Semblance

Semblance is a WebAssembly interpreter written in Rust. The implementation is a rather direct translation of the WebAssembly Core Specification into Rust code. Everything is much easier to grok if you are familiar with the spec.

I wanted to understand the WebAssembly core specification in detail, and this project is the result of that. As such, this is not production quality software. There are many places where I've skipped input validation in an effort to move forward quickly. One should be careful to only evaluate trusted WebAssembly modules with this interpreter.

I've progressed far enough to have implemented every opcode in the WebAssembly 2.0 specification except for the vector instructions.

Build Instructions

Install Rust

Compile the interpreter by running cargo build --release. The executable will be found at target/release/semblance.

cargo build --release

Usage Instructions

semblance <MODULE> [OPTIONS]

Options:
    -h, --help                      Print this help text
    -I, --invoke <FN> [ARGS...]     Invoke an exported function

Currently, the runtime provides an implementation of void puts(char *str); that can be used to write to standard output.

extern void puts(char *str);

void hello() {
    puts("Hello, World!");
}

Compile this C code to WebAssembly with Clang:

clang \
    --target=wasm32 \
    -O3 \
    # Don't include the standard library
    -nostdlib \
    # Don't include an entry point
    -Wl,--no-entry \
    # Export all symbols in the resulting module
    -Wl,--export-all \
    # Allow any undefined symbols (like puts) to be
    # found in the environment, provided by the runtime
    -Wl,--allow-undefined \
    -o hello.wasm \
    hello.c

Run hello.wasm:

./target/release/semblance hello.wasm --invoke hello

This should output:

Hello, World!

Contributing

Since this project is primarily for my personal learning purposes, I am currently not accepting any pull requests. However, I am pretty inexperienced writing C and would happily accept a code review from someone willing to provide feedback.