dynamic-loader

A simple dynamic library loader for Linux capable of loading simple binaries using simple shared libraries.

Features

• Actually readable codebase for a dynamic loader
• Loads simple shared objects (no glibc or musl support*)
• Calls initializers and finalizers
• Performs relocations at load-time (no lazy loading supported yet)
• Supports dlopen(), dlsym(), and dlclose()
• no support for threads or TLS, though rudimentary thread-safety is supported in the loader

* glibc and musl provide their own dynamic library loader that they expect to be loaded with:

• ld-linux-x86_64.so.2 (aka ld.so) for glibc
• ld-musl-x86_64.so.1 (a symlink to musl's libc.so) for musl

Dependencies

• libmusl.a and musl-gcc (arch: musl, debian: musl musl-tools)
• elf.h (arch: part of base, debian: libelf-dev)

Building

• cmake -B build
• cd build
• make
• run build/loader/libloader.so or any of the binaries in build/tests/bin/

Resources

This loader was written mostly by using specifications from refspecs.linuxbase.org, mainly the ELF spec and the x86_64 processor supplement.

For some parts of the early loader startup code (mainly the code in crt/), I also read the source code of musl libc to get a better understanding of typical ways to handle early loader init. To understand ELF symbol hash tables, I used multiple sources, but this blog post on flagpenguin.me was most useful.