[CVE-2020-16120] ovl: verify permissions in ovl_path_open()

struct file *ovl_path_open(struct path *path, int flags)

{

	return dentry_open(path, flags | O_NOATIME, current_cred());

	struct inode *inode = d_inode(path->dentry);

	int err, acc_mode;

	if (flags & ~(O_ACCMODE | O_LARGEFILE))

		BUG();

	switch (flags & O_ACCMODE) {

	case O_RDONLY:

		acc_mode = MAY_READ;

		break;

	case O_WRONLY:

		acc_mode = MAY_WRITE;

		break;

	default:

		BUG();

	}

	err = inode_permission(inode, acc_mode | MAY_OPEN);

	if (err)

		return ERR_PTR(err);

	/* O_NOATIME is an optimization, don't fail if not permitted */

	if (inode_owner_or_capable(inode))

		flags |= O_NOATIME;

	return dentry_open(path, flags, current_cred());

}

/* Caller should hold ovl_inode->lock */