Pre-RFC: To effortlessly develop seL4 systems in rust

[kent-mcleod2]

@nspin

I’ve noted that build systems for Rust seL4 userspaces would be simpler if the elfloader didn’t have to be rebuilt to embed the root task image. I’ve proposed adding support for the elfloader to be passed the root task image at runtime as a potential solution. Now, I’ve come up with an idea for what I believe might be a stronger solution.

My idea is to first build the elfloader without a CPIO payload, as before. Then, at a different phase of the build, once the CPIO payload, including the root task, is assembled, the empty elfloader image is rendered into a new ELF file along with the CPIO payload. A few variables in the elfloader part of this new aggregate image are then modified to point to the payload.

I agree that removing the need to rebuild the elfloader from source each time would simplify development.

Just briefly documenting an additional approach before I forget about it: merge all of the object files into an intermediate object file using -Wl,--relocatable. Then linking in the CPIO archive would only require linking the single elfloader.o with a cpio.o file + a platform specific linker script.
CMake snippet: support creating relocatable object target in add_library (#16977) · Issues · CMake / CMake · GitLab