elf-from-scratch

This was a challenge to myself: build an ELF64 file from scratch without using a toolchain (other than an assembler to get raw x86_64 instructions)

main.py will write to stdout an executable ELF64 file with a single program header table entry, and no sections.

main.py currently hardcodes the program to write- it's "hello world". See asm/hello.asm for a program listing.

go.sh will run the main script, call readelf -a on the output, and run the ELF64 file.

Here's what readelf has to say about the produced ELF:

	ELF Header:
		Magic:   7f 45 4c 46 02 01 01 00 00 00 00 00 00 00 00 00
		Class:                             ELF64
		Data:                              2's complement, little endian
		Version:                           1 (current)
		OS/ABI:                            UNIX - System V
		ABI Version:                       0
		Type:                              EXEC (Executable file)
		Machine:                           Advanced Micro Devices X86-64
		Version:                           0x1
		Entry point address:               0x400078
		Start of program headers:          64 (bytes into file)
		Start of section headers:          0 (bytes into file)
		Flags:                             0x0
		Size of this header:               64 (bytes)
		Size of program headers:           56 (bytes)
		Number of program headers:         1
		Size of section headers:           0 (bytes)
		Number of section headers:         0
		Section header string table index: 0

	There are no sections in this file.

	There are no sections to group in this file.

	Program Headers:
		Type           Offset             VirtAddr           PhysAddr
									 FileSiz            MemSiz              Flags  Align
		LOAD           0x0000000000000000 0x0000000000400000 0x0000000000400000
									 0x000000000000002f 0x000000000000002f  RWE    0x4

	There is no dynamic section in this file.

	There are no relocations in this file.

	The decoding of unwind sections for machine type Advanced Micro Devices X86-64 is not currently supported.

	Dynamic symbol information is not available for displaying symbols.

	No version information found in this file.

TODO

• Why does my program header table entry have to have an offset of 0? I get segfaults otherwise :(
  • Might be due to this - seems like p_offset must be page aligned?

references

Links I found useful while making this:

• ELF-64 Object File Format
• Linux X86_64 syscalls table

useful commands

see util/ folder for handy scripts using the below commands.

nasm assembler

# compile only x86_64 instructions
nasm -f bin -o out.bin in.asm

# compile to linkable elf
nasm -f elf64 -o out.o in.asm

link out.o file without stdlib and gcc cruft:

gcc out.o -o out.elf -nostartfiles -nostdinc -nostdlib