[MIPS] Restore the trusted stack in NaClSyscallSeg.

src/trusted/service_runtime/arch/mips/nacl_syscall.S

 /*
  * Copyright 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 "native_client/src/trusted/service_runtime/arch/mips/sel_rt.h"
 #include "native_client/src/trusted/service_runtime/nacl_config.h"
 
 .text
@@ skipping 52 matching lines @@
  * When service runtime serves a system call, it first creates a structure which
  * utilizes these arguments. The structure is created by Decoder functions in
  * nacl_syscall_handlers.c. (nacl_syscall_handlers.c is an automatically
  * generated file and placed in
  * scons-out//gen/native_client/src/trusted/service_runtime).
  */
 
   /* Place a0-a3 argument registers in the reserved 16-byte region in caller's
      stack frame (o32 ABI), and place return address on top of that. */
 
   .cpload $t9

[Mark Seaborn, 2012/11/01 18:01:57]

Can you add a comment here saying that this restores $gp to the trusted value
that we rely on below?

I know normally the reader of assembly code would be expected to know what
standard constructs mean, but:

- We are short on MIPS experts in the NaCl project :-)
- .cpload doesn't seem to be documented in the GNU as manual:
http://sourceware.org/binutils/docs-2.23/as/index.html
- In trusted context switch code like this, it's more important to explicitly
account for every register value we rely on.

[petarj, 2012/11/02 12:37:52]

Done.

 
   sw    $a3, 12($sp)
   sw    $a2,  8($sp)
   sw    $a1,  4($sp)
   sw    $a0,  0($sp)
   /* Save return address for returning to untrusted code. */
   sw    $ra, -4($sp)
   /* Save return address that indicates which trampoline was called. */
   sw    $t5, -8($sp)
 
@@ skipping 15 matching lines @@
 
   sw $s0, NACL_THREAD_CONTEXT_OFFSET_S0($a1)
   sw $s1, NACL_THREAD_CONTEXT_OFFSET_S1($a1)
   sw $s2, NACL_THREAD_CONTEXT_OFFSET_S2($a1)
   sw $s3, NACL_THREAD_CONTEXT_OFFSET_S3($a1)
   sw $s4, NACL_THREAD_CONTEXT_OFFSET_S4($a1)
   sw $s5, NACL_THREAD_CONTEXT_OFFSET_S5($a1)
   sw $s6, NACL_THREAD_CONTEXT_OFFSET_S6($a1)
   sw $s7, NACL_THREAD_CONTEXT_OFFSET_S7($a1)
   sw $t8, NACL_THREAD_CONTEXT_OFFSET_T8($a1)
-  sw $gp, NACL_THREAD_CONTEXT_OFFSET_GLOBAL_PTR($a1)
   sw $sp, NACL_THREAD_CONTEXT_OFFSET_STACK_PTR($a1)
   sw $fp, NACL_THREAD_CONTEXT_OFFSET_FRAME_PTR($a1)
 
   /* Restore the thread index. */
   srl   $a0, $a0, 2
 
-  /* Reserve 16-byte for arguments (o32 ABI) */
-  addiu $sp, $sp, -16
+  /* Restore the trusted stack */
+  lw $sp, NACL_THREAD_CONTEXT_OFFSET_TRUSTED_STACK_PTR($a1)
+
   lw    $t9,%call16(NaClSyscallCSegHook)($gp)
   jr    $t9
   nop
 
   /* NOTREACHED */
 
   /*
    * If the thread returns, which must not happen, it will be halted
    * by the following instruction
    */
 
   NACL_HALT