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// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
// A fuzz target that consumes a Zlib-compressed input.
// This test verifies that we can find this bug with a custom mutator.
#include <cstddef>
#include <cstdint>
#include <cstdio>
#include <cstdlib>
#include <zlib.h>
// The fuzz target.
// Uncompress the data, crash on input starting with "FU".
// Good luck finding this w/o a custom mutator. :)
extern "C" int LLVMFuzzerTestOneInput(const uint8_t *Data, size_t Size) {
uint8_t Uncompressed[100];
size_t UncompressedLen = sizeof(Uncompressed);
if (Z_OK != uncompress(Uncompressed, &UncompressedLen, Data, Size))
return 0;
if (UncompressedLen < 2) return 0;
if (Uncompressed[0] == 'F' && Uncompressed[1] == 'U')
abort(); // Boom
return 0;
// Forward-declare the libFuzzer's mutator callback.
extern "C" size_t
LLVMFuzzerMutate(uint8_t *Data, size_t Size, size_t MaxSize);
// The custom mutator:
// * deserialize the data (in this case, uncompress).
// * If the data doesn't deserialize, create a properly serialized dummy.
// * Mutate the deserialized data (in this case, just call LLVMFuzzerMutate).
// * Serialize the mutated data (in this case, compress).
extern "C" size_t LLVMFuzzerCustomMutator(uint8_t *Data, size_t Size,
size_t MaxSize, unsigned int Seed) {
uint8_t Uncompressed[100];
size_t UncompressedLen = sizeof(Uncompressed);
size_t CompressedLen = MaxSize;
if (Z_OK != uncompress(Uncompressed, &UncompressedLen, Data, Size)) {
// The data didn't uncompress.
// So, it's either a broken input and we want to ignore it,
// or we've started fuzzing from an empty corpus and we need to supply
// out first properly compressed input.
uint8_t Dummy[] = {'H', 'i'};
if (Z_OK != compress(Data, &CompressedLen, Dummy, sizeof(Dummy)))
return 0;
// fprintf(stderr, "Dummy: max %zd res %zd\n", MaxSize, CompressedLen);
return CompressedLen;
UncompressedLen =
LLVMFuzzerMutate(Uncompressed, UncompressedLen, sizeof(Uncompressed));
if (Z_OK != compress(Data, &CompressedLen, Uncompressed, UncompressedLen))
return 0;
return CompressedLen;