v8-based WebAssembly Sandbox

Overview

This is a proof of concept for a JavaScript + WebAssembly standalone sandbox implemented in C++ utilizing the V8 engine.

Building

1. First of all, you have to build the V8 monolith library (libv8_monolith.a). Instructions can be found here
2. A compiler supporting C++17 along with the filesystem library has to be used for building, e.g. g++-8.
3. Inside the project root:

mkdir build
cd build
cmake .. \
    -DCMAKE_CXX_COMPILER={path to an appropriate compiler (see 2. above)} \
    -DV8_INCLUDE_DIR={path to the v8 include/ dir} \
    -DV8_MONOLITH_LIB_PATH={path to the compiled libv8_monolith.a from 1. above}
make

4. The above should leave you with a working wasm-sandbox executable.

Running

Example:

./wasm-sandbox -O output/ -I input/ -j program.js -w program.wasm\
    -o output_file1 -o output_file2 -- program_arg1 --program_flag1 program_arg2

Flags:

• -O : Path to the output directory. All files specified by -o will be copied from the program virtual filesystem to this directory.
• -I : Path to the input directory. The contents of this directory will be mapped as / in the program's virtual filesystem (which is also the CWD of the program).
• -w : Path to the WebAssembly program file produced by the compiler (e.g. Emscripten).
• -j : Path to the JavaScript program file produced by the compiler (e.g. Emscripten).
• -o [repeated] : Path to an output file produced by the program inside its virtual filesystem. Each file specified by this flag will be copied to the directory specified by -O after the program finishes. If any of the files specified by -o cannot be found after the program finishes, an exception will be raised.
• -- : Anything after this will be passed to the program being run as arguments.

Program restrictions

Currently only programs compiled by Emscripten are supported. Furthermore, the following flags have to be used while compiling via emcc:

• -s BINARYEN_ASYNC_COMPILATION=0 - without this, the program will not actually run, but it won't show any errors (quite difficult to debug). This is due to the fact that the sandbox doesn't implement any sort of asynchronicity on top of V8, so the WebAssembly compilation has to be done synchronously.
• -s MEMFS_APPEND_TO_TYPED_ARRAYS=1 - without this, writing bigger (the actual size seems to depend on the program) files will fail with OOM errors.

If the program uses any substantial amount of memory, the following flags may also be useful:

• -s ALLOW_MEMORY_GROWTH=1
• -s TOTAL_MEMORY=1073741824 (or another number, though there are restrictions). It seems to sometimes be required to specify this flag even though -s ALLOW_MEMORY_GROWTH=1 is specified as well (see the flite example).

Example:

Let's say test.c takes a string argument, reads in.txt file and writes the following string: Hello world!\n{contents of the input file}\n{the argument}\n to out.txt file.

First, we compile the program using Emscripten:

emcc test.c -o test.js -s BINARYEN_ASYNC_COMPILATION=0 -s MEMFS_APPEND_TO_TYPED_ARRAYS=1 -s ALLOW_MEMORY_GROWTH=1

This produces 2 files, test.js and test.wasm. Then, we create directories for the program's input and output:

mkdir input output
echo "interesting file contents" > input/in.txt

Finally, we can run the program inside the sandbox:

./wasm-sandbox -I input -O output -j test.js -w test.wasm -o out.txt -- interesting_program_arg

And the we can read the results:

cat output/out.txt
> Hello world!
> interesting file contents
> interesting_program_arg

The actual program can be found in examples/test, both as source and compiled into JS+WASM.

Wasm store

More examples of precompiled Wasm binaries can be found in golemfactory/wasm-store repo.

Possible TODOs

• Allow outputing variable number of files (perhaps whole directories?).
• Sync to a newer release of V8.
• Figure out if there is a more performant way of linking V8 than libv8_monolith.a.
• Create an abstract Integrator interface, which EmscriptenIntegrator will implement, if any different integrators are needed in the future, targeting different compilers than Emscripten.