Project NM

Instructions

The goal of this project is to recode the GNU nm binary, without options for the mandatory part, with a few for bonuses (see below).
As usual at 42 we are not allowed any library. The only functions that we are allowed are: open, close, mmap, munmap, write, fstat, malloc, free, exit, perror, strerror, getpagesize.

This project relies heavily on data structures and parsing of a binary file.
While it may seem simple at first, it requires a lot of understanding of the ELF filetype as well as a lot of error control.

To any student who would like to go this path:

While you already met elf files all along your cursurs perhaps you have never really tried to look at how it was built. Therefore a good first step would be man elf.

You will learn about:

ELF header

	typedef struct {
	unsigned char e_ident[EI_NIDENT];
	uint16_t		e_type;
	uint16_t		e_machine;
	uint32_t		e_version;
	ElfN_Addr	 e_entry;
	ElfN_Off		e_phoff;
	ElfN_Off		e_shoff;
	uint32_t		e_flags;
	uint16_t		e_ehsize;
	uint16_t		e_phentsize;
	uint16_t		e_phnum;
	uint16_t		e_shentsize;
	uint16_t		e_shnum;
	uint16_t		e_shstrndx;
	} ElfN_Ehdr;

Program header

	typedef struct {
		uint32_t	p_type;
		Elf32_Off	p_offset;
		Elf32_Addr p_vaddr;
		Elf32_Addr p_paddr;
		uint32_t	p_filesz;
		uint32_t	p_memsz;
		uint32_t	p_flags;
		uint32_t	p_align;
	} Elf32_Phdr;

	typedef struct {
		uint32_t	p_type;
		uint32_t	p_flags;
		Elf64_Off	p_offset;
		Elf64_Addr p_vaddr;
		Elf64_Addr p_paddr;
		uint64_t	p_filesz;
		uint64_t	p_memsz;
		uint64_t	p_align;
	} Elf64_Phdr

Section header

	typedef struct {
		uint32_t	sh_name;
		uint32_t	sh_type;
		uint32_t	sh_flags;
		Elf32_Addr sh_addr;
		Elf32_Off	sh_offset;
		uint32_t	sh_size;
		uint32_t	sh_link;
		uint32_t	sh_info;
		uint32_t	sh_addralign;
		uint32_t	sh_entsize;
	} Elf32_Shdr;

	typedef struct {
		uint32_t	sh_name;
		uint32_t	sh_type;
		uint64_t	sh_flags;
		Elf64_Addr sh_addr;
		Elf64_Off	sh_offset;
		uint64_t	sh_size;
		uint32_t	sh_link;
		uint32_t	sh_info;
		uint64_t	sh_addralign;
		uint64_t	sh_entsize;
	} Elf64_Shdr

String and symbol tables

	typedef struct {
		uint32_t		st_name;
		Elf32_Addr	st_value;
		uint32_t		st_size;
		unsigned char st_info;
		unsigned char st_other;
		uint16_t		st_shndx;
	} Elf32_Sym;

	typedef struct {
		uint32_t		st_name;
		unsigned char st_info;
		unsigned char st_other;
		uint16_t		st_shndx;
		Elf64_Addr	st_value;
		uint64_t		st_size;
	} Elf64_Sym;

And other types like relocation entries, Dynamic tags and notes

This is a daunting task, and obviously no errors or crash are allowed, but any file can be sent.

This file is a good entrypoint before writting your own packer and later on viruses.

Bonuses:

In order to score a higher grade than normally possible, the following options were to be implemented

    -a : display all symbols even debugger-only symbols
	-g : display only external (global) symbols
	-u : display only undefined symbols (external to each object file)
	-r : reverse sort
	-p : no-sort

Sorting tips:

I highly suggest before losing your mind on sorting algorithms to read a bit about locales LC_COLLATE and/or LC_ALL

Categorising the symbols:

As usual at 42, we are not allowed any libs in this exercise, therefore we are not allowed to rely on binary file descriptor library libbfd that handles this part for nm. Fully understanding the categories (I have to say "arbitrary") used in nm was a real challenge and led to a ton of trial/error with custom built binaries. Good luck to you if you go down this path.