Example of code optimisation (remove libc)

README.md

root@laptop [02:00:10] [~/Documents/pentest/cours] 
-> # ls -alh opti printf-libc
-rwxr-xr-x 1 root root 1,5K juin  12 14:00 opti
-rwxr-xr-x 1 root root 726K juin  12 14:00 printf-libc

root@laptop [02:00:26] [~/Documents/pentest/cours] 
-> # ./opti arg1 arg2 arg3    
./opti
arg1
arg2
arg3

root@laptop [02:00:42] [~/Documents/pentest/cours] 
-> # ./printf-libc arg1 arg2 arg3
./printf-libc
arg1
arg2
arg3

opti.c

/*

gcc opti.c -o opti -nostdlib -nostdinc -static -fno-asynchronous-unwind-tables -O1
ls -alh opti -> 1,5K

// strip --strip-all ./a.out // -> remove debug symbols (func name, etc) -> 928

*/

#define STDOUT 1
#define WRITE 1
#define EXIT 60

int main_perso(int argc, char *argv[]);

static inline long syscall1(long syscall, long arg1);
static inline long syscall3(long syscall, long arg1, long arg2, long arg3);
int write(int fd, char *str);
int print(char *str);
void exit(int status);

/*
void _start(){
	int *argc = getArgcPtr();
	int *argv = argc + 8;
	int ret = main_perso(argc, argv);
	exit(ret);
}
*/

asm(
		".global _start\n"
		"_start:\n"
		"   xorl %ebp,%ebp\n"	   // mark outermost stack frame
		"   movq 0(%rsp),%rdi\n"	// get argc
		"   lea 8(%rsp),%rsi\n"	 // argv = %rbp + 8
		"   call main_perso\n"	   // call our main_perso
		"   movq %rax,%rdi\n"	   // take the main return code and use it as first argument for...
		"   call exit\n");


int main_perso(int argc, char *argv[]) {
	
	for(int i = 0; i < argc; ++i) {
		print(argv[i]);
		print("\n");
	}

	return 0;
}

int slen(char *str){
	int count = 0;
	while(str[count++] != 0);
	return count-1;
}

static inline long syscall1(long syscall, long arg1) {
	long ret;
	__asm__ __volatile__("syscall"
		: "=a" (ret)
		: "0" (syscall), "D" (arg1)
		: "memory");
	return ret;
}

static inline long syscall3(long syscall, long arg1, long arg2, long arg3) {
	long ret;
	__asm__ __volatile__("syscall"
		: "=a" (ret)
		: "0" (syscall), "D" (arg1), "S"(arg2), "d"(arg3)
		: "cc", "rcx", "r11", "memory");
	return ret;
}

int write(int fd, char *str){
	return syscall3(WRITE, fd, (long)str, slen(str));
}

int print(char *str){
	return write(STDOUT, str);
}

void exit(int status) {
	syscall1(EXIT, status);
}

printf-libc.c

/*
gcc printf-libc.c -o printf-libc -static
ls -alh printf-libc -> 726K
*/
#include <stdio.h>

int main(int argc, char const *argv[])
{
	for (int i = 0; i < argc; ++i)
	{
		printf("%s\n", argv[i]);
	}
	
	return 0;
}