MemorySanitizerBootstrappingClang

Introduction

MemorySanitizer code itself can not be tested with MemorySanitizer for obvious reasons, but the rest of Clang/LLVM can. Instructions on this page show how to do it.

This guide presumes you have a recent-enough Clang binary installed on your system to successfully compile LLVM and tools. If you don't, you can download a pre-built toolchain from the LLVM website or compile one yourself using this guide.

If you're going to compile Clang yourself, make sure you also build LLD and compiler_rt, as they'll be necessary here.

Note: The following instructions were adapted from the buildbot bootstrap process.

Building libcxx and libcxxabi

MSan requires that all code in the process is instrumented (see handling-external-code). Luckily, the only external dependency of Clang is the C++ standard library (and, of course, libc, but MSan almost takes care of it).

First, clone the LLVM source from github and enter a new build directory:

git clone --depth=1 https://github.com/llvm/llvm-project
cd llvm-project

LIBCXX_DIR="$PWD/build_libcxx"
mkdir $LIBCXX_DIR
cd $LIBCXX_DIR

Second, configure the CMake project:

cmake -GNinja ../llvm \
	-DCMAKE_BUILD_TYPE=Release \
	-DLLVM_ENABLE_PROJECTS="libcxx;libcxxabi" \
	-DCMAKE_C_COMPILER=clang \
	-DCMAKE_CXX_COMPILER=clang++ \
	-DLLVM_USE_SANITIZER=MemoryWithOrigins

This will generate a Ninja build configuration in the current build directory. If you don't want to use Ninja, omit -GNinja above and it will generate a Makefile instead.

Lastly, build the libraries:

cmake --build . -- cxx cxxabi

This will place the relevant build artifacts in ./lib and ./include.

Building clang

Go back up to the root llvm-project directory, and create one more build directory:

CLANG_DIR="$PWD/build_clang"
mkdir $CLANG_DIR
cd $CLANG_DIR

Then, configure CMake again. This time, we're going to specify the libc++ we just built in the previous section, using the build directory $LIBCXX_DIR we used previously.

cmake -GNinja ../llvm \
	-DCMAKE_BUILD_TYPE=Release \
	-DLLVM_ENABLE_PROJECTS="clang;lld" \
	-DCMAKE_C_COMPILER=clang \
	-DCMAKE_CXX_COMPILER=clang++ \
	-DLLVM_USE_SANITIZER=MemoryWithOrigins \
	-DLLVM_ENABLE_LIBCXX=ON \
	-DCMAKE_CXX_FLAGS=" -nostdinc++ -isystem $LIBCXX_DIR/include -isystem $LIBCXX_DIR/include/c++/v1" \
	-DCMAKE_EXE_LINKER_FLAGS=" -stdlib=libc++ -Wl,--rpath=$LIBCXX_DIR/lib -L$LIBCXX_DIR/lib -fsanitize=memory"

Then, build the binaries:

cmake --build . -- clang lld

This will place the built binaries in ./bin.

Note: building all targets in the instrumented tree will attempt to link newly built MSan runtime with MSan runtime from the previous stage, which is not a good idea.