Re: Understanding Linux addr space, malloc, and heap

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

Arjan van de Ven wrote:
> On Fri, 2005-10-21 at 09:45 -0400, Vincent W. Freeh wrote:
> 
>>Thanks for your quick response.  It basically confirmed that I observed 
>>what I thought I did.  However, I am no closer to solving my problem.  I 
>>cannot mprotect data that I malloc beyond the first 65 pages.
> 
> 
> you can't mprotect malloc() memory period ..

Actually, I can and do.  Simple program at end.

> 
>>  Why is 
>>that?  Can that be fixed?  Second, why does mprotect silently fail?  I 
>>could live with it failing--but I cannot deal with a call the "works" 
>>but doesn't work.
> 
> 
> need more info :)
> 

I call mprotect and it return 0--meaning it succeeded.  But the 
permissions on the page remain rw.  So it fails to change the 
permissions, but doesn't give any indication of this.

Thanks,
vince.

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

int main(int argc, char *argv[])
{
   void *p;
   int pgsize = getpagesize();

   p = malloc(1024);
   mprotect((void*)((unsigned)p & ~(pgsize-1)), 1024, PROT_NONE);
   printf("\t*p = %d\n", *(int *)p);
   return 0;
}