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gtirb-semantics

Introduction

This codebase serves to tie several verification tools together. The GTIRB intermediate representation produced by the Datalog Disassembler is deserialised using Google Protocol Buffers. This is then dismantled and the ASLi ASL Interpreter is used to add instruction semantics for each instruction opcode. These are then reserialised back into the original IR protobufs alongside the original data produced by DDisasm (.gts file). The semantic information itself is also printed to stdout.

Requirements

To build and run this, you will need:

  • ASLp — following its readme, install dependencies then follow the steps for use in other projects.
  • ddisasm — installing via the Nix package is recommended.
  • protoc — installed by the system package manager.
  • various OCaml libraries — install these with opam install --deps-only ./*.opam
  • Scala / SBT (optional) — only needed for the "retrieve" tool, install with Coursier (see also: BASIL docs).

As a reference for these steps on Ubuntu 20.04, you may see scripts/build-all.sh (however, using this script directly is not advised).

Usage

dune exec gtirb_semantics [INPUT GTIRB FILE] [OUTPUT GTS FILE]

GTIRB files can be obtained with:

ddisasm ./a.out --ir a.gtirb

GTIRB Specifics

The serialised output is almost identical to that produced by ddisasm except with a few differences:

  • The file does not have the 8-byte magic prefix used by GTIRB.
  • The semantic information provided by ASLi has been added as an auxdata record for each compilation module. The semantic information JSON data is structured as so: 
    {
    uuid : [
        [
            opcode_0_semantics
        ],
        [
            opcode_1_semantics
        ],
        ...
    ]
}
    Where uuid is the base64 string of a UUID corresponding to a code block within the GTIRB structure. Each opcode_n_semantics are readable strings of the ASL AST.

Use with other tools

Some boilerplate Scala code has been provided in extras/retrieve. This minimal solution deserialises a .gts file and retrieves the IPCFG, text sections for each module, and semantic information for each module.

A GTIRB spelunking tool has been provided in extras/spelunking. It is runnable with python3 spelunk.py gtirb_file search_key where search_key is any of the below:

Search Key Target
cfg Interprocess control flow graph for the entire binary
code Code blocks from each compilation module
data Data blocks from each compilation module
functions Function blocks from each compilation module's auxdata
instrs Dumps instructions as they appear in each texct section, may be endian-inverted
symbols Symbols from each compilation module
texts Text sections from each compilation module

This has been provided to make it easier to extract relevant information from the GTIRB IR when developing future tools. Alternatively, see the --json option in ddisasm for producing a readable JSON representation of the GTIRB IR, although this will be extremely verbose.

This tool is easily extendable to accommodate any increased spelunking needs in the future. It is important to note that the spelunker will not recognise .gts files due to the structural differences.

scripts/debug-gts.py is a tool for converting the .gts into a human-readable JSON format, with instruction names and opcode alongside each block of semantics.

scripts/proto-json.py converts to/from GTIRB/gts and a JSON format. This can be useful for exploring the GTIRB output with tools such as jq.

Disassembly Pipeline

An example pipeline of disassembly -> instruction lifting -> semantic info -> compression -> serialisation -> deserialisation -> decompression is located in scripts/pipeline.sh. This will disassemble an example ARM64 binary and produce:

  • The initial GTIRB IR.
  • The GTIRB IR with compressed semantic information.
  • The raw semantic information without context.
  • The semantic information in the aforementioned JSON structure.

These will be located in example.gtirb, example.gts, example.ast and example.ast.json respectively within extras/example-bin. This can be run from the scripts subdirectory with ./fulltest.sh. This script assumes the directory layout as produced by build-all.sh. As such, if the scala code located in extras/retrieve is modified, then it must be copied to a directory basli adjacent to gtirb-semantics, as per the layout created by scripts/build-all.sh in order to avoid modifying fulltest.sh itself.