how can I protect a heap memory in linux?

Question

I want to make a chunck of heap memory read-only. For that I have tried with memalign() with mprotect().But from the memalignment what can I get , memalign allocates memory away from the process heap.

I want to make some portion of the heap read-only. Any help on that ?

malloc()->mmap()->mprotect() a hypothetical thought , but not sure if that can help ... Any sample code to implement above ?

I need to protect the memory address within the heap. with malloc() i get address around 0x10012008 whereas with mmap() it is 0xf7ec9000.My intention is to make a part of heap-meory to be read only to catch any trampler that might try to run through that heap.

Answer 1

You should use mmap() directly and drop malloc() entirely. And, depending on your needs, you may not need mprotect() at all:

ptr = mmap(NULL, length, PROT_READ, MAP_PRIVATE | MAP_ANONYMOUS, 0, 0);

On recent kernels and libc implementations this will allocate the requested amount of memory with the specified protection mode - in this case the allocated memory area can only be read, but not written. If you only need a bunch of zero pages, that would do. Otherwise, the resulting area will be aligned properly and you can use mprotect() to unprotect it for short periods of time in a controlled manner...

Answer 2

Yes, mmap and mprotect are the right functions. I do not understand what's the problem with your current approch, i.e., what you mean by "For that I have tried with memalign() with mprotect().But from the memalignment what can I get , memalign allocates memory away from the process heap."

Below is an example how to create a write-protected memory area:

#include <fcntl.h>  
#include <signal.h>
#include <stdio.h>
#include <string.h>
#include <sys/mman.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <unistd.h>

static int alloc_size;
static char* memory;

void segv_handler (int signal_number)  {
 printf ("memory accessed!\n");
 mprotect (memory, alloc_size, PROT_READ | PROT_WRITE);
} 

int main () {
 int fd;
 struct sigaction sa;

 /* Install segv_handler as the handler for SIGSEGV. */
 memset (&sa, 0, sizeof (sa));
  sa.sa_handler = &segv_handler;
 sigaction (SIGSEGV, &sa, NULL);

 /* Allocate one page of memory by mapping /dev/zero. Map the memory
 as write-only, initially. */
  alloc_size = getpagesize ();
 fd = open ("/dev/zero", O_RDONLY);
  memory = mmap (NULL, alloc_size, PROT_WRITE, MAP_PRIVATE, fd, 0);
  close (fd);
  /* Write to the page to obtain a private copy. */
  memory[0] = 0;
 /* Make the memory unwritable. */
  mprotect (memory, alloc_size, PROT_NONE);

 /* Write to the allocated memory region. */
 memory[0] = 1;

  /* All done; unmap the memory. */
 printf ("all done\n");
 munmap (memory, alloc_size);
 return 0;
}