Re: Understanding Linux addr space, malloc, and heap

["Vincent W. Freeh" <vin@csc.ncsu.edu>]

Clearly, it was a mistake to post that code.  I had no idea so many 
people would point out the bleeding obvious.

Here is a more elaborate version--that does the same thing, but more 
lines of code.  In it malloc'd memory is mprotect'd.  The program 
generates a SIGSEGV, a page fault.

----------------
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <sys/mman.h>

#include <limits.h>    /* for PAGESIZE */
#ifndef PAGESIZE
#define PAGESIZE 4096
#endif

int
main(void)
{
  char *p;
  char c;

  /* Allocate a buffer; it will have the default
     protection of PROT_READ|PROT_WRITE. */
  p = malloc(1024+PAGESIZE-1);
  if (!p) {
    perror("Couldn’t malloc(1024)");
    exit(errno);
  }

  /* Align to a multiple of PAGESIZE, assumed to be a power of two */
  p = (char *)(((int) p + PAGESIZE-1) & ~(PAGESIZE-1));

  c = p[666];         /* Read; ok */
  p[666] = 42;        /* Write; ok */

  /* Mark the buffer read-only. */
  if (mprotect(p, 1024, PROT_READ)) {
    perror("Couldn’t mprotect");
    exit(errno);
  }

  c = p[666];         /* Read; ok */
  p[666] = 42;        /* Write; program dies on SIGSEGV */

  exit(0);
}


Arjan van de Ven wrote:
> > But I can't mprotect the 66th page I malloc.  And mprotect fails SILENTLY!
>
>
> I'm not convinced it does that.. not until the bugs are out of the
> code.... since right now it mprotects the wrong stuff, which sometimes
> overlaps with what you malloced, sometimes not.
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