Wasabi

Tutorial at PLDI 2019

We offered a tutorial on how to use Wasabi for dynamically analyzing WebAssembly at PLDI 2019. Although the conference is now over, all the material is online at http://wasabi.software-lab.org/tutorial-pldi2019/. In particular, the slides are available here and the hands-on tasks are in this repo under tutorial/.

Installation from Source

• Dependencies and tools

  • Git, CMake, and GCC or Clang for building the dependencies (those for sure, but possibly more)
  • Firefox >= 52 (which is what I use, or Chrome >= 57) for running WebAssembly
  • WebAssembly Binary Toolkit (WABT): wat2wasm/wasm2wat for converting Wasm binaries to/from text, wasm-objdump for inspecting binaries, and wasm-interp for a simple interpreter. (See https://github.com/WebAssembly/wabt#cloning.)

  git clone --recursive https://github.com/WebAssembly/wabt
  cd wabt
  make
  
  # add binaries to $PATH, e.g., by appending the following line to ~/.profile or ~/.bashrc
  export PATH="path/to/your/wabt/bin:$PATH"
  
  # test
  wat2wasm
  > usage: wat2wasm [options] filename

  • Emscripten: emcc for compiling C/C++ programs to WebAssembly. (See https://emscripten.org/docs/getting_started/downloads.html.)

  git clone https://github.com/emscripten-core/emsdk.git
  cd emsdk
  ./emsdk install latest
  ./emsdk activate latest
  
  # add emcc to $PATH, e.g., by appending the following line to ~/.profile or ~/.bashrc
  # WARNING unfortunately, this also exports a quite old emscripten-included clang version,
  # so maybe do this manually before using emscripten and not for all shells. 
  source path/to/your/emsdk/emsdk_env.sh
  
  # test
  emcc --version
  > emcc (Emscripten gcc/clang-like replacement) 1.38.1

  • Rust (>=1.31.0 stable): cargo as Rust's package manager and build tool (no need to call rustc manually) and rustup for managing different Rust toolchain versions. (See https://www.rust-lang.org/tools/install.)

  curl https://sh.rustup.rs -o rustup-init.sh
  # follow instructions (typically just enter 1 to proceed)
  # should automatically change ~/.profile to include the binaries in $PATH
  sh rustup-init.sh
  
  # test
  cargo --version
  > cargo 1.31.0-stable

• Wasabi itself

git clone https://github.com/danleh/wasabi.git
cd wasabi
# download dependencies from https://crates.io, compile with optimizations, make wasabi binary available in $PATH
cargo install --path .

# test
wasabi
> Error: expected at least one argument
> Usage: wasabi <input_wasm_file> [<output_dir>]

Alternative Setup via Docker

• Thanks to ctfhacker for the Dockerfile contribution.
• After having cloned this repo, you build the Docker image with

docker build --rm -t wasabi .

• Once built, you can use the container via (assuming you have a hello.wasm file in your working directory)

ls
> hello.wasm
docker run --rm -t -v `pwd`:/data  wasabi /data/hello.wasm /data

Usage Tutorial

• Create WebAssembly programs

  • Manually:

  ;; paste into hello-manual.wat
  (module
    (import "host" "print" (func $i (param i32)))
    (func $somefun
      i32.const 42
      call $i)
    (export "somefun" (func $somefun))
  )

  # assemble binary Wasm file
  wat2wasm hello-manual.wat
  
  # run binary (imported function host.print is provided by the interpreter)
  wasm-interp --host-print --run-all-exports hello-manual.wasm
  > called host host.print(i32:42) =>
  > somefun() =>

  • From C with Emscripten:

  // paste into hello.c
  #include <stdio.h>
  int main(int argc, char const *argv[]) {
      printf("Hello, world!\n");
      return 0;
  }

  # emscripten produces asm.js by default, so use WASM=1 flag
  # note that this generates 3 files: 
  # - hello.wasm: actual binary
  # - hello.js: glue code for compiling and running WebAssembly in the browser, uses fetch() to get hello.wasm
  # - hello.html: website that emulates a console, includes hello.js
  emcc hello.c -s WASM=1 -o hello.html
  
  # (necessary for Chrome, optional for Firefox:) some-origin policy disallows getting 
  # hello.wasm file from inside hello.js unless it is served from an actual webserver
  # emrun is just a minimal webserver provided by emscripten  
  emrun --no_browser --port 8080 .
  
  # browse to local webserver with Firefox or Chrome
  firefox http://localhost:8080/hello.html
  chromium-browser http://localhost:8080/hello.html
  
  # (optional:) inspect the produced binary with wasm2wat or wasm-objdump
  wasm2wat hello.wasm -o hello.wat
  wasm-objdump hello.wasm -hdx | less

• Apply Wasabi to WebAssembly programs in the browser

  • Step 1: Instrument

    # start with C to Wasm (via Emscripten) project from previous point, that is:  
    ls
    > hello.c  hello.html  hello.js  hello.wasm
    
    # instrument hello.wasm, produces 2 files in out/:
    # - out/hello.wasm: instrumented binary, with imported hooks and calls to these hooks inserted between instructions
    # - out/hello.wasabi.js: Wasabi loader, runtime, and generated program-dependent JavaScript (low-level monomorphized hooks and statically extracted information about the binary)
    wasabi hello.wasm
    
    # replace original binary with instrumented one and copy generated JavaScript 
    mv hello.wasm hello.orig.wasm
    cp out/* .
    
    # insert Wasabi-generated JavaScript into emscripten-generated HTML harness (FIXME hacky)
    sed -i '/<script async type="text\/javascript" src="hello.js"><\/script>/a <script src="hello.wasabi.js"></script>' hello.html
    
    # should still work as before instrumentation
    emrun --no_browser --port 8080 .
    firefox http://localhost:8080/hello.html

  • Step 2: Analyze

    # use example analysis that just logs all instructions with their inputs and results
    cp /path/to/wasabi/analyses/log-all.js .
    
    # include analysis in emscripten-generated HTML harness (FIXME hacky)
    # NOTE analysis must be loaded *after* the Wasabi-generated JavaScript
    sed -i '/<script src="hello.wasabi.js"><\/script>/a <script src="log-all.js"></script>' hello.html
    
    # run in browser again, see lots of output on JavaScript console
    emrun --no_browser --port 8080 .
    firefox http://localhost:8080/hello.html

• Running WebAssembly and instrumenting with Wasabi for Node.js: TODO