Wed 10 Jan 2018 11:20 - 11:45 at Bunker Hill - Strings Chair(s): Zachary Tatlock

The theory of strings with concatenation has been widely argued as the basis of constraint solving for verifying string-manipulating programs. However, this theory is far from adequate for expressing many string constraints that are also needed in practice; for example, the use of regular constraints (pattern matching against a regular expression), and the string-replace function (replacing either the first occurrence or all occurrences of a pattern'' string constant/variable/regular expression by areplacement'' string constant/variable), among many others. Both regular constraints and the string-replace function are crucial for such applications as analysis of JavaScript (or more generally HTML5 applications) against cross-site scripting (XSS) vulnerabilities, which motivates us to consider a richer class of string constraints. The importance of the string-replace function (especially the replace-all facility) is increasingly recognised, which can be witnessed by the incorporation of the function in the input languages of several string constraint solvers.

Recently, it was shown that any theory of strings containing the string-replace function (even the most restricted version where pattern/replacement strings are both constant strings) becomes undecidable if we do not impose some kind of straight-line (aka acyclicity) restriction on the formulas. Despite this, the straight-line restriction is still practically sensible since this condition is
typically met by string constraints that are generated by symbolic execution. In this paper, we provide the first systematic study of straight-line string constraints with the string-replace function and the regular constraints as the basic operations. We show that a large class of such constraints (i.e. when only a constant string or a regular expression is permitted in the pattern) is decidable. We note that the string-replace function, even under this restriction, is sufficiently powerful for expressing the concatenation operator and much more (e.g. extensions of regular expressions with string variables). This gives us the most expressive decidable logic containing concatenation, replace, and regular constraints under the same umbrella. Our decision procedure for the straight-line fragment follows an automata-theoretic approach, and is modular in the sense that the string-replace terms are removed one by one to generate more and more regular constraints, which can then be discharged by the state-of-the-art string constraint solvers. We also show that this fragment is, in a way, a maximal decidable subclass of the straight-line fragment with string-replace and regular constraints. To this end, we show undecidability results for the following two extensions: (1) variables are permitted in the pattern parameter of the replace function, (2) length constraints are permitted.

Wed 10 Jan

POPL-2018-papers
10:30 - 12:10: Research Papers - Strings at Bunker Hill
Chair(s): Zachary TatlockUniversity of Washington
POPL-2018-papers10:30 - 10:55
Talk
Anders MiltnerPrinceton University, Kathleen FisherTufts University, Benjamin C. PierceUniversity of Pennsylvania, David WalkerPrinceton University, Steve ZdancewicUniversity of Pennsylvania
POPL-2018-papers10:55 - 11:20
Talk
Jeevana Priya InalaMIT, Rishabh SinghMicrosoft Research
POPL-2018-papers11:20 - 11:45
Talk
Taolue ChenBirkbeck, University of London, Yan ChenState Key Laboratory of Computer Science, Institute of Software, Chinese Academy of Sciences & University of Chinese Academy of Sciences, Matthew HagueRoyal Holloway, University of London, Anthony Widjaja LinOxford University, Zhilin WuState Key Laboratory of Computer Science, Institute of Software, Chinese Academy of Sciences
POPL-2018-papers11:45 - 12:10
Talk
Lukas HolikBrno University of Technology, Anthony Widjaja LinOxford University, Petr JankuBrno University of Technology, Philipp RuemmerUppsala University, Tomas VojnarBrno University of Technology