Beyond reachability: Shape abstraction in the presence of pointer arithmetic

Cristiano Calcagno, Dino Distefano, Peter W. O'Hearn, Hongseok Yang
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Read: 24 April 2021

Static Analysis
Springer Berlin Heidelberg
Berlin, Heidelberg
Pages 182-203
Note(s): separation logic, abstract interpretation

This paper combines separation logic, abstract interpretation and some custom abstract domains/transformations to show how separation logic can be used to automatically verify low-level memory manipulation code of the kind found in memory allocators.

They describe the verification of seven allocators (3 pairs of malloc-free implementations plus another) that use acyclic or cyclic free lists and that do/don’t coalesce adjacent free blocks. Their tools runs in time varying from a small fraction of a second to 9 minutes (for “malloc_K&R”).

The technique revolves around abstract interpretation using

  • a carefully chosen subset of separation logic terms as an abstract domain extended with predicates for describing the kinds of memory shapes found in memory allocators

  • an abstraction function that is specifically designed to handle
    1. the type of memory shapes found in memory allocators (i.e., blocks of memory with all the metadata in the first few words)
    2. the types of reasoning found in memory allocators (i.e., merging adjacent blocks of memory and splitting large blocks of memory)
  • a magic constant “n” (chosen as 4 in this paper) that captures the maximum offset in objects that they need to reason about

The core of the paper is really about going back and forth between separation logic, logical reasoning, etc. and abstract interpretation, widening operators, etc.: choosing particular representations or abstraction operators based on the impact on abstract interpretation and justifying / relating that choice to separation logic reasoning steps.

[Aside: I saw a talk by Josh Berdine (a frequent collaborator with the paper authors) at POPL 2020 about a new separation logic based tool where he moved continuously between the two modes. IIRC, every slide contained separation logic judgements but most of the words he uttered were about this operational, abstract-interpretation-based interpretation of the logic.]

Although the choice of abstract domains, etc. enables automatic reasoning about the programs of interest, it is not clear how well the particular choices that they make would work on other code. It seems that this is a demonstration of the potential of this combination of spatial reasoning and abstract interpretation rather than a general purpose tool. (See [calcagno:popl:2009] for a somewhat more general tool that builds on what was learned in this and other papers from the time.)

Separation logic