def get_role_name(session, role_arn):
    """
    Get function role name based on role_arn
    
    Parameters
    ----------
    session : :class:`Session`
        AWS Boto3 Session. See :class:`Session` for creation.
    role_arn : str
        Role arn
    
    Returns
    -------
    str
        Role name.
        Returns None if no role name is found for role arn.
    """
    if role_arn is None:
        return None
    iam = session.client('iam')
    roles = iam.list_roles()
    role = [role for role in roles['Roles'] if role['Arn'] == role_arn]
    role_name = role[0] if len(role) else None
    return role_name
