Fuzz testing Rust in Fuchsia

Fuchsia's Rust toolchain supports fuzzing Rust crates using LLVM's libFuzzer. Most of the information in C/C++ fuzzing guide still applies. This document only focuses on the details specific to Rust.

Write a Rust fuzzer

You need to implement a fuzz target function and annotate it with the [#fuzz] attribute from the fuchsia-fuzzing crate.

This function should typically be located near the code that it tests. It is analogous to a fuzz target function in C or C++, and is used by libFuzzer to search for inputs that cause panics or other errors.

The function should use its inputs to exercise an API to be tested. There are two ways to write the inputs to the function. For each of the examples below, assume you want to test code like the following:

struct ToyStruct {
    n: u8,
    s: String,

fn toy_example(input: ToyStruct) -> Result<u8, &'static str>;

Automatically transform arbitrary inputs

You can create a fuzz target function that takes one or more inputs with the Arbitrary trait from the arbitrary crate. This is the recommended approach.

To write a fuzz target function that automatically transforms arbitrary inputs:

  1. If needed, implement the Arbitrary trait for the types used by your test code. This can be done "by hand" following the crate's instructions, but the recommended way is to automatically derive the trait.

    For example, in your src/lib.rs:

    use arbitrary:Arbitrary;
    struct ToyStruct { ... }
  2. Create a function with the #[fuzz] attribute that passes the necessary parameters to the code you wish to test.

    For example, in your src/lib.rs:

    use fuchsia_fuzzing::fuzz;
    fn toy_example_arbitrary(input: ToyStruct) {
        let _ = toy_example(input);

Manually transform a byte slice to API inputs

If the code you wish to test already operates on bytes, or if it's not possible to implement the Arbitrary trait for your inputs, you can create a fuzz target function that uses the bytes provided by the fuzzer engine directly. As before, this function needs the #[fuzz] attribute. It should take a reference to byte slice as its single parameter, i.e. &[u8].

For example, in your src/lib.rs:

use fuchsia_fuzzing::fuzz;

fn toy_example_u8(input: &[u8]) {
    if input.len() == 0 {
    let n = input[0];
    if let Ok(s) = std::str::from_utf8(input) {
        let _ = toy_example(ToyStruct{n, s: s.to_string(),});

Build a Rust fuzzer

The rustc_fuzzer GN template generates a GN target that compiles the Rust fuzz target function into a C object file that it then links with libFuzzer.

To build a Rust fuzzer:

  1. Add a rustc_fuzzer GN target to the crate's BUILD.gn.

    When choosing where and how to add this target, consider the following:

    • It is recommended to have the fuzzer name match the fuzz target function name, and to include the fuzz target function in a Rust library, i.e. in src/lib.rs. You may leave the body of the template empty when following these recommendations.

      For example, using the toy_example_arbitrary example above, you would add the following to your BUILD.gn:

      rustc_fuzzer("toy_example_arbitrary") {
    • If the fuzz target function name differs from the fuzzer name, you must provide it with the rustfunction parameter.

      For example, using the toy_example_u8 example above, you would add the following to your BUILD.gn:

      rustc_fuzzer("toy_example_raw_bytes") {
          rustfunction = "toy_example_u8"
    • If the code to be tested cannot be easily factored into a library, a Rust binary can be used with two implications:

      • You must exclude the main function from compilation, along with any items not used when fuzzing, e.g. imports only used in main.

      For example:

      use only::used::in::main;
      fn main() { ... }
      • You must explicitly provide the fuzz target function to the rustc_fuzzer with the source_root parameter.

      For example, in your BUILD.gn:

      rustc_fuzzer("toy_example_with_main") {
          source_root = "src/main.rs"
  2. Add the fuzzer to a new or existing fuzzers_package GN target to bundle it into a deployable package. You'll want to add it to a fuzzer_profile that tells the build to add Rust instrumentation.

    For example:

    fuzzers_package("example_fuzzers") {
      fuzzer_profiles = [
          fuzzers = [
          sanitizers = [ "rust-asan" ]

After this, you can continue following the instructions in Fuzz testing in Fuchsia with LibFuzzer.

To use the rust-asan-fuzzer variant, configure your build with the following:

fx set [other-args...] --fuzz-with rust-asan`

Run a rust fuzzer

To run the fuzzer, see the Quick-start guide for how to use the fx fuzz commands.


The code in this document is taken from the complete Rust fuzzer example in //examples/fuzzer/rust.