DynamicRoleHierarchy.php

<?php

namespace Acme\SecurityBundle\Role;

use Symfony\Component\Security\Core\Role\RoleHierarchyInterface;
use Symfony\Component\Security\Core\Role\RoleHierarchy;

use Acme\SecurityBundle\Entity\RoleRepository;

class DynamicRoleHierarchy implements RoleHierarchyInterface
{
    protected $roleRepository;
    protected $roleHierarchy = null;

    public function __construct(RoleRepository $roleRepository)
    {
        $this->roleRepository = $roleRepository;
    }

    public function getReachableRoles(array $roles)
    {
        if (null === $this->roleHierarchy) {
            $this->roleHierarchy = new RoleHierarchy($this->fetchRoleHierarchy());
        }

        return $this->roleHierarchy->getReachableRoles($roles);
    }

    protected function fetchRoleHierarchy()
    {
        $hierarchy = array();
        $this->roleRepository;
        // do your stuff with $this->roleRepository

        return $hierarchy;
    }
}

services.xml

<!-- 
Probably you should not overwrite native Symfony services.
There will be better add your DynamicRoleHierarchy to RoleHierarchyVoter via CompilerPass
-->

<?xml version="1.0" encoding="UTF-8"?>

<container xmlns="http://symfony.com/schema/dic/services"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://symfony.com/schema/dic/services http://symfony.com/schema/dic/services/services-1.0.xsd">

    <services>
        <service id="security.role_hierarchy" class="Acme\SecurityBundle\Role\DynamicRoleHierarchy" public="false">
            <argument type="service" id="your.role.repository" />
        </service>
    </services>
</container>