookup_fast(nd, &path, &inode, &seq);
	if (unlikely(err <= 0)) {
		if (err < 0)
			return err;
		path.dentry = lookup_slow(&nd->last, nd->path.dentry,
					  nd->flags);
		if (IS_ERR(path.dentry))
			return PTR_ERR(path.dentry);

		path.mnt = nd->path.mnt;
		err = follow_managed(&path, nd);
		if (unlikely(err < 0))
			return err;

		if (unlikely(d_is_negative(path.dentry))) {
			path_to_nameidata(&path, nd);
			return -ENOENT;
		}

		seq = 0;	/* we are already out of RCU mode */
		inode = d_backing_inode(path.dentry);
	}

	if (flags & WALK_PUT)
		put_link(nd);
	err = should_follow_link(nd, &path, flags & WALK_GET, inode, seq);
	if (unlikely(err))
		return err;
	path_to_nameidata(&path, nd);
	nd->inode = inode;
	nd->seq = seq;
	return 0;
}

static int walk_component(struct nameidata *nd, int flags)
{
	struct path path;
	struct inode *inode;
	unsigned seq;
	int err;
	/*
	 * "." and ".." are special - ".." especially so because it has
	 * to be able to know about the current root directory and
	 * parent relationships.
	 */
	if (unlikely(nd->last_type != LAST_NORM)) {
		err = handle_dots(nd, nd->last_type);
		if (flags & WALK_PUT)
			put_link(nd);
		return err;
	}
	err = lookup_fast(nd, &path, &inode, &seq);
	if (unlikely(err <= 0)) {
		if (err < 0)
			return err;
		path.dentry = lookup_slow(&nd->last, nd->path.dentry,
					  nd->flags);
		if (IS_ERR(path.dentry))
			return PTR_ERR(path.dentry);

		path.mnt = nd->path.mnt;
		err = follow_managed(&path, nd);
		if (unlikely(err < 0))
			return err;

		if (unlikely(d_is_negative(path.dentry))) {
			path_to_nameidata(&path, nd);
			return -ENOENT;
		}

		seq = 0;	/* we are already out of RCU mode */
		inode = d_backing_inode(path.dentry);
	}

	if (flags & WALK_PUT)
		put_link(nd);
	err = should_follow_link(nd, &path, flags & WALK_GET, inode, seq);
	if (unlikely(err))
		return err;
	path_to_nameidata(&path, nd);
	nd->inode = inode;
	nd->seq = seq;
	return 0;
}
