Tiny gcc compiled binary experiment

Tiny gcc compiled binary experiment

.gitignore

main

main.c

int main();

void exit(int x) {
	asm("movl %0, %%ebx" : : "r"(x) :);
	asm("movl $1, %eax");
	asm("int $0x80");
	while(1);
}

void _start() {
	exit(main());
}

int main() {
	return 42;
}

Makefile

BITS ?= 32

.PHONY: main clean test

main:
	gcc -o main main.c -nodefaultlibs -nostdlib -m$(BITS) -nostartfiles -Os -s -static -Wl,--build-id=none -Wl,--nmagic
	strip --remove-section=.comment --remove-section=.eh_frame --strip-all main
	sstrip -z main
	python strip.py main

clean:
	rm main

test: main
	@stat --printf="Size: %s Byte\n" main
	@./main; if [ $${?} = 42 ]; then echo "Seems good :)"; else echo "Seems broken :-/"; fi

strip.py

#!/usr/bin/env python3

import sys
import struct

binary = open(sys.argv[1], "rb").read()

is_64bit = binary[0x04] == 0x02

elf_header_names = ['e_ident[EI_MAG]', 'e_ident[EI_CLASS]', 'e_ident[EI_DATA]', 'e_ident[EI_VERSION]', 'e_ident[EI_OSABI]', 'e_ident[EI_ABIVERSION]', 'e_ident[EI_RESERVED]', 'e_type', 'e_machine', 'e_version', 'e_entry', 'e_phoff', 'e_shoff', 'e_flags', 'e_ehsize', 'e_phentsize', 'e_phnum', 'e_shentsize', 'e_shnum', 'e_shstrndx']

elf_header = {}
program_header = {}

if is_64bit:
    elf_header_format = "<IBBBBB7sHHIQQQIHHHHHH"
    program_header_format = "<IIQQQQQQ"
    program_header_names = ['p_type', 'p_flags', 'p_offset', 'p_vaddr', 'p_paddr', 'p_filesz', 'p_memsz', 'p_align']
else:
    elf_header_format = "<IBBBBB7sHHIIIIIHHHHHH"
    program_header_format = "<IIIIIIII"
    program_header_names = ['p_type', 'p_offset', 'p_vaddr', 'p_paddr', 'p_filesz', 'p_memsz', 'p_flags', 'p_align']

elf_header_length = struct.calcsize(elf_header_format)
program_header_length = struct.calcsize(program_header_format)

elf_header_values = list(struct.unpack(elf_header_format, binary[0x00:0x00+elf_header_length]))
program_header_values = list(struct.unpack(program_header_format, binary[elf_header_length:elf_header_length+program_header_length]))

for i in range(len(elf_header_names)):
    elf_header[elf_header_names[i]] = elf_header_values[i]

for i in range(len(program_header_names)):
    program_header[program_header_names[i]] = program_header_values[i]

program_offset = elf_header_length + elf_header['e_phnum'] * program_header_length
program = binary[program_offset:program_offset+program_header['p_filesz']]

elf_header['e_entry'] -= program_header_length
elf_header['e_phnum'] = 1
elf_header['e_ident[EI_RESERVED]'] = b"METTIGL"

program_header['p_offset'] -= program_header_length
program_header['p_vaddr'] -= program_header_length
program_header['p_paddr'] -= program_header_length

for i, e_name in enumerate(elf_header_names):
    elf_header_values[i] = elf_header[e_name]

for i, p_name in enumerate(program_header_names):
    program_header_values[i] = program_header[p_name]

new_elf_header = struct.pack(elf_header_format, *elf_header_values)
new_program_header = struct.pack(program_header_format, *program_header_values)
new_binary = new_elf_header + new_program_header + program

print("ELF header size: %d" % len(new_elf_header))
print("Program header size: %d" % len(new_program_header))
print("Program size: %d" % len(program))
print("Total size: %d" % len(new_binary))

open(sys.argv[1], "wb").write(new_binary)