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:

• Never call abort(), exit(), or use global state that persists across calls
• Handle all inputs gracefully (crash = bug found)
• Keep the target fast: the fuzzer calls it millions of times

2. Build with libFuzzer

# Clang (libFuzzer is built into Clang)
clang -fsanitize=fuzzer,address -g -O1 \
    fuzz_parser.c myparser.c -o fuzz_parser

# With UBSan too
clang -fsanitize=fuzzer,address,undefined -g -O1 \
    fuzz_parser.c myparser.c -o fuzz_parser

-fsanitize=fuzzer links libFuzzer and provides main(). Do not provide your own main() in the fuzz target.

3. Run libFuzzer

# Create corpus directory
mkdir -p corpus

# Seed with known-good inputs (greatly accelerates coverage)
cp tests/inputs/* corpus/

# Run the fuzzer
./fuzz_parser corpus/ -max_len=65536 -timeout=10

# Run for a time limit
./fuzz_parser corpus/ -max_total_time=3600

# Run with specific number of jobs (parallel)
./fuzz_parser corpus/ -jobs=4 -workers=4

# Minimise a corpus (remove redundant inputs)
./fuzz_parser -merge=1 corpus_min/ corpus/