What is the ruling on placing a stone, cement, or a reed pole at the head and feet of the deceased in the grave?

The Shari'ah has forbidden building over graves and commanded their demolition, based on the Prophet's (peace and blessings be upon him) prohibition of plastering graves and building over them, and his command to Ali ibn Abi Talib to level every elevated grave. However, the Shari'ah permits placing a marker on a grave for identification, based on the Prophet's (peace and blessings be upon him) action when he placed a stone at the head of Uthman ibn Maz'un's grave to mark his brother's grave.