The Design of a Self-Compiling C Transpiler Targeting POSIX Shell

Software supply chain attacks are increasingly frequent and can be hard to guard against. Reproducible builds ensure that generated artifacts (executable programs) can be reliably created from their source code. However, the tools used by the build process are also vulnerable to supply chain attacks so a complete solution must also include reproducible builds for the various compilers used. With this problem as our main motivation we explore the use of the widely available POSIX shell as the only trusted pre-built binary for the reproducible build process. We have developed pnut, a C to POSIX shell transpiler written in C that generates human-readable shell code. Because the compiler is self-applicable, it is possible to distribute a human-readable shell script implementing a C compiler that depends only on the existence of a POSIX compliant shell such as bash, ksh, zsh, etc. Together, pnut and the shell serve as the seed for a chain of builds that create increasingly capable compilers up to the most recent version of the GNU Compiler Collection (GCC) that is a convenient basis to build any other required tool in the toolchain. The end result is a complete build toolchain built only from a shell and human-readable source files. We discuss the level of C language support needed to achieve our goal, the generation of portable POSIX shell code from C, and the performance of the compiler and generated code.