README

Introduction

This repository contains work relating to an ongoing project (under the aegis of the wider REMS project) to formalise the linking of ELF object code, and related notions. Toward this end, this repository contains:

• A formalisation of the core ELF file format, the de facto standard executable and linkable file format on Linux and related systems, written in Lem. This formalisation has been tested against approximately 5,000 ELF binaries found "in the wild" on various different platforms.
• A partial formalisation of various aspects of the platform Application Binary Interfaces for AMD64, Power64, AArch64, and X86-32, as well as a partial formalisation of the GNU extensions for ELF that the Linux operating system expects. These formalisations have been formalised on an ad hoc basis, as they were needed and may (almost certainly will be) incomplete.
• An executable linker/link-checker built atop the aforementioned ELF and ABI formalisations for AMD64, capable of linking complex link-jobs such as bzip2, derived from an OCaml extraction of the Lem models above.
• A sample proof of correctness for AMD64 relocation, using an Isabelle/HOL extraction of the linker, ELF model, and ABI formalisations mentioned above.

The ELF formalisation (and parts of the ABI formalisation) are currently also being used as a subcomponent of the ppcmem2 architectural exploration tool, for parsing ELF files and setting up initial machine states.

Contributors

The main contributors to the formalisation are: Stephen Kell, Dominic Mulligan and Peter Sewell.

Current clients of the formalisation are: Shaked Flur, Kathy Gray, Christopher Pulte, and Susmit Sarkar, authors of the ppcmem2 tool.

This work is funded by REMS: Rigorous Engineering of Mainstream Systems, EPSRC Programme Grant EP/K008528/1.

Directory structure

The top-level directory structure is as follows:

• 'auto_generated' contains proof assistant theories automatically extracted from the base models by the Lem tool. Note that these theories are extracted on an ad hoc basis and are not guaranteed to be in lock-step with the latest specification from which they are derived.
• 'doc' contains various specifications, standards documents and/or other useful documents necessary for the formalisation.
• 'src' contains the latest formalisation. It is further divided into the following subdirectories:
  • 'abis' contains Lem code specific to the formalisation of Application Binary Interfaces. Each ABI considered has a further dedicated subdirectory below 'abis'.
  • 'adaptors' contains Lem code providing a simplified interface to various external tools that rely on the formalisation in 'src'. For example, the file src/adaptors/sail_interface.lem presents a simplified interface for extracting executable process information to the ppcmem2 project.
• 'prf' contains a copy of Fox's X64 HOL4 specification, and the sample Isabelle proof for AMD64 relocation.
• 'talks' contains the LaTeX source code of talks and other publicity material relating to the linking formalisation.
• 'test' contains example ELF binaries for multiple platforms/ABIs to test the various formalisations against.
• 'validation' contains the validation scripts used for mass-validation of the ELF and ABI specifications against existing ELF binaries.

Build process

To build the model from a fresh git checkout (assuming your current working directory is linksem), cd into src and type make.

This will build the Lem model (extracting OCaml files) and build these files with the OCaml compiler.

Known issues

1. Currently a bug in Lem prevents 'error.lem' from being extracted correctly to OCaml. Therefore any changes to 'error.lem' require special attention, namely hand editing 'error.ml', the Lem generated ML file, to add parentheses around the infix monadic bind operator '>>=' to avoid an OCaml parse error.
2. Another bug in Lem prevents us from writing large numeric constants in their most natural form. Rather, any constant that exceeds the range of the OCaml 'int' type must be split into smaller constants that are added or multiplied together, due to the Lem parser using 'int' throughout. Where this happens the total of the computation (i.e. the constant's value) is noted in a comment.
3. Isabelle extractions of the Lem models require a small amount of hand editing to get Isabelle to accept them, due to various bugs in Lem.

All other issues, infelicities or missing pieces of formalisation should be noted in a camldoc-style comment at the top of the relevant Lem file.