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PhD Thesis in Computer Science

The final version of the manuscript is available here.

The defence took place online on March 30th, 2021 in front of a jury of:

Advisor | Ralf Treinen | Professeur | Université de Paris |

Advisor | Yann Régis-Gianas | Maître de Conférences | Université de Paris |

Reviewer | Andreas Podelski | Professor | University of Freiburg |

Reviewer | Stéphane Demri | Directeur de Recherche | CNRS |

Examiner | Julia Lawall | Directrice de Recherche | Inria |

Examiner | Roberto Di Cosmo | Professeur | Université de Paris |

Examiner | Michael Greenberg | Assistant Professor | Pomona College |

A replay of the presentation is available here [MQ;LQ]. The slides are available here.

This thesis aims at applying techniques from deductive program verification and analysis of tree transformations to the problem of analysing Shell scripts. In particular, we aim at analysing Shell scripts that are used in software installation in the Debian GNU/Linux distribution. The final goal is to build a proof-of-concept tool able to read Debian packages – the way Debian has to distribute software – and report on their quality and on the potential bugs they might have.

Shell is a scripting language providing control structures around Unix utility calls. Unix utilities are objects that can perform all kind of transformation on Unix filesystems. We model Unix filesystems using feature trees and transformations of Unix filesystems using formulas in a feature tree logic named FTS. We describe these modelisations extensively and discuss their validity. The control structures of Shell scripts are converted to control structures in an intermediary language that has clearly defined semantics. This involves the definition of this intermediary language, the design of a static parser for Shell scripts and of a conversion that respects the semantics of both languages. The semantics of Shell scripts is then computed using symbolic execution of the aforementioned intermediary language, using a database of specifications of Unix utility calls as formulas of FTS. The result is, for each potential trace of execution of a Shell script, a formula of FTS describing the filesystem transformation this trace performs.

The main part of the thesis then focuses on decidability of formulas of FTS. The goal is to be able to detect traces of execution of Shell scripts that cannot happen and to check properties on the Shell scripts, such as “if the script fails, then it must not have performed any transformation”. A first, theoretical, part aims at showing that the full first-order theory of FTS is decidable. This goes by first reasoning only on Σ₁-formulas of FTS and defining a system of rules R₁ that transforms Σ₁-formulas. We show that we can use R₁ to decide the satisfiability of Σ₁-formulas as well as some other properties. We then extend the reasoning from Σ₁-formulas to first-order formulas of FTS using properties of R₁ and weak quantifier eliminations. We conclude by stating that the first-order theory of FTS is indeed decidable. A second, more practical, part aims at designing efficient decision procedures for a subset of FTS rich enough to express the semantics of Unix utilities and Shell scripts. This goes by focusing on conjunctive formulas and improving on R₁. This results in a system R₂ which is more efficient on conjunctive formulas but would not have the required properties to prove decidability of the first-order. We then show how R₂ can be implemented efficiently and that it can be extended without loss of efficiency to support specific forms of Σ₁-formulas.

Finally, this thesis describes the applications of the theoretical work to the implementation of a toolchain able to analyse all software packages in the Debian distribution and report on them. We describe our analysis and the bugs that we have found during the whole project. This thesis takes place within the CoLiS project, ANR-15-CE25-0001, taking place from October 2015 to March 2021.