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# `wasm-smith`
**A WebAssembly test case generator.**
[](https://docs.rs/wasm-smith/)
[](https://crates.io/crates/wasm-smith)
[](https://crates.io/crates/wasm-smith)
* [Features](#features)
* [Usage](#usage)
* [With `cargo fuzz` and `libfuzzer-sys`](#with-cargo-fuzz-and-libfuzzer-sys)
* [As a Command Line Tool](#as-a-command-line-tool)
## 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`](https://docs.rs/arbitrary/*/arbitrary/trait.Arbitrary.html)
trait**: Easy to use with [`cargo
fuzz`](https://github.com/rust-fuzz/cargo-fuzz) and
[`libfuzzer-sys`](https://github.com/rust-fuzz/libfuzzer)!
* **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:
```shell
$ cargo fuzz add my_wasm_smith_fuzz_target
```
Next, add `wasm-smith` to your dependencies:
```shell
$ cargo add wasm-smith
```
Then, define your fuzz target so that it takes arbitrary `wasm_smith::Module`s
as an argument, convert the module into serialized Wasm bytes via the `to_bytes`
method, and then feed it into your system:
```rust
// 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:
```shell
$ cargo fuzz run my_wasm_smith_fuzz_target
```
> **Note:** Also check out [the `validate` fuzz
> target](https://github.com/bytecodealliance/wasm-tools/blob/main/fuzz/src/validate.rs)
> 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`:
```shell
$ cargo install wasm-tools
```
Convert some arbitrary input into a valid Wasm module:
```shell
$ head -c 100 /dev/urandom | wasm-tools smith -o test.wasm
```
Finally, run your tool on the generated Wasm module:
```shell
$ my-wasm-tool test.wasm
```
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