A tiny x86_x64 JIT whiout use mmap on Linux

Question

I am implementing a JIT (for studies) and I would like to know if it is possible to run opcodes, without using mmap, since I am 'playing' on an operating system that MMAP does not have the MAP_ANONYMOUS definition for.

The JIT code:

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

int main () {
  // Hexadecimal x86_64 machine code for: int mul (int a, int b) { return a * b; }
  unsigned char code [] = {
    0x55,                   // push rbp
    0x48, 0x89, 0xe5,       // mov  rbp, rsp
    0x89, 0x7d, 0xfc,       // mov  DWORD PTR [rbp-0x4],edi
    0x89, 0x75, 0xf8,       // mov  DWORD PTR [rbp-0x8],esi
    0x8b, 0x75, 0xfc,       // mov  esi,DWORD PTR [rbp-04x]
    0x0f, 0xaf, 0x75, 0xf8, // imul esi,DWORD PTR [rbp-0x8]
    0x89, 0xf0,             // mov  eax,esi
    0x5d,                   // pop  rbp
    0xc3                    // ret
  };

  // allocate executable memory via sys call
  void* mem = mmap(NULL, sizeof(code), PROT_WRITE | PROT_EXEC, MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);

  // copy runtime code into allocated memory
  memcpy(mem, code, sizeof(code));

  // typecast allocated memory to a function pointer
  int (*func) () = mem;

  // call function pointer
  printf("%d * %d = %d\n", 5, 11, func(5, 11));

  // Free up allocated memory
  munmap(mem, sizeof(code));
}

Brazil

If it possible if the page containing the array is executable. Is it? – Gills

Answer 1

If I recall correctly, before the invention of MAP_ANONYMOUS, the standard way to map non-file-backed memory was to open /dev/zero and map it like a file. If your system has mmap at all, I would expect this to work.

Otherwise, you can allocate memory any way you like, and use mprotect to make it executable. Note that you will want to ensure that your code is aligned and padded to page boundaries, so that you don't inadvertently change the protections on other parts of the program. posix_memalign is a good way to do this, or the older valloc.

Otology