1 Tracking Bad Apples : Reporting the Origin of Null and Undefined Value Errors ∗

Programs sometimes crash due to unusable values, for example, when Java and C# programs dereference null pointers and when C and C++ programs use undefined values to affect program behavior. A stack trace produced on such a crash identifies the effect of the unusable value, not its cause, and is often not much help to the programmer. This paper presents efficient origin tracking of unusable values; it shows how to record where these values come into existence, correctly propagate them, and report them if they cause an error. The key idea is value piggybacking: when the original program stores an unusable value, value piggybacking instead stores origin information in the spare bits of the unusable value. Modest compiler support alters the program to propagate these modified values through operations such as assignments and comparisons. We evaluate two implementations: the first tracks null pointer origins in a JVM, and the second tracks undefined value origins in a memorychecking tool built with Valgrind. These implementations show that origin tracking via value piggybacking is fast and often useful, and in the Java case, has low enough overhead for use in a production environment. ∗ This work is supported by an Intel fellowship, NSF CCF-0429859, NSF CCR-0311829, NSF EIA-0303609, DARPA F33615-03-C-4106, Intel, IBM, and Microsoft. Any opinions, findings and conclusions expressed herein are the authors’ and do not necessarily reflect those of the sponsors. Permission to make digital or hard copies of all or part of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and the full citation on the first page. To copy otherwise, to republish, to post on servers or to redistribute to lists, requires prior specific permission and/or a fee. OOPSLA’07, October 21–25, 2007, Montréal, Québec, Canada. Copyright c © 2007 ACM 978-1-59593-786-5/07/0010. . . $5.00