Code review comments from jeffpc
7029 want per-process exploit mitigation features (secflags)
7030 want basic address space layout randomization (aslr)
7031 noexec_user_stack should be a secflag
7032 want a means to forbid mappings around NULL.

*** 22,33 ****
  /*
   * Copyright 2008 Sun Microsystems, Inc.  All rights reserved.
   * Use is subject to license terms.
   */
  
- #pragma ident   "%Z%%M% %I%     %E% SMI"
- 
  #pragma weak _sbrk = sbrk
  #pragma weak _brk = brk
  
  #include "lint.h"
  #include <synch.h>
--- 22,31 ----
*** 38,53 ****
  #include <inttypes.h>
  #include <unistd.h>
  #include "mtlib.h"
  #include "libc.h"
  
! extern int _end;
! void *_nd = &_end;
  mutex_t __sbrk_lock = DEFAULTMUTEX;
  
! extern int _brk_unlocked(void *);
! extern void *_sbrk_unlocked(intptr_t);
  
  /*
   * The break must always be at least 8-byte aligned
   */
  #if (_MAX_ALIGNMENT < 8)
--- 36,50 ----
  #include <inttypes.h>
  #include <unistd.h>
  #include "mtlib.h"
  #include "libc.h"
  
! void *_nd = NULL;
  mutex_t __sbrk_lock = DEFAULTMUTEX;
  
! extern intptr_t _brk_unlocked(void *);
! void *_sbrk_unlocked(intptr_t);
  
  /*
   * The break must always be at least 8-byte aligned
   */
  #if (_MAX_ALIGNMENT < 8)
*** 85,96 ****
   *   - the addend is positive and brk + addend > ULONG_MAX
   */
  void *
  _sbrk_unlocked(intptr_t addend)
  {
!         char *old_brk = BRKALIGN(_nd);
!         char *new_brk = BRKALIGN(old_brk + addend);
  
          if ((addend > 0 && new_brk < old_brk) ||
              (addend < 0 && new_brk > old_brk)) {
                  errno = ENOMEM;
                  return ((void *)-1);
--- 82,100 ----
   *   - the addend is positive and brk + addend > ULONG_MAX
   */
  void *
  _sbrk_unlocked(intptr_t addend)
  {
!         char *old_brk;
!         char *new_brk;
! 
!         if (_nd == NULL) {
!                 _nd = (void *)_brk_unlocked(0);
!         }
! 
!         old_brk = BRKALIGN(_nd);
!         new_brk = BRKALIGN(old_brk + addend);
  
          if ((addend > 0 && new_brk < old_brk) ||
              (addend < 0 && new_brk > old_brk)) {
                  errno = ENOMEM;
                  return ((void *)-1);
*** 116,126 ****
  {
          uintptr_t old_brk;
          uintptr_t ret_brk;
          uintptr_t high_brk;
          uintptr_t new_brk;
!         int brk_result;
  
          if (!primary_link_map) {
                  errno = ENOTSUP;
                  return ((void *)-1);
          }
--- 120,130 ----
  {
          uintptr_t old_brk;
          uintptr_t ret_brk;
          uintptr_t high_brk;
          uintptr_t new_brk;
!         intptr_t brk_result;
  
          if (!primary_link_map) {
                  errno = ENOTSUP;
                  return ((void *)-1);
          }
*** 132,141 ****
--- 136,148 ----
          low_align = MAX(low_align, ALIGNSZ);
          high_align = MAX(high_align, ALIGNSZ);
  
          lmutex_lock(&__sbrk_lock);
  
+         if (_nd == NULL)
+                 _nd = (void *)_brk_unlocked(0);
+ 
          old_brk = (uintptr_t)BRKALIGN(_nd);
          ret_brk = P2ROUNDUP(old_brk, low_align);
          high_brk = ret_brk + min_size;
          new_brk = P2ROUNDUP(high_brk, high_align);
  
*** 161,171 ****
  }
  
  int
  brk(void *new_brk)
  {
!         int result;
  
          if (!primary_link_map) {
                  errno = ENOTSUP;
                  return (-1);
          }
--- 168,187 ----
  }
  
  int
  brk(void *new_brk)
  {
!         intptr_t result;
! 
!         /*
!          * brk(2) will return the current brk if given an argument of 0, so we
!          * need to fail it here
!          */
!         if (new_brk == 0) {
!                 errno = ENOMEM;
!                 return (-1);
!         }
  
          if (!primary_link_map) {
                  errno = ENOTSUP;
                  return (-1);
          }