hey! don't do this!

cursed.c

#include <stdio.h>
#include <string.h>
#include <stdint.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/io.h>
#include <sys/mman.h>
#include "elf.h"

void *address_of(char *elf, char *name) {
	int fd = open(elf, O_RDONLY);

	struct stat s;
	fstat(fd, &s);

	uint8_t *buf = mmap(0, s.st_size, PROT_READ, MAP_PRIVATE, fd, 0);

	Elf64_Ehdr *ehdr = (Elf64_Ehdr *) buf;
	Elf64_Shdr *shdr = (Elf64_Shdr *) (buf + ehdr->e_shoff);
	for (int i = 0; i < ehdr->e_shnum; ++i) {
		if (shdr[i].sh_type == SHT_SYMTAB) {
			char *strtab = (char *) (buf + shdr[shdr[i].sh_link].sh_offset);
			Elf64_Sym *symtab = (Elf64_Sym *) (buf + shdr[i].sh_offset);
			int symtab_len = shdr[i].sh_size / sizeof(Elf64_Sym);

			for (int i = 0; i < symtab_len; ++i) {
				if (strcmp(strtab + symtab[i].st_name, name) == 0) {
					return (void *) symtab[i].st_value;
				}
			}
		}
	}

	return NULL;
}

void *func_of(char *elf, char *name) {
	int main(int argc, char *argv[]);
	return address_of(elf, name) + ((void *) main - address_of(elf, "main"));
}

void foo() {
	printf("welcome to foo\n");
}

int bar(int x) {
	return 2 * x + 1;
}

int main(int argc, char *argv[]) {
	void (*foo2)() = func_of(argv[0], "foo");
	printf("foo (compiled) = %p\n", foo);
	printf("foo (dynamic)  = %p\n", foo2);
	foo2();

	int (*bar2)(int) = func_of(argv[0], "bar");
	printf("bar (compiled) = %p\n", bar);
	printf("bar (dynamic)  = %p\n", bar2);
	printf("bar(1) (compiled) = %d\n", bar(1));
	printf("bar(1) (dynamic)  = %d\n", bar2(1));

	return 0;
}