alterstep / wain

:sparkles: wain: WebAssembly INterpreter from scratch in Safe Rust with zero dependency

wain

wain is a WebAssembly INterpreter written in Rust from scratch with zero dependency. An implementation of WebAssembly.

Features:

• No unsafe code. Memory safety and no undefined behavior are guaranteed
• No external runtime dependency. It means no unsafe dependency and less binary size
• Efficiency. Avoid unnecessary allocations and run instructions as fast as possible without unsafe code
• Modular implementation. Binary format parser, text format parser, validator, executor are developed as separate libraries

Note that this project is in progress. Before v1.0.0 means experimental. Not all of the features are implemented yet. Current status is that all the MVP implementations have been done and many tasks are remaining.

Roadmap to v1.0.0 (priority order):

• Add sufficient tests to all libraries and fuzz tests for parsers
• Pass all spec tests
• Add benchmarks to track performance
• Introduce internal representation to execute instructions more efficiently
• Add documentation for every public APIs

Please see the task board for current progress.

This project started for fun and understanding Wasm deeply.

Installation

wain crate is not published yet. Please clone this repository and build the project by cargo build.

Minimum supported Rust version is 1.42.0.

$ cargo install wain
$ wain --help

If you don't want to run text format code, it can be omitted:

# Only run binary format files
$ cargo install wain --no-default-features --features binary

Usage

wain command

Run a binary format Wasm source

$ wain examples/hello/hello.wasm
Hello, world

Run a text format Wasm source:

$ wain examples/hello/hello.wat
Hello, world

Please see examples directory for more examples.

Current restrictions are as follows:

• Only int putchar(int) and int getchar() are implemented as external functions by default
• wain can run only one module at once. It means that importing things from other modules does not work yet
• Many extensions like threads, WASI support, SIMD support, ... are not implemented yet

As libraries

wain consists of multiple crates.

• wain: Command line tool to execute given Wasm sources
• wain-ast: Abstract syntax tree definition. Implementation of Wasm structure spec. This syntax tree is common for both binary format and text format
• wain-syntax-binary: Parser for Wasm binary format (.wasm files). Implementation for Wasm binary format spec. It parses &[u8] value into wain_ast::Root abstract syntax tree
• wain-syntax-text: Parser for Wasm text format (.wat files). Implementation for Wasm text format spec. It parses &str value into wain_ast::Root abstract syntax tree
• wain-validate: Validator of a Wasm abstract syntax tree. Implementation of Wasm validation spec
• wain-exec: Executor which interprets a Wasm abstract syntax tree. Implementation of Wasm execution spec. It directly interprets a syntax tree for now, but it would translate it into an internal representation and execute it for efficiency in the future

wain-* crates are libraries as modular implementation of WebAssembly.

Here is an example code to run the interpreter from Rust.

extern crate wain_syntax_binary;
extern crate wain_validate;
extern crate wain_exec;

use std::fs;
use std::process::exit;
use wain_syntax_binary::parse;
use wain_validate::validate;
use wain_exec::{execute, Run};

let source = fs::read("foo.wasm").unwrap();
let tree = parse(&source).unwrap();

if let Err(err) = validate(&tree) {
    eprintln!("This .wasm file is invalid!: {}", err);
    exit(1);
}

match execute(tree.module) {
    Ok(run) => {
        if let Run::Warning(msg) = run {
            eprintln!("Warning: {}, msg);
        }
    }
    Err(trap) => eprintln!("Execution was trapped: {}", trap),
}

By default, only putchar and getchar in env module are supported as external functions. But you can implement your own struct which implements wain_exec::Importer for defining external functions from Rust side.

extern crate wain_exec;
use wain_exec::{Machine, Stack, Memory, Importer}

struct YourOwnImporter {
    // ...
}

impl Importer for YourOwnImporter {
    fn call(&mut self, name: &str, stack: &mut Stack, memory: &mut Memory) -> Result<(), ImportError> {
        // Implement your own function call. `name` is a name of function and you have full access
        // to stack and linear memory. Pop values from stack for getting arguments and push value to
        // set return value.
        // Note: Consistency between imported function signature and implementation of this method
        // is your responsibility.
    };
}

let ast = ...; // Parse abstract syntax tree and validate it

let mut machine = Machine::instantiate(&ast.module, YourOwnImporter{ /* ... */ }).unwrap();
let run = machine.execute().unwrap();

Future works

• WASI support
• Wasm features after MVP support (threads, SIMD, multiple return values, ...)
• Compare benchmarks with other Wasm implementations
• Self-hosting interpreter. Compile wain into Wasm and run it by itself

How it works

Here I note some points on each phase of interpretation.

Parsing

wain-syntax-binary parses .wasm binary file into wain_ast::Root abstract syntax tree following binary format spec. Wasm binary format is designed to be parsed by basic LL(1) parser. So parsing is very straightforward. Parser implementation is smaller than 1000 lines.

In contrast, implementation of parsing text format is more complicated. wain-syntax-text parses .wat text file into wain_ast::Root abstract syntax tree following text format spec.

1. Lex and parse .wat file into WAT sytnax tree which is dedicated for text format resolving many syntax sugars. Since multiple modules can be put in .wat file, it can be parsed into multiple trees
2. Translate the WAT syntax trees into common Wasm syntax trees (wain_ast::Root) resolving identifiers. Identifiers may refer things not defined yet (forward references) so .wat file cannot be parsed into common Wasm syntax trees directly
3. Compose a single module from the multiple Wasm syntax trees following spec

Validation

Validation is done by traversing a given Wasm syntax tree in wain-validate crate. Conforming spec, following things are validated:

• In Wasm, every reference is an index. It validates all indices are not out of bounds
• Wasm is designed to check stack operations statically. It validates instructions sequences with emulating stack state
• Type check is best-effort due to polymorphic instruction select. Since almost all instructions are not polymorphic, almost all type checks can be done in validation

Conforming the spec, wain validates instructions after unreachable instruction. For example,

(unreachable) (i64.const 0) (i32.add)

i32.add is invalid because it should take two i32 values from stack but at least one i64 value is in the stack.

Execution

wain-exec crate interprets a Wasm syntax tree conforming spec. Thanks to validation, checks at runtime are minimal (e.g. function signature on indirect call).

1. Allocate memory, table, global variables. Initialize stack
2. Interpret syntax tree nodes pushing/popping values to/from stack

Currently wain interprets a Wasm syntax tree directly. I'm planning to define internal representation which can be interpreted faster.

Entrypoint is 'start function' which is defined either

1. Function set in start section
2. Exported function named _start in export section

The 1. is a standard entrypoint but Clang does not emit start section. Instead it handles _start function as entrypoint. wain implements both entrypoints (1. is prioritized).

License

the MIT license