125 releases (45 breaking)

0.229.0 Apr 17, 2025
0.227.1 Mar 7, 2025
0.222.0 Dec 18, 2024
0.221.0 Nov 27, 2024
0.1.11 Nov 23, 2020

#236 in WebAssembly

Download history 17570/week @ 2025-01-05 21628/week @ 2025-01-12 17650/week @ 2025-01-19 17631/week @ 2025-01-26 17875/week @ 2025-02-02 18029/week @ 2025-02-09 17652/week @ 2025-02-16 18221/week @ 2025-02-23 20055/week @ 2025-03-02 18629/week @ 2025-03-09 19215/week @ 2025-03-16 15622/week @ 2025-03-23 20928/week @ 2025-03-30 19006/week @ 2025-04-06 15468/week @ 2025-04-13 16727/week @ 2025-04-20

73,590 downloads per month
Used in 6 crates

Apache-2.0…

2.5MB
52K SLoC

wasm-smith

A WebAssembly test case generator.

[

Features

  • Always valid: All generated Wasm modules pass validation. wasm-smith gets past your wasm parser and validator, exercising the guts of your Wasm compiler, runtime, or tool.

  • Supports the full WebAssembly language: Doesn't have blind spots or unimplemented instructions.

  • Implements the Arbitrary trait: Easy to use with cargo fuzz and libfuzzer-sys!

  • Deterministic: Given the same input seed, always generates the same output Wasm module, so you can always reproduce test failures.

  • Plays nice with mutation-based fuzzers: Small changes to the input tend to produce small changes to the output Wasm module. Larger inputs tend to generate larger Wasm modules.

Usage

With cargo fuzz and libfuzzer-sys

First, use cargo fuzz to define a new fuzz target:

$ cargo fuzz add my_wasm_smith_fuzz_target

Next, add wasm-smith to your dependencies:

$ cargo add wasm-smith

Then, define your fuzz target so that it takes arbitrary wasm_smith::Modules as an argument, convert the module into serialized Wasm bytes via the to_bytes method, and then feed it into your system:

// fuzz/fuzz_targets/my_wasm_smith_fuzz_target.rs

#![no_main]

use libfuzzer_sys::fuzz_target;
use wasm_smith::Module;

fuzz_target!(|module: Module| {
    let wasm_bytes = module.to_bytes();

    // Your code here...
});

Finally, start fuzzing:

$ cargo fuzz run my_wasm_smith_fuzz_target

Note: Also check out the validate fuzz target defined in this repository. Using the wasmparser crate, it checks that every module generated by wasm-smith validates successfully.

As a Command Line Tool

Install the CLI tool via cargo:

$ cargo install wasm-tools

Convert some arbitrary input into a valid Wasm module:

$ head -c 100 /dev/urandom | wasm-tools smith -o test.wasm

Finally, run your tool on the generated Wasm module:

$ my-wasm-tool test.wasm

Dependencies

~1.3–2.6MB
~48K SLoC