Expositor: Scriptable Time-Travel Debugging with First Class Traces

We present Expositor, a new debugging environment that combines scripting and time-travel debugging to allow programmers to automate complex debugging tasks. The fundamental abstraction provided by Expositor is the execution trace, which is a time-indexed sequence of program state snapshots. Programmers can manipulate traces as if they were simple lists with operations such as map and filter. Under the hood, Expositor efficiently implements traces as lazy, sparse interval trees whose contents are materialized on demand. Expositor also provides a novel data structure, the edit hash array mapped trie, which is a lazy implementation of sets, maps, multisets, and multimaps that enables programmers to maximize the efficiency of their debugging scripts. We have used Expositor to debug a stack overflow and to unravel a subtle data race in Firefox. We believe that Expositor represents an important step forward in improving the technology for diagnosing complex, hard-to-understand bugs.

This is joint work with Yit Phang Khoo and Jeffrey S. Foster.

Michael W. Hicks is a professor in the Computer Science department at the University of Maryland, College Park, and is affiliated with the Maryland Cybersecurity Center (MC2). His research focuses on using programming languages and analyses to improve the security, reliability, and availability of software. Noteworthy among his research accomplishments is the development of analysis and compilation tools for enabling software to be safely updated while it runs. He has explored the design of new programming languages and analysis tools for automatically discovering or remediating software flaws and security vulnerabilities. He has recently been exploring new approaches to privacy preserving computation, and on means to debug programs more effectively.