Using Valgrind to detect undefined value errors with bit-precision Julian Seward Nicholas Nethercote OpenWorks LLP Department of Computer Sciences Cambridge, UK University of Texas at Austin
[email protected] [email protected] Abstract We present Memcheck, a tool that has been implemented with the dynamic binary instrumentation framework Val- grind. Memcheck detects a wide range of memory errors 1.1 Basic operation and features in programs as they run. This paper focuses on one kind Memcheck is a dynamic analysis tool, and so checks pro- of error that Memcheck detects: undefined value errors. grams for errors as they run. Memcheck performs four Such errors are common, and often cause bugs that are kinds of memory error checking. hard to find in programs written in languages such as C, First, it tracks the addressability of every byte of mem- C++ and Fortran. Memcheck's definedness checking im- ory, updating the information as memory is allocated and proves on that of previous tools by being accurate to the freed. With this information, it can detect all accesses to level of individual bits. This accuracy gives Memcheck unaddressable memory. a low false positive and false negative rate. Second, it tracks all heap blocks allocated with The definedness checking involves shadowing every malloc(), new and new[]. With this information it bit of data in registers and memory with a second bit can detect bad or repeated frees of heap blocks, and can that indicates if the bit has a defined value. Every value- detect memory leaks at program termination. creating operation is instrumented with a shadow oper- Third, it checks that memory blocks supplied as argu- ation that propagates shadow bits appropriately.