delay_puts.c

#include <stdio.h>
#include <stddef.h>
#include <string.h>
#include <sys/mman.h>

typedef int (delayed_puts_t)(void);
extern delayed_puts_t puts_helper;
extern size_t puts_helper_size;

__asm__ (
    "puts_helper:\n"
    "endbr64\n"
    "mov puts_helper_size(%rip), %rdi\n"
    "jmp *puts_helper_size+8(%rip)\n"
    "puts_helper_size:\n"
    ".8byte .-puts_helper"
);

delayed_puts_t *delay_puts(const char *s) {
    int (*fp)(const char *) = puts;
    delayed_puts_t *mem = mmap(NULL, puts_helper_size + sizeof s + sizeof fp, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0);
    memcpy(mem, &puts_helper, puts_helper_size);
    memcpy(mem + puts_helper_size, &s, sizeof s);
    memcpy(mem + puts_helper_size + sizeof s, &fp, sizeof fp);
    mprotect(mem, puts_helper_size + sizeof s + sizeof fp, PROT_READ|PROT_EXEC);
    return mem;
}

int main(void) {
    delayed_puts_t *puts_world = delay_puts("world");
    delayed_puts_t *puts_hello = delay_puts("hello");
    puts_hello();
    puts_world();
    return 0;
}