Instrumenting JVMs at the machine code level

Abstract — Adding small code snippets at key points to existing code fragments is called instrumentation. It is an established technique to debug certain otherwise hard to solve faults, such as memory management issues and data races. Dynamic instru-mentation can already be used to analyse code which is loaded or even generated at run time. With the advent of environments such as the Java Virtual Machine with optimizing Just-In-Time compilers, a new obstacle arises: self-modifying code. In order to instrument this kind of code correctly, one must be able to detect modifications and adapt the instrumentation code accordingly. In this paper we propose an innovative technique that uses the hardware page protection mechanism of modern processors to detect such modifications. We also show how an instrumentor can adapt the instrumented version depending on the kind of modificiations as well as an experimental evaluation of said techniques. I