cargo-fuzz

# cargo-fuzz

cargo-fuzz is the de facto choice for fuzzing Rust projects when using Cargo. It uses libFuzzer as the backend and provides a convenient Cargo subcommand that automatically enables relevant compilation flags for your Rust project, including support for sanitizers like AddressSanitizer.

## When to Use

cargo-fuzz is currently the primary and most mature fuzzing solution for Rust projects using Cargo.

| Fuzzer | Best For | Complexity |
|--------|----------|------------|
| cargo-fuzz | Cargo-based Rust projects, quick setup | Low |
| AFL++ | Multi-core fuzzing, non-Cargo projects | Medium |
| LibAFL | Custom fuzzers, research, advanced use cases | High |

**Choose cargo-fuzz when:**
- Your project uses Cargo (required)
- You want simple, quick setup with minimal configuration
- You need integrated sanitizer support
- You're fuzzing Rust code with or without unsafe blocks

## Quick Start

```rust
#![no_main]

use libfuzzer_sys::fuzz_target;

fn harness(data: &[u8]) {
    your_project::check_buf(data);
}

fuzz_target!(|data: &[u8]| {
    harness(data);
});
```

Initialize and run:
```bash
cargo fuzz init
# Edit fuzz/fuzz_targets/fuzz_target_1.rs with your harness
cargo +nightly fuzz run fuzz_target_1
```

## Installation

cargo-fuzz requires the nightly Rust toolchain because it uses features only available in nightly.

### Prerequisites

- Rust and Cargo installed via [rustup](https://rustup.rs/)
- Nightly toolchain

### Linux/macOS

```bash
# Install nightly toolchain
rustup install nightly

# Install cargo-fuzz
cargo install cargo-fuzz
```

### Verification

```bash
cargo +nightly --version
cargo fuzz --version
```

## Writing a Harness

### Project Structure

cargo-fuzz works best when your code is structured as a library crate. If you have a binary project, split your `main.rs` into:

```text
src/main.rs  # Entry point (main function)
src/lib.rs   # Code to fuzz (public functions)
Cargo.toml
```

Initialize fuzzing:
```bash
cargo fuzz init
```

This creates:
```text
fuzz/
├── Cargo.toml
└── fuzz_targets/
    └── fuzz_target_1.rs
```

### Harness Structure

```rust
#![no_main]

use libfuzzer_sys::fuzz_target;

fn harness(data: &[u8]) {
    // 1. Validate input size if needed
    if data.is_empty() {
        return;
    }

    // 2. Call target function with fuzz data
    your_project::target_function(data);
}

fuzz_target!(|data: &[u8]| {
    harness(data);
});
```

### Harness Rules

| Do | Don't |
|----|-------|
| Structure code as library crate | Keep everything in main.rs |
| Use `fuzz_target!` macro | Write custom main function |
| Handle `Result::Err` gracefully | Panic on expected errors |
| Keep harness deterministic | Use random number generators |

> **See Also:** For detailed harness writing techniques and structure-aware fuzzing with the
> `arbitrary` crate, see the **fuzz-harness-writing** technique skill.

## Structure-Aware Fuzzing

cargo-fuzz integrates with the [arbitrary](https://github.com/rust-fuzz/arbitrary) crate for structure-aware fuzzing:

```rust
// In your library crate
use arbitrary::Arbitrary;

#[derive(Debug, Arbitrary)]
pub struct Name {
    data: String
}
```

```rust
// In your fuzz target
#![no_main]
use libfuzzer_sys::fuzz_target;

fuzz_target!(|data: your_project::Name| {
    data.check_buf();
});
```

Add to your library's `Cargo.toml`:
```toml
[dependencies]
arbitrary = { version = "1", features = ["derive"] }
```

## Running Campaigns

### Basic Run

```bash
cargo +nightly fuzz run fuzz_target_1
```

### Without Sanitizers (Safe Rust)

If your project doesn't use unsafe Rust, disable sanitizers for 2x performance boost:

```bash
cargo +nightly fuzz run --sanitizer none fuzz_target_1
```

Check if your project uses unsafe code:
```bash
cargo install cargo-geiger
cargo geiger
```

### Re-executing Test Cases

```bash
# Run a specific test case (e.g., a crash)
cargo +nightly fuzz run fuzz_target_1 fuzz/artifacts/fuzz_target_1/crash-<hash>

# Run all corpus entries without fuzzing
cargo +nightly fuzz run fuzz_target_1 fuzz/corpus/fuzz_target_1 -- -runs=0
```

### Using Dictionaries

```bash
cargo +nightly fuzz run fuzz_target_1 -- -dict=./dict.dict
```

### Interpreting Output

| Output | Meaning |
|--------|---------|
| `NEW` | New coverage-increasing input discovered |
| `pulse` | Periodic status update |
| `INITED` | Fuzzer initialized successfully |
| Crash with stack trace | Bug found, saved to `fuzz/artifacts/` |

Corpus location: `fuzz/corpus/fuzz_target_1/`
Crashes location: `fuzz/artifacts/fuzz_target_1/`

## Sanitizer Integration

### AddressSanitizer (ASan)

ASan is enabled by default and detects memory errors:

```bash
cargo +nightly fuzz run fuzz_target_1
```

### Disabling Sanitizers

For pure safe Rust (no unsafe blocks in your code or dependencies):

```bash
cargo +nightly fuzz run --sanitizer none fuzz_target_1
```

**Performance impact:** ASan adds ~2x overhead. Disable for safe Rust to improve fuzzing speed.

### Checking for Unsafe Code

```bash
cargo install cargo-geiger
cargo geiger
```

> **See Also:** For detailed sanitizer configuration, flags, and troubleshooting,
> see the **address-sanitizer** technique skill.

## Coverage Analysis

cargo-fuzz integrates with Rust's coverage tools to analyze fuzzing effectiveness.

### Prerequisites

```bash
rustup toolchain install nightly --component llvm-tools-preview
cargo install cargo-binutils
cargo install rustfilt
```

### Generating Coverage Reports

```bash
# Generate coverage data from corpus
cargo +nightly fuzz coverage fuzz_target_1
```

Create coverage generation script:

```bash
cat <<'EOF' > ./generate_html
#!/bin/sh
if [ $# -lt 1 ]; then
    echo "Error: Name of fuzz target is required."
    echo "Usage: $0 fuzz_target [sources...]"
    exit 1
fi
FUZZ_TARGET="$1"
shift
SRC_FILTER="$@"
TARGET=$(rustc -vV | sed -n 's|host: ||p')
cargo +nightly cov -- show -Xdemangler=rustfilt \
  "target/$TARGET/coverage/$TARGET/release/$FUZZ_TARGET" \
  -instr-profile="fuzz/cov