Help with mprotect and writing on code section

hi,
I am trying to write on the code section of an executing C program. I tried using mprotect to get PROT_WRITE access to a section in memory and tried to write bytecode on it. It doesnt seem to write on that section. The code is:

Code:

---

#ifndef PAGESIZE
#define PAGESIZE 4096
#endif
unsigned char buffer[1024];

int blank();
int main()
{
 unsigned  long *p;
 unsigned char c;
 unsigned long *d;
 int i;
 p = ␣

p = (unsigned long *)(((int) p + PAGESIZE-1) & ~(PAGESIZE-1));
 if (mprotect(p,12,PROT_WRITE))
  {
  perror("Could not mprotect");
  exit(errno);
  }
  p[0] = 0xccccc3c9; // basically I was trying to make it return without executing
  p[1] = 0x08a14097;
d= ␣
printf(" written values %x %x",p[0],p[1]);
printf(" written values %x %x",d[0],d[1]);
.....
return 0;
}

int blank()
{
  int index=0;
  int i;
  unsigned char *p;
  p = &main;
  printf("here");
 for (i=0;i<=203;i++)
  buffer[index++] = p[i];
  buffer[index]='\0';
  return 0;
}

---

The first print returns: ccccc3c9 8a14097.
The second print returns: 83e58955 45c718ec // these are the values in blank() on objdump of the binary.

Can someone help me on this, or is there any easier way to make a running program write byte code onto itself?
Regards,
Raghu

I figured a small portion of the problem. p starts pointing to a different location after the alignment.
so I did the following:

Code:

---

#ifndef PAGESIZE
#define PAGESIZE 4096
#endif
unsigned char buffer[1024];
int blank();

int main()
{
 unsigned  long *p;
 unsigned char c;
 unsigned long *d;
 int i;
 p = ␣
 d  = (unsigned long *)(((int) p + PAGESIZE-1) & ~(PAGESIZE-1));

 if (mprotect(d,1024,PROT_WRITE))
  {
  perror("Could not mprotect");
  exit(errno);
  }
  printf("before");
  printf("address contained in p %x add cotained in d %x", p, d);
 fflush(NULL);
 p[0] = 0xcc;
  printf("after");
 fflush(NULL);
 return 0;
 }
int blank()
{
}

---

This results in
p = 8048590
d = 8049000
Can someone tell me how to align d such that it does not come from one page after p?
Thanks

And what exactly are you hoping to learn from this?

Assuming you are modifying x86 machine code [with 0xcc being int3, I expect that you are], perhaps you should use unsigned char *, instead of unsigned long?

Also, you may want to add some more parenthesis to your alignment calculation [assuming you are insisting on using unsigned long *], so that when you add 1024, you are not actually adding 1024 * sizeof(unsigned long).

I too ponder what you are actually wanting to achieve by doing this...

--
Mats