contrib/fuzz/FuzzedDataProvider.h

dirstate-v2: Introduce a docket file

.hg/dirstate now only contains some metadata to point to a separate data file
named .hg/dirstate.{}.d with a random hexadecimal identifier. For now every
update creates a new data file and removes the old one, but later we’ll
(usually) append to an existing file.

Separating into two files allows doing the "write to a temporary file then
atomically rename into destination" dance with only a small docket file,
without always rewriting a lot of data.

Differential Revision: https://phab.mercurial-scm.org/D11088

//===- FuzzedDataProvider.h - Utility header for fuzz targets ---*- C++ -* ===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
// A single header library providing an utility class to break up an array of
// bytes. Whenever run on the same input, provides the same output, as long as
// its methods are called in the same order, with the same arguments.
//===----------------------------------------------------------------------===//

#ifndef LLVM_FUZZER_FUZZED_DATA_PROVIDER_H_
#define LLVM_FUZZER_FUZZED_DATA_PROVIDER_H_

#include <algorithm>
#include <climits>
#include <cstddef>
#include <cstdint>
#include <cstring>
#include <initializer_list>
#include <string>
#include <type_traits>
#include <utility>
#include <vector>

// In addition to the comments below, the API is also briefly documented at
// https://github.com/google/fuzzing/blob/master/docs/split-inputs.md#fuzzed-data-provider
class FuzzedDataProvider
{
      public:
	// |data| is an array of length |size| that the FuzzedDataProvider wraps
	// to provide more granular access. |data| must outlive the
	// FuzzedDataProvider.
	FuzzedDataProvider(const uint8_t *data, size_t size)
	    : data_ptr_(data), remaining_bytes_(size)
	{
	}
	~FuzzedDataProvider() = default;

	// Returns a std::vector containing |num_bytes| of input data. If fewer
	// than |num_bytes| of data remain, returns a shorter std::vector
	// containing all of the data that's left. Can be used with any byte
	// sized type, such as char, unsigned char, uint8_t, etc.
	template <typename T> std::vector<T> ConsumeBytes(size_t num_bytes)
	{
		num_bytes = std::min(num_bytes, remaining_bytes_);
		return ConsumeBytes<T>(num_bytes, num_bytes);
	}

	// Similar to |ConsumeBytes|, but also appends the terminator value at
	// the end of the resulting vector. Useful, when a mutable
	// null-terminated C-string is needed, for example. But that is a rare
	// case. Better avoid it, if possible, and prefer using |ConsumeBytes|
	// or |ConsumeBytesAsString| methods.
	template <typename T>
	std::vector<T> ConsumeBytesWithTerminator(size_t num_bytes,
	                                          T terminator = 0)
	{
		num_bytes = std::min(num_bytes, remaining_bytes_);
		std::vector<T> result =
		    ConsumeBytes<T>(num_bytes + 1, num_bytes);
		result.back() = terminator;
		return result;
	}

	// Returns a std::string containing |num_bytes| of input data. Using
	// this and
	// |.c_str()| on the resulting string is the best way to get an
	// immutable null-terminated C string. If fewer than |num_bytes| of data
	// remain, returns a shorter std::string containing all of the data
	// that's left.
	std::string ConsumeBytesAsString(size_t num_bytes)
	{
		static_assert(sizeof(std::string::value_type) ==
		                  sizeof(uint8_t),
		              "ConsumeBytesAsString cannot convert the data to "
		              "a string.");

		num_bytes = std::min(num_bytes, remaining_bytes_);
		std::string result(
		    reinterpret_cast<const std::string::value_type *>(
		        data_ptr_),
		    num_bytes);
		Advance(num_bytes);
		return result;
	}

	// Returns a number in the range [min, max] by consuming bytes from the
	// input data. The value might not be uniformly distributed in the given
	// range. If there's no input data left, always returns |min|. |min|
	// must be less than or equal to |max|.
	template <typename T> T ConsumeIntegralInRange(T min, T max)
	{
		static_assert(std::is_integral<T>::value,
		              "An integral type is required.");
		static_assert(sizeof(T) <= sizeof(uint64_t),
		              "Unsupported integral type.");

		if (min > max)
			abort();

		// Use the biggest type possible to hold the range and the
		// result.
		uint64_t range = static_cast<uint64_t>(max) - min;
		uint64_t result = 0;
		size_t offset = 0;

		while (offset < sizeof(T) * CHAR_BIT && (range >> offset) > 0 &&
		       remaining_bytes_ != 0) {
			// Pull bytes off the end of the seed data.
			// Experimentally, this seems to allow the fuzzer to
			// more easily explore the input space. This makes
			// sense, since it works by modifying inputs that caused
			// new code to run, and this data is often used to
			// encode length of data read by |ConsumeBytes|.
			// Separating out read lengths makes it easier modify
			// the contents of the data that is actually read.
			--remaining_bytes_;
			result =
			    (result << CHAR_BIT) | data_ptr_[remaining_bytes_];
			offset += CHAR_BIT;
		}

		// Avoid division by 0, in case |range + 1| results in overflow.
		if (range != std::numeric_limits<decltype(range)>::max())
			result = result % (range + 1);

		return static_cast<T>(min + result);
	}

	// Returns a std::string of length from 0 to |max_length|. When it runs
	// out of input data, returns what remains of the input. Designed to be
	// more stable with respect to a fuzzer inserting characters than just
	// picking a random length and then consuming that many bytes with
	// |ConsumeBytes|.
	std::string ConsumeRandomLengthString(size_t max_length)
	{
		// Reads bytes from the start of |data_ptr_|. Maps "\\" to "\",
		// and maps "\" followed by anything else to the end of the
		// string. As a result of this logic, a fuzzer can insert
		// characters into the string, and the string will be lengthened
		// to include those new characters, resulting in a more stable
		// fuzzer than picking the length of a string independently from
		// picking its contents.
		std::string result;

		// Reserve the anticipated capaticity to prevent several
		// reallocations.
		result.reserve(std::min(max_length, remaining_bytes_));
		for (size_t i = 0; i < max_length && remaining_bytes_ != 0;
		     ++i) {
			char next = ConvertUnsignedToSigned<char>(data_ptr_[0]);
			Advance(1);
			if (next == '\\' && remaining_bytes_ != 0) {
				next =
				    ConvertUnsignedToSigned<char>(data_ptr_[0]);
				Advance(1);
				if (next != '\\')
					break;
			}
			result += next;
		}

		result.shrink_to_fit();
		return result;
	}

	// Returns a std::vector containing all remaining bytes of the input
	// data.
	template <typename T> std::vector<T> ConsumeRemainingBytes()
	{
		return ConsumeBytes<T>(remaining_bytes_);
	}

	// Returns a std::string containing all remaining bytes of the input
	// data. Prefer using |ConsumeRemainingBytes| unless you actually need a
	// std::string object.
	std::string ConsumeRemainingBytesAsString()
	{
		return ConsumeBytesAsString(remaining_bytes_);
	}

	// Returns a number in the range [Type's min, Type's max]. The value
	// might not be uniformly distributed in the given range. If there's no
	// input data left, always returns |min|.
	template <typename T> T ConsumeIntegral()
	{
		return ConsumeIntegralInRange(std::numeric_limits<T>::min(),
		                              std::numeric_limits<T>::max());
	}

	// Reads one byte and returns a bool, or false when no data remains.
	bool ConsumeBool()
	{
		return 1 & ConsumeIntegral<uint8_t>();
	}

	// Returns a copy of the value selected from the given fixed-size
	// |array|.
	template <typename T, size_t size>
	T PickValueInArray(const T (&array)[size])
	{
		static_assert(size > 0, "The array must be non empty.");
		return array[ConsumeIntegralInRange<size_t>(0, size - 1)];
	}

	template <typename T>
	T PickValueInArray(std::initializer_list<const T> list)
	{
		// TODO(Dor1s): switch to static_assert once C++14 is allowed.
		if (!list.size())
			abort();

		return *(list.begin() +
		         ConsumeIntegralInRange<size_t>(0, list.size() - 1));
	}

	// Returns an enum value. The enum must start at 0 and be contiguous. It
	// must also contain |kMaxValue| aliased to its largest (inclusive)
	// value. Such as: enum class Foo { SomeValue, OtherValue, kMaxValue =
	// OtherValue };
	template <typename T> T ConsumeEnum()
	{
		static_assert(std::is_enum<T>::value,
		              "|T| must be an enum type.");
		return static_cast<T>(ConsumeIntegralInRange<uint32_t>(
		    0, static_cast<uint32_t>(T::kMaxValue)));
	}

	// Returns a floating point number in the range [0.0, 1.0]. If there's
	// no input data left, always returns 0.
	template <typename T> T ConsumeProbability()
	{
		static_assert(std::is_floating_point<T>::value,
		              "A floating point type is required.");

		// Use different integral types for different floating point
		// types in order to provide better density of the resulting
		// values.
		using IntegralType =
		    typename std::conditional<(sizeof(T) <= sizeof(uint32_t)),
		                              uint32_t, uint64_t>::type;

		T result = static_cast<T>(ConsumeIntegral<IntegralType>());
		result /=
		    static_cast<T>(std::numeric_limits<IntegralType>::max());
		return result;
	}

	// Returns a floating point value in the range [Type's lowest, Type's
	// max] by consuming bytes from the input data. If there's no input data
	// left, always returns approximately 0.
	template <typename T> T ConsumeFloatingPoint()
	{
		return ConsumeFloatingPointInRange<T>(
		    std::numeric_limits<T>::lowest(),
		    std::numeric_limits<T>::max());
	}

	// Returns a floating point value in the given range by consuming bytes
	// from the input data. If there's no input data left, returns |min|.
	// Note that |min| must be less than or equal to |max|.
	template <typename T> T ConsumeFloatingPointInRange(T min, T max)
	{
		if (min > max)
			abort();

		T range = .0;
		T result = min;
		constexpr T zero(.0);
		if (max > zero && min < zero &&
		    max > min + std::numeric_limits<T>::max()) {
			// The diff |max - min| would overflow the given
			// floating point type. Use the half of the diff as the
			// range and consume a bool to decide whether the result
			// is in the first of the second part of the diff.
			range = (max / 2.0) - (min / 2.0);
			if (ConsumeBool()) {
				result += range;
			}
		} else {
			range = max - min;
		}

		return result + range * ConsumeProbability<T>();
	}

	// Reports the remaining bytes available for fuzzed input.
	size_t remaining_bytes()
	{
		return remaining_bytes_;
	}

      private:
	FuzzedDataProvider(const FuzzedDataProvider &) = delete;
	FuzzedDataProvider &operator=(const FuzzedDataProvider &) = delete;

	void Advance(size_t num_bytes)
	{
		if (num_bytes > remaining_bytes_)
			abort();

		data_ptr_ += num_bytes;
		remaining_bytes_ -= num_bytes;
	}

	template <typename T>
	std::vector<T> ConsumeBytes(size_t size, size_t num_bytes_to_consume)
	{
		static_assert(sizeof(T) == sizeof(uint8_t),
		              "Incompatible data type.");

		// The point of using the size-based constructor below is to
		// increase the odds of having a vector object with capacity
		// being equal to the length. That part is always implementation
		// specific, but at least both libc++ and libstdc++ allocate the
		// requested number of bytes in that constructor, which seems to
		// be a natural choice for other implementations as well. To
		// increase the odds even more, we also call |shrink_to_fit|
		// below.
		std::vector<T> result(size);
		if (size == 0) {
			if (num_bytes_to_consume != 0)
				abort();
			return result;
		}

		std::memcpy(result.data(), data_ptr_, num_bytes_to_consume);
		Advance(num_bytes_to_consume);

		// Even though |shrink_to_fit| is also implementation specific,
		// we expect it to provide an additional assurance in case
		// vector's constructor allocated a buffer which is larger than
		// the actual amount of data we put inside it.
		result.shrink_to_fit();
		return result;
	}

	template <typename TS, typename TU> TS ConvertUnsignedToSigned(TU value)
	{
		static_assert(sizeof(TS) == sizeof(TU),
		              "Incompatible data types.");
		static_assert(!std::numeric_limits<TU>::is_signed,
		              "Source type must be unsigned.");

		// TODO(Dor1s): change to `if constexpr` once C++17 becomes
		// mainstream.
		if (std::numeric_limits<TS>::is_modulo)
			return static_cast<TS>(value);

		// Avoid using implementation-defined unsigned to signer
		// conversions. To learn more, see
		// https://stackoverflow.com/questions/13150449.
		if (value <= std::numeric_limits<TS>::max()) {
			return static_cast<TS>(value);
		} else {
			constexpr auto TS_min = std::numeric_limits<TS>::min();
			return TS_min + static_cast<char>(value - TS_min);
		}
	}

	const uint8_t *data_ptr_;
	size_t remaining_bytes_;
};

#endif // LLVM_FUZZER_FUZZED_DATA_PROVIDER_H_
// no-check-code since this is from a third party