get_key.c

// This code demonstrate how to break a simple disassembler.
// you may need to build with "-no-pie"
// gcc -o get_key get_key.c -no-pie
#include <stdio.h>

const char reveal_key[] = "The key is '%x'\n";

int get_key(int argc) {
    int key = 0;
  __asm__(
      // sub 1 because argc is almost never 0
      "    sub $1, %1\n"
      // makeshift switch statement based on the argc
      "    cmp $2, %1\n"
      "    ja end\n"
      "    shl $2, %1\n"
      "    add $offsets, %1\n"
      "    mov $0,%%rdi\n"
      "    mov (%1),%%edi\n"
      "    jmp *%%rdi\n"
      // I will put the jmp table in the middle of the function LOL
      "offsets:\n"
      "    .long zero\n"
      "    .long one\n"
      "    .long two\n"
      // just data that is impossible to disassembly with x86_64
      "    .long 0xFFFFB8FF\n"
      "    .long 0xFFFFB8FF\n"
      "    .long 0xFFFFB8FF\n"
      "    .long 0xFFFFB8FF\n"
      "zero:\n"
      "    mov $0xDEADBEFF, %0\n"
      "    jmp end\n"
      "one:\n"
      "    mov $0xDEC0C0FF, %0\n"
      "    jmp end\n"
      "two:\n"
      "    mov $0x1337, %0\n"
      "end:\n"
      : "=r" (key)
      : "r" (argc)
      : "%rdi", "%edi");
  return key;
}

int main(int argc, char **argv) {
  printf("argc %d\n", argc);
  int key = get_key(argc);
  printf("key %x\n", key);
  return 0;
}