closure in C, inspired by libffcall

mycallback.c

#include <stdint.h>
#include <stdio.h>
#include <string.h>
#include <sys/mman.h>
#include <unistd.h>

#include "mycallback.h"

static uint8_t codes[] = {
  0x48, 0xc7, 0xc0, 0x00, 0x00, 0x00, 0x00, /* mov $0x00000000,%rax */
  0x48, 0x89, 0xfe,                         /* mov %rdi, %rsi */
  0x48, 0xc7, 0xc7, 0x00, 0x00, 0x00, 0x00, /* mov $0x00000000,%rdi */
  0xff, 0xe0,                               /* jmpq *%rax */
};

void *alloc_callback(void *func, void *data) {
  const size_t codes_len = sizeof(codes);
  const long pageSize = sysconf(_SC_PAGESIZE);

  uint8_t *callback = mmap(NULL, codes_len, PROT_READ|PROT_WRITE, MAP_ANONYMOUS|MAP_PRIVATE, -1, 0);
  if (callback == MAP_FAILED) {
    perror("mmap");
    return NULL;
  }

  memcpy(callback, codes, codes_len);

  callback[3] = (uintptr_t)func&0xFFu;
  callback[4] = ((uintptr_t)func>>8)&0xFFu;
  callback[5] = ((uintptr_t)func>>16)&0xFFu;
  callback[6] = ((uintptr_t)func>>24)&0xFFu;

  callback[13] = (uintptr_t)data&0xFFu;
  callback[14] = ((uintptr_t)data>>8)&0xFFu;
  callback[15] = ((uintptr_t)data>>16)&0xFFu;
  callback[16] = ((uintptr_t)data>>24)&0xFFu;

  if (mprotect((void*)((uintptr_t)callback&~(pageSize-1)), pageSize, PROT_READ|PROT_EXEC) < 0) {
    perror("mprotect");
    return NULL;
  }

  return callback;
}

void free_callback(void *func) {
  if (munmap(func, sizeof(codes)) < 0) {
    perror("munmap");
  }
}

mycallback.h

#ifndef _CALLBACK_H
#define _CALLBACK_H

extern void *alloc_callback(void *func, void *data);
extern void free_callback(void *func);

#endif /* _CALLBACK_H */

mycallback_main.c

#include <stdio.h>
#include <stdlib.h>

#include "mycallback.h"

static int sum(int *data, int arg) {
  *data += arg;
  return *data;
}

int main() {
  int *data1 = (int *)malloc(sizeof(*data1));
  *data1 = 1;
  int *data2 = (int *)malloc(sizeof(*data2));
  *data2 = 1;

  int (*mysum1)(int) = alloc_callback(sum, data1);
  if (mysum1 == NULL) {
    printf("failed to allocate callback.\n");
    return 1;
  }

  int (*mysum2)(int) = alloc_callback(sum, data2);
  if (mysum2 == NULL) {
    printf("failed to allocate callback.\n");
    return 1;
  }

  mysum1(2);
  mysum1(3);
  int sum1 = mysum1(4);

  mysum2(2);
  mysum2(4);
  int sum2 = mysum2(8);

  printf("sum1: %d\n", sum1);
  printf("sum2: %d\n", sum2);

  free_callback(mysum1);
  free_callback(mysum2);
 
  return 0;
}