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This directory contains two utilities for fuzzing Clang: clang-fuzzer and clang-proto-fuzzer. Both use libFuzzer to generate inputs to clang via coverage-guided mutation. The two utilities differ, however, in how they structure inputs to Clang. clang-fuzzer makes no attempt to generate valid C++ programs and is therefore primarily useful for stressing the surface layers of Clang (i.e. lexer, parser). clang-proto-fuzzer uses a protobuf class to describe a subset of the C++ language and then uses libprotobuf-mutator to mutate instantiations of that class, producing valid C++ programs in the process. As a result, clang-proto-fuzzer is better at stressing deeper layers of Clang and LLVM. =================================== Building clang-fuzzer =================================== Within your LLVM build directory, run CMake with the following variable definitions: - CMAKE_C_COMPILER=clang - CMAKE_CXX_COMPILER=clang++ - LLVM_USE_SANITIZE_COVERAGE=YES - LLVM_USE_SANITIZER=Address Then build the clang-fuzzer target. Example: cd $LLVM_SOURCE_DIR mkdir build && cd build cmake .. -GNinja -DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++ \ -DLLVM_USE_SANITIZE_COVERAGE=YES -DLLVM_USE_SANITIZER=Address ninja clang-fuzzer ====================== Running clang-fuzzer ====================== bin/clang-fuzzer CORPUS_DIR ======================================================= Building clang-proto-fuzzer (Linux-only instructions) ======================================================= Install the necessary dependencies: - binutils // needed for libprotobuf-mutator - liblzma-dev // needed for libprotobuf-mutator - libz-dev // needed for libprotobuf-mutator - docbook2x // needed for libprotobuf-mutator - Recent version of protobuf [3.3.0 is known to work] Within your LLVM build directory, run CMake with the following variable definitions: - CMAKE_C_COMPILER=clang - CMAKE_CXX_COMPILER=clang++ - LLVM_USE_SANITIZE_COVERAGE=YES - LLVM_USE_SANITIZER=Address - CLANG_ENABLE_PROTO_FUZZER=ON Then build the clang-proto-fuzzer and clang-proto-to-cxx targets. Optionally, you may also build clang-fuzzer with this setup. Example: cd $LLVM_SOURCE_DIR mkdir build && cd build cmake .. -GNinja -DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++ \ -DLLVM_USE_SANITIZE_COVERAGE=YES -DLLVM_USE_SANITIZER=Address \ -DCLANG_ENABLE_PROTO_FUZZER=ON ninja clang-proto-fuzzer clang-proto-to-cxx This directory also contains a Dockerfile which sets up all required dependencies and builds the fuzzers. ============================ Running clang-proto-fuzzer ============================ bin/clang-proto-fuzzer CORPUS_DIR Arguments can be specified after -ignore_remaining_args=1 to modify the compiler invocation. For example, the following command line will fuzz LLVM with a custom optimization level and target triple: bin/clang-proto-fuzzer CORPUS_DIR -ignore_remaining_args=1 -O3 -triple \ arm64apple-ios9 To translate a clang-proto-fuzzer corpus output to C++: bin/clang-proto-to-cxx CORPUS_OUTPUT_FILE =================== llvm-proto-fuzzer =================== Like, clang-proto-fuzzer, llvm-proto-fuzzer is also a protobuf-mutator based fuzzer. It receives as input a cxx_loop_proto which it then converts into a string of valid LLVM IR: a function with either a single loop or two nested loops. It then creates a new string of IR by running optimization passes over the original IR. Currently, it only runs a loop-vectorize pass but more passes can easily be added to the fuzzer. Once there are two versions of the input function (optimized and not), llvm-proto-fuzzer uses LLVM's JIT Engine to compile both functions. Lastly, it runs both functions on a suite of inputs and checks that both functions behave the same on all inputs. In this way, llvm-proto-fuzzer can find not only compiler crashes, but also miscompiles originating from LLVM's optimization passes. llvm-proto-fuzzer is built very similarly to clang-proto-fuzzer. You can run the fuzzer with the following command: bin/clang-llvm-proto-fuzzer CORPUS_DIR To translate a cxx_loop_proto file into LLVM IR do: bin/clang-loop-proto-to-llvm CORPUS_OUTPUT_FILE To translate a cxx_loop_proto file into C++ do: bin/clang-loop-proto-to-cxx CORPUS_OUTPUT_FILE Note: To get a higher number of executions per second with llvm-proto-fuzzer it helps to build it without ASan instrumentation and with the -O2 flag. Because the fuzzer is not only compiling code, but also running it, as the inputs get large, the time necessary to fuzz one input can get very high. Example: cmake .. -GNinja -DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++ \ -DCLANG_ENABLE_PROTO_FUZZER=ON -DLLVM_USE_SANITIZE_COVERAGE=YES \ -DCMAKE_CXX_FLAGS="-O2" ninja clang-llvm-proto-fuzzer clang-loop-proto-to-llvm