Symbolic Execution for (Almost) Free: Hijacking an Existing Implementation to Perform Symbolic Execution

Symbolic execution of a language is traditionally achieved by replacing the language’s interpreter with an entirely new interpreter. This may be an unnecessary burden, and it is tempting instead to try to use as much of the existing interpret infrastructure as possible, both for handling aspects of the computation that are not symbolic, and for propagating symbolic ones. This approach was used to implement Rubicon, a bounded verification system for Ruby on Rails web applications, in less than 1000 lines of Ruby code. Rubicon uses symbolic execution to derive verification conditions from Rails applications and an off-theshelf solver to check them. Despite its small size, Rubicon has been used to find previously unknown bugs in open-source Rails applications. The key idea is to encode symbolic values and operations in a library written in the target language itself, overriding only a small part of the standard interpreter. We formalize this approach, showing that replacing a few key operators with symbolic versions in a standard interpreter gives the same effect as replacing the entire interpreter with a symbolic one.