100% chametz sh’avar ha-Pesach is forbidden for use. What about chametz that’s only “chometz m’drabbanan”?
The Shulchan Aruch (end of 447) writes that for chametz derabanan there is no prohibition on deriving benefit after Pesach.
The Magen Avraham adds that it is likewise permitted in eating.
However, the Rivash, which is the source of the halachah, writes explicitly that it is forbidden to eat the chametz, and this is clearly how the Rivash understood the Ran (the original source). The Chok Yaakov points out the difficulty of the Magen Avraham.
Therefore, in view of the explicit ruling of the Rivah (siman 3) one should in practice be stringent for eating the chametz, though it is permitted in deriving benefit.