CODEWITHSUNDEEP
cassemblyx86segmentation-faultmmap
xorsema
xorsema

Reputation: 21

mmap related segmentation fault

I am writing an emulator and I need to write machine code directly to memory, then jump to (call) it. I allocate the memory with mmap, and then write the code to it. Anything other than a "ret" or "nop" instruction segfaults. I know mmap returns without error, and I have isolated the problem in an example I put together to illustrate the issue.

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

int main()
{
  uint8_t *data = mmap(NULL, 3 * sizeof(uint8_t), PROT_EXEC|PROT_WRITE, MAP_ANONYMOUS|MAP_PRIVATE, -1, 0), *p;
  p = data;
  if(data) {
    *p++ = 0xb8; //mov $1, %eax
    *p++ = 0x01;
    *p++ = 0xC3; //ret
  } else
    perror("mmap");

  uint8_t (*fp)();
  fp = (void*) (data);
  printf("%u\n",(uint8_t) fp());
  return 0;
}

That example produces the problem. Help?

EDIT:I should mention I'm on Linux 2.6, x86.

Upvotes: 0

Views: 1591

Answers (1)

Tobias Schlegel
Tobias Schlegel

Reputation: 3970

Your opcode is incorrect, this works:

if(data) {
    *p++ = 0xb8; //mov $1, %eax
    *p++ = 0x01;
    *p++ = 0x00;
    *p++ = 0x00;
    *p++ = 0x00;
    *p++ = 0xC3; //ret
}

0xb8 is moving a 32bit immediate into eax, so you have to specify all 4 bytes.

Upvotes: 5

Related Questions