CHERI RISC-V Port of musl C Library

This is based on the morello port of musl libc at https://git.morello-project.org/morello/musl-libc with RISC-V customization added.

Building musl-libc for CHERI RISC-V Linux

Prerequisites

In order to build musl you will need:

• The CHERI-enabled musl sources
• The CHERI-enabled LLVM compiler
• Linux Kernel headers from a CHERI-enabled Linux Kernel

The build is slightly complicated by the fact that LLVM's compiler runtime must be compiled using libc headers from musl but the musl build needs this compiler runtime for its linking step.

Build Process Overview

1. Compile clang and lld
2. Setup a sysroot environment with musl and linux kernel headers
3. Build and install the compiler runtime from the LLVM project
4. Build and install musl
5. Compile your first test program.

In the rest of this document we will assume that the following environment variables are set:

• INSTALL: Installation directory of the LLVM toolchain
• SYSROOT: The sysroot directory used for cross compiling
• TARGET=riscv64-unknown-linux-gnu
• MARCH=-march=rv64imafdc_zcherihybrid_zcherilevels_zish4add
• MABI=-mabi=l64pc128d

Compile clang and lld

Please refer to the LLVM build instructions for details on how to build LLVM for CHERI. Please make sure you include these options in your cmake command line:

	-DCMAKE_INSTALL_PREFIX=$INSTALL
	-DCMAKE_C_COMPILER_TARGET=$TARGET
	-DLLVM_ENABLE_PROJECTS="clang;llvm;lld"
	-DCLANG_DEFAULT_RTLIB="compiler-rt"

Setup the sysroot Environment

To install the kernel headers, run the command below from the kernel's top-level directory. There is no need to configure or build the kernel before.

	make ARCH=riscv INSTALL_HDR_PATH=$SYSROOT/usr headers_install

To build the musl headers go to the musl source directory and do

	mkdir build_headers && cd build_headers
	../configure CC="$INSTALL/bin/clang -target $TARGET" \
		--enable-shared --prefix=/usr
	make install-headers DESTDIR=$SYSROOT

Build and install the Compiler Runtime

As we want to cross compile the compiler runtime with the CHERI compiler we need a toolchain.cmake file that points to the LLVM compiler. You can create one like this:

	TCFLAGS="-target $TARGET \
		$MARCH $MABI -mlittle-endian \
		--sysroot=$SYSROOT \
		-fuse-ld=lld -nostdlib"
	cat <<EOF >toolchain.cmake
	set(CMAKE_SYSTEM_NAME Linux)
	set(CMAKE_SYSTEM_PROCESSOR riscv64)
	set(CMAKE_C_COMPILER $INSTALL/bin/clang)
	set(CMAKE_CXX_COMPILER $INSTALL/bin/clang++)
	set(CMAKE_ASM_COMPILER $INSTALL/bin/clang)
	set(CMAKE_SYSROOT $SYSROOT)
	set(CMAKE_C_FLAGS "$TCFLAGS")
	set(CMAKE_CXX_FLAGS "$TCFLAGS")
	set(CMAKE_ASM_FLAGS "$TCFLAGS")
	set(CMAKE_C_LINK_FLAGS "$TCFLAGS")
	set(CMAKE_CXX_LINK_FLAGS "$TCFLAGS")
	EOF

Then go the LLVM source directory and build the compiler runtime like this:

	cmake -G Ninja -B buildrt -S $PWD/compiler-rt/ \
		-DCMAKE_TOOLCHAIN_FILE=$PWD/toolchain.cmake \
		-DCMAKE_BUILD_TYPE=Release \
		-DCMAKE_INSTALL_PREFIX=$($INSTALL/bin/clang -print-resource-dir) \
		-DCOMPILER_RT_STANDALONE_BUILD=OFF \
		-DCOMPILER_RT_DEFAULT_TARGET_ONLY=ON \
		-DCMAKE_C_COMPILER_TARGET=riscv64-unknown-linux-gnu \
		-DCOMPILER_RT_BUILD_CRT=ON \
		-DCOMPILER_RT_BUILD_STANDALONE_LIBATOMIC=ON \
		-DLLVM_ENABLE_PER_TARGET_RUNTIME_DIR=ON \
		-DCMAKE_POSITION_INDEPENDENT_CODE=ON \
		-DLLVM_LIBDIR_SUFFIX= \
		-DLLVM_CMAKE_DIR=$INSTALL/lib/cmake/llvm \
		-DCOMPILER_RT_INCLUDE_TESTS=OFF \
		-DCOMPILER_RT_BUILD_XRAY=OFF \
		-DCOMPILER_RT_BUILD_SANITIZERS=OFF \
		-DCOMPILER_RT_BUILD_MEMPROF=OFF \
		-DCOMPILER_RT_BUILD_LIBFUZZER=OFF \
		-DCOMPILER_RT_BUILD_PROFILE=OFF \
		-DCOMPILER_RT_BUILD_CTX_PROFILE=OFF
	ninja -C buildrt install

Build and install musl

In the musl source directory run

	INC=$( $INSTALL/bin/clang -print-resource-dir)/include
	XFLAGS="-target $TARGET \
		$MARCH $MABI -mlittle-endian \
		--sysroot=$SYSROOT \
		-isystem $INC \
		-fuse-ld=lld"
	./configure \
		CC="$INSTALL/bin/clang $XFLAGS" \
		LD=$INSTALL/bin/lld \
		--enable-shared --enable-static \
		--prefix=/
	make \
		CC="$INSTALL/bin/clang $XFLAGS" \
		LD=$INSTALL/bin/lld \
		DESTDIR=$SYSROOT \
		install

Build your first Binary

Now you should be able to cross compile a standard HelloWorld.c like this:

	$INSTALL/bin/clang -target $TARGET $MARCH $MABI \
		--sysroot $SYSROOT -fuse-ld=lld \
		-o HelloWorld HelloWorld.c