Safe self-modifying code

Methods, systems, and apparatus, including computer programs encoded on a computer storage medium, for runtime language-independent sandboxing of software. In one aspect, a system implements an extended Software Fault Isolation (SFI) software sandboxing system configured to provide a user-mode program interface for receiving runtime requests for modifying verifiably safe executable machine code. Requests can include dynamic code creation, dynamic code deletion, and atomic modification of machine code instructions. A runtime modification of a verifiably safe executable memory region is made in response to each received runtime request, and code within the modified memory region has a guarantee of safe execution.