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

@@ -20,12 +20,13 @@
  */
 /*
  * Copyright 2006 Sun Microsystems, Inc.  All rights reserved.
  * Use is subject to license terms.
  */
-
-#pragma ident   "%Z%%M% %I%     %E% SMI"
+/*
+ * Copyright (c) 2013, Joyent Inc. All rights reserved.
+ */
 
 #include <stdio.h>
 #include <stdlib.h>
 #include <unistd.h>
 #include <ctype.h>

@@ -350,10 +351,20 @@
          * 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);