[MIPS] Leave room for frame header on stack for MIPS arch

src/trusted/service_runtime/posix/nacl_signal.c

 /*
  * Copyright (c) 2012 The Native Client Authors. All rights reserved.
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include <errno.h>
 #include <signal.h>
 #include <stddef.h>
 #include <string.h>
@@ skipping 253 matching lines @@
   frame->context.frame_ptr = regs->r11;
   regs->lr = kReturnAddr;
   regs->r0 = context_user_addr;  /* Argument 1 */
   regs->prog_ctr = NaClUserToSys(nap, nap->exception_handler);
   regs->stack_ptr = NaClUserToSys(nap, new_stack_ptr);
 #elif NACL_ARCH(NACL_BUILD_ARCH) == NACL_mips
   frame->context.frame_ptr = regs->frame_ptr;
   regs->return_addr = kReturnAddr;
   regs->a0 = context_user_addr;
   regs->prog_ctr = NaClUserToSys(nap, nap->exception_handler);
-  regs->stack_ptr = NaClUserToSys(nap, new_stack_ptr);
+  regs->stack_ptr = NaClUserToSys(nap, new_stack_ptr) - NACL_STACK_ARG_SIZE;

[Mark Seaborn, 2013/03/06 00:26:58]

The adjustment should be applied before NaClUserToSys() for more safety.

I think it would be cleaner to add this adjustment via an extra field in
NaClExceptionFrame in service_runtime/nacl_exception.h.

[petarj, 2013/03/07 14:52:48]

Done, I added the adjustment just after the alignment adjustment.
Extra field in NaClExceptionFrame would not be transparent enough, and we could
not apply the same kind of adjustment for NaClSysThreadCreate.

+  regs->t9 = regs->prog_ctr;

[Mark Seaborn, 2013/03/06 00:26:58]

Can you add a comment?  e.g. "PIC functions assume that r9 contains the
function's address on entry"

[petarj, 2013/03/07 14:52:48]

Done.

 #else
 # error Unsupported architecture
 #endif
 
 #if NACL_ARCH(NACL_BUILD_ARCH) == NACL_x86
   frame->return_addr = kReturnAddr;
   regs->flags &= ~NACL_X86_DIRECTION_FLAG;
 #endif
 
   return 1;
@@ skipping 175 matching lines @@
   unsigned int a;
 
   /* Remove all handlers */
   for (a = 0; a < NACL_ARRAY_SIZE(s_Signals); a++) {
     if (sigaction(s_Signals[a], &s_OldActions[a], NULL) != 0) {
       NaClLog(LOG_FATAL, "Failed to unregister handler for %d.\n\tERR:%s\n",
                           s_Signals[a], strerror(errno));
     }
   }
 }