Verification of Programs Sensitive to Heap Layout

Abstract

Most C and C++ programs use dynamically allocated memory (often known as a heap) to store and organize their data. In practice, it can be useful to compare addresses of different heap objects, for instance, to store them in a binary search tree or a sorted array. However, comparisons of pointers to distinct objects are inherently ambiguous: The address order of two objects can be reversed in different executions of the same program, due to the nature of the allocation algorithm and other external factors. This poses a significant challenge to program verification, since a sound verifier must consider all possible behaviors of a program, including an arbitrary reordering of the heap. A naive verification of all possibilities, of course, leads to a combinatorial explosion of the state space: For this reason, we propose an under-approximating abstract domain that can be soundly refined to consider all relevant heap orderings. We have implemented the proposed abstract domain and evaluated it against several existing software verification tools on a collection of pointer-manipulating programs. In many cases, existing tools only consider a single fixed heap order, which is a source of unsoundness. We demonstrate that using our abstract domain, this unsoundness can be repaired at only a very modest performance cost. Additionally, we show that, even though many verifiers ignore it, ambiguous behavior is present in a considerable fraction of programs from software verification competition (sv-comp).