OS-3561 lxbrand emulation library should execute on alternate stack
OS-3558 lxbrand add support for full in-kernel syscall handling
OS-3545 lx_syscall_regs should not walk stack
OS-3868 many LTP testcases now hang
OS-3901 lxbrand lx_recvmsg fails to translate control messages when 64-bit
Reviewed by: Jerry Jelinek <jerry.jelinek@joyent.com>
Reviewed by: Patrick Mooney <patrick.mooney@joyent.com>
Reviewed by: Bryan Cantrill <bryan@joyent.com>

*** 18,27 ****
--- 18,30 ----
   *
   * CDDL HEADER END
   */
  
  /*
+  * Copyright 2015 Joyent, Inc.
+  */
+ /*
   * Copyright 2010 Sun Microsystems, Inc.  All rights reserved.
   * Use is subject to license terms.
   */
  
  /*      Copyright (c) 1984, 1986, 1987, 1988, 1989 AT&T */
*** 108,121 ****
--- 111,129 ----
          getfpasrs(lwp, ucp->uc_mcontext.asrs);
          if (ucp->uc_mcontext.fpregs.fpu_en == 0)
                  ucp->uc_flags &= ~UC_FPU;
          ucp->uc_mcontext.gwins = (gwindows_t *)NULL;
  
+         if (mask != NULL) {
                  /*
                   * Save signal mask.
                   */
                  sigktou(mask, &ucp->uc_sigmask);
+         } else {
+                 ucp->uc_flags &= ~UC_SIGMASK;
+                 bzero(&ucp->uc_sigmask, sizeof (ucp->uc_sigmask));
+         }
  }
  
  
  void
  restorecontext(ucontext_t *ucp)
*** 410,424 ****
--- 418,437 ----
  
          if (ucp->uc_mcontext.fpregs.fpu_en == 0)
                  ucp->uc_flags &= ~UC_FPU;
          ucp->uc_mcontext.gwins = (caddr32_t)NULL;
  
+         if (mask != NULL) {
                  /*
                   * Save signal mask (the 32- and 64-bit sigset_t structures are
                   * identical).
                   */
                  sigktou(mask, (sigset_t *)&ucp->uc_sigmask);
+         } else {
+                 ucp->uc_flags &= ~UC_SIGMASK;
+                 bzero(&ucp->uc_sigmask, sizeof (ucp->uc_sigmask));
+         }
  }
  
  int
  getsetcontext32(int flag, void *arg)
  {