question for the unix gurus about memory protection, etc.

Sunday, 23 September 2001

        Examine the following code:

#if defined (WIN32_NATIVE) || defined (CYGWIN)

/* Return whether all bytes in the specified memory block can be read. */
int
debug_can_access_memory (void *ptr, Memory_Count len)
{
  return !IsBadReadPtr (ptr, len);
}

#else /* !(defined (WIN32_NATIVE) || defined (CYGWIN)) */

/* #### There must be a better way!!!! */

static JMP_BUF memory_error_jump;

static SIGTYPE
debug_memory_error (int signum)
{
  EMACS_REESTABLISH_SIGNAL (signum, debug_memory_error);
  EMACS_UNBLOCK_SIGNAL (signum);
  LONGJMP (memory_error_jump, 1);
}

/* Return whether all bytes in the specified memory block can be read. */
int
debug_can_access_memory (void *ptr, Memory_Count len)
{
  /* Use volatile to protect variables from being clobbered by longjmp. */
  SIGTYPE (*volatile old_sigbus) (int);
  SIGTYPE (*volatile old_sigsegv) (int);
  volatile int old_errno = errno;
  volatile int retval = 1;

  if (!SETJMP (memory_error_jump))
    {
      old_sigbus =
	(SIGTYPE (*) (int)) EMACS_SIGNAL (SIGBUS, debug_memory_error);
      old_sigsegv =
	(SIGTYPE (*) (int)) EMACS_SIGNAL (SIGSEGV, debug_memory_error);

      if (len > 1)
	/* If we can, try to avoid problems with super-optimizing compilers
	   that might decide that memcmp (ptr, ptr, len) can be optimized
	   away since its result is always 1. */
	memcmp (ptr, (char *) ptr + 1, len - 1);
      else
	memcmp (ptr, ptr, len);
    }
  else
    retval = 0;
  EMACS_SIGNAL (SIGBUS, old_sigbus);
  EMACS_SIGNAL (SIGSEGV, old_sigsegv);
  errno = old_errno;

  return retval;
}

#endif /* defined (WIN32_NATIVE) || defined (CYGWIN) */

My general solution for Unix, while [hopefully, i.e. i haven't tested it]
something that actually works, is clearly pretty clunky.  Can anyone suggest a
better way, even if it only works on some systems?

I notice, for example, that there's an mprotect[] system call under Linux and
others that lets you change the protections of a page of memory; but there's no
corresponding call I could find to retrieve the current memory settings.

ben

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