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// Copyright (c) 2011 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
// Fuzz testing for EncodedProgram serialized format and assembly.
// We would like some assurance that if an EncodedProgram is malformed we will
// not crash. The EncodedProgram could be malformed either due to malicious
// attack to due to an error in patch generation.
// We try a lot of arbitrary modifications to the serialized form and make sure
// that the outcome is not a crash.
#include "courgette/encoded_program.h"
#include <stddef.h>
#include <memory>
#include "base/test/test_suite.h"
#include "courgette/assembly_program.h"
#include "courgette/base_test_unittest.h"
#include "courgette/courgette.h"
#include "courgette/program_detector.h"
#include "courgette/streams.h"
class DecodeFuzzTest : public BaseTest {
void FuzzExe(const char *) const;
void FuzzByte(const std::string& buffer, const std::string& output,
size_t index) const;
void FuzzBits(const std::string& buffer, const std::string& output,
size_t index, int bits_to_flip) const;
// Returns true if could assemble, false if rejected.
bool TryAssemble(const std::string& buffer, std::string* output) const;
// Loads an executable and does fuzz testing in the serialized format.
void DecodeFuzzTest::FuzzExe(const char* file_name) const {
std::string file1 = FileContents(file_name);
const void* original_buffer = file1.c_str();
size_t original_length = file1.length();
std::unique_ptr<courgette::AssemblyProgram> program;
const courgette::Status parse_status =
courgette::ParseDetectedExecutable(original_buffer, original_length,
EXPECT_EQ(courgette::C_OK, parse_status);
std::unique_ptr<courgette::EncodedProgram> encoded;
const courgette::Status encode_status = Encode(*program, &encoded);
EXPECT_EQ(courgette::C_OK, encode_status);
courgette::SinkStreamSet sinks;
const courgette::Status write_status =
WriteEncodedProgram(encoded.get(), &sinks);
EXPECT_EQ(courgette::C_OK, write_status);
courgette::SinkStream sink;
bool can_collect = sinks.CopyTo(&sink);
size_t length = sink.Length();
std::string base_buffer(reinterpret_cast<const char*>(sink.Buffer()), length);
std::string base_output;
bool ok = TryAssemble(base_buffer, &base_output);
// Now we have a good serialized EncodedProgram in |base_buffer|. Time to
// fuzz.
// More intense fuzzing on the first part because it contains more control
// information like substeam lengths.
size_t position = 0;
for ( ; position < 100 && position < length; position += 1) {
FuzzByte(base_buffer, base_output, position);
// We would love to fuzz every position, but it takes too long.
for ( ; position < length; position += 900) {
FuzzByte(base_buffer, base_output, position);
// FuzzByte tries to break the EncodedProgram deserializer and assembler. It
// takes a good serialization of and EncodedProgram, flips some bits, and checks
// that the behaviour is reasonable. It has testing checks for unreasonable
// behaviours.
void DecodeFuzzTest::FuzzByte(const std::string& base_buffer,
const std::string& base_output,
size_t index) const {
printf("Fuzzing position %d\n", static_cast<int>(index));
// The following 10 values are a compromize between run time and coverage of
// the 255 'wrong' values at this byte position.
// 0xFF flips all the bits.
FuzzBits(base_buffer, base_output, index, 0xFF);
// 0x7F flips the most bits without changing Varint32 framing.
FuzzBits(base_buffer, base_output, index, 0x7F);
// These all flip one bit.
FuzzBits(base_buffer, base_output, index, 0x80);
FuzzBits(base_buffer, base_output, index, 0x40);
FuzzBits(base_buffer, base_output, index, 0x20);
FuzzBits(base_buffer, base_output, index, 0x10);
FuzzBits(base_buffer, base_output, index, 0x08);
FuzzBits(base_buffer, base_output, index, 0x04);
FuzzBits(base_buffer, base_output, index, 0x02);
FuzzBits(base_buffer, base_output, index, 0x01);
// FuzzBits tries to break the EncodedProgram deserializer and assembler. It
// takes a good serialization of and EncodedProgram, flips some bits, and checks
// that the behaviour is reasonable.
// There are EXPECT calls to check for unreasonable behaviour. These are
// somewhat arbitrary in that the parameters cannot easily be derived from first
// principles. They may need updating as the serialized format evolves.
void DecodeFuzzTest::FuzzBits(const std::string& base_buffer,
const std::string& base_output,
size_t index, int bits_to_flip) const {
std::string modified_buffer = base_buffer;
std::string modified_output;
modified_buffer[index] ^= bits_to_flip;
bool ok = TryAssemble(modified_buffer, &modified_output);
if (ok) {
// We normally expect TryAssemble to fail. But sometimes it succeeds.
// What could have happened? We changed one byte in the serialized form:
// * If we changed one of the copied bytes, we would see a single byte
// change in the output.
// * If we changed an address table element, all the references to that
// address would be different.
// * If we changed a copy count, we would run out of data in some stream,
// or leave data remaining, so should not be here.
// * If we changed an origin address, it could affect all relocations based
// off that address. If no relocations were based off the address then
// there will be no changes.
// * If we changed an origin address, it could cause some abs32 relocs to
// shift from one page to the next, changing the number and layout of
// blocks in the base relocation table.
// Generated length could vary slightly due to base relocation table layout.
// In the worst case the number of base relocation blocks doubles, approx
// 12/4096 or 0.3% size of file.
size_t base_length = base_output.length();
size_t modified_length = modified_output.length();
ptrdiff_t diff = base_length - modified_length;
if (diff < -200 || diff > 200) {
EXPECT_EQ(base_length, modified_length);
size_t changed_byte_count = 0;
for (size_t i = 0; i < base_length && i < modified_length; ++i) {
changed_byte_count += (base_output[i] != modified_output[i]);
if (index > 60) { // Beyond the origin addresses ...
EXPECT_NE(0U, changed_byte_count); // ... we expect some difference.
// Currently all changes are smaller than this number:
EXPECT_GE(45000U, changed_byte_count);
bool DecodeFuzzTest::TryAssemble(const std::string& buffer,
std::string* output) const {
std::unique_ptr<courgette::EncodedProgram> encoded;
bool result = false;
courgette::SourceStreamSet sources;
bool can_get_source_streams = sources.Init(buffer.c_str(), buffer.length());
if (can_get_source_streams) {
const courgette::Status read_status =
ReadEncodedProgram(&sources, &encoded);
if (read_status == courgette::C_OK) {
courgette::SinkStream assembled;
const courgette::Status assemble_status =
Assemble(encoded.get(), &assembled);
if (assemble_status == courgette::C_OK) {
const void* assembled_buffer = assembled.Buffer();
size_t assembled_length = assembled.Length();
output->assign(reinterpret_cast<const char*>(assembled_buffer),
result = true;
return result;
TEST_F(DecodeFuzzTest, All) {
int main(int argc, char** argv) {
return base::TestSuite(argc, argv).Run();