Re: back into the thread....

[Sterling Augustine <saugustine@google.com>]

This feature clearly works.

On Tue, Nov 12, 2013 at 2:08 PM, Mark Manning <mark4th@gmail.com> wrote:
> Got a reply from someone here about my problems with gdb but i cannot
> figure out how to reply all and googles reply always top posts my
> replies.  Also i still have the same issue in that i cannot execute
> code that is not part of the original executables object.  any
> additional code written into memory by my compiler is not executable
> yet the person who replied to me said that it SHOULD be possible as he
> does it all the time.
>
> this is a version of gdb running on an arm target (beagle board xm)
> under a gentoo linux install - is this a bug injected into gdb by some
> gentoo snafu?

If gdb doesn't think the memory is accessible, then it probably isn't.

The following test-case works perfectly well for gdb. You may want to
be sure that you are following all the correct steps in your code
generator, particularly the posix_memalign and mprotect. Otherwise
your code will take an unexpected segfault. I can set a breakpoint at
dst and stepi through it no problem.

The contents of bytes comes from compiling

int foo(int x) { return x; }

at O2 and then copying the resulting bytes into the array. You would
want to do something similar to get ARM results. Be sure it doesn't
have relocations.

=====

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

const char bytes[] = { 0x89, 0xf8, 0xc3 };
#define EXEC_BYTES sizeof(bytes)

typedef int(*function_ptr)(int);

int main(int argc, char *argv[])
{
  int test_val;
  int return_val;
  function_ptr dst = malloc(EXEC_BYTES);
  if (posix_memalign((void **) &dst, 4096*8, EXEC_BYTES) != 0) {
    printf("can't allocate.\n");
    exit (-1);
  }
  if (mprotect(dst, EXEC_BYTES, PROT_READ|PROT_WRITE|PROT_EXEC) != 0) {
    printf("can't mprotect\n");
    exit (-1);
  }

  if (argc > 1)
    test_val = atoi(argv[1]);

  memcpy(dst, bytes, EXEC_BYTES);

  return_val = dst(test_val);
  printf("return val was %d\n", return_val);
  return 0;

}