OS-1804 agent lwp clobbers amd64 abi stack redzone
Reviewed by: Robert Mustacchi <rm@joyent.com>

*** 20,31 ****
   */
  /*
   * Copyright 2006 Sun Microsystems, Inc.  All rights reserved.
   * Use is subject to license terms.
   */
! 
! #pragma ident   "%Z%%M% %I%     %E% SMI"
  
  #include <stdio.h>
  #include <stdlib.h>
  #include <unistd.h>
  #include <ctype.h>
--- 20,32 ----
   */
  /*
   * Copyright 2006 Sun Microsystems, Inc.  All rights reserved.
   * Use is subject to license terms.
   */
! /*
!  * Copyright (c) 2013, Joyent Inc. All rights reserved.
!  */
  
  #include <stdio.h>
  #include <stdlib.h>
  #include <unistd.h>
  #include <ctype.h>
*** 350,359 ****
--- 351,370 ----
           * Begin with the current stack pointer.
           */
  #ifdef _LP64
          if (model == PR_MODEL_LP64) {
                  sp = P->status.pr_lwp.pr_reg[R_SP] + STACK_BIAS;
+ #if defined(__amd64)
+                 /*
+                  * To offset the expense of computerised subtraction, the AMD64
+                  * ABI allows a process the use of a 128-byte area beyond the
+                  * location pointed to by %rsp.  We must advance the agent's
+                  * stack pointer by at least the size of this region or else it
+                  * may corrupt this temporary storage.
+                  */
+                 sp -= STACK_RESERVE64;
+ #endif
                  sp = PSTACK_ALIGN64(sp);
          } else {
  #endif
                  sp = (uint32_t)P->status.pr_lwp.pr_reg[R_SP];
                  sp = PSTACK_ALIGN32(sp);