ELF Fun (🧝?)

I am working on a compiler and wanted to write the object file from scratch instead of using assembly as an intermediary language.

This is the result of messing around while following a tutorial for libelf.

Structure

There are 4 programs that I've written, each more useful than the last!

phdr - Prints all the Program Headers of an ELF

• Not useful to my end goal since relocatable ELFs don't have Program Headers, but still useful to know.

shdr - Prints all the Section Headers + some data associated of an ELF

• Will print the symbol table and the section header string table data too
• Some other random bits will get printed too because I was debugging my generated binary which brings me too...

write - Writes an object file/relocatable ELF with a small amount of assembly

• This is the thing I was after
• Program generated exits with unique status code to make sure its running
• Linkable with ld

prog - Prints the symbols of a file

• Made this so that I could debug why I couldn't print symbols in my program
• Turned it into a fun exercise in manually indexing the strtab buffer to map the symbol names instead of using elf_strptr()

Compiling

make

All binaries are in the out/ folder.

All objects are in the obj/ folder.

Compatability

This commit fixed the problem with certain linkers not working.

Thank god to LLVM's lld which told me what I was doing wrong.

Tested Linkers

• ld ✅
• lld ✅
• mold ✅
• Maybe 🫵 YOU could find a linker that fails...

Thanks to...

• Joseph Koshy's libelf by Example
• libelf
• OSDev Wiki's ELF Tutorial
• k3170's article Introduction to the ELF Format (Part VI) : The Symbol Table and Relocations (Part 1)
• And many random forums I stumbled across while hacking this together