Fuzzing

Fuzzing

Purpose

Guide agents through setting up and running coverage-guided fuzz testing: libFuzzer (in-process) and AFL++ (fork-based), with sanitizer integration and CI pipeline setup.

Triggers

• "How do I fuzz-test my parser/deserializer?"
• "What is a fuzz target / how do I write one?"
• "How do I set up libFuzzer?"
• "How do I use AFL++ on my program?"
• "How do I run fuzzing in CI?"
• "Fuzzer found a crash — how do I reproduce it?"

Workflow

1. Write a fuzz target (libFuzzer)

A fuzz target is a function that accepts arbitrary bytes and exercises the code under test.

// fuzz_parser.c
#include <stdint.h>
#include <stddef.h>
#include "myparser.h"

// Entry point called by libFuzzer with random data
int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
    // Must not abort/exit on invalid input (that's expected)
    // Must not read outside [data, data+size)

    MyParser *p = parser_create();
    if (p) {
        parser_feed(p, (const char *)data, size);
        parser_destroy(p);
    }
    return 0;  // Always return 0 (non-zero means discard input)
}

Key rules: