[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 88 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)

[Mark Seaborn, 2012/11/01 01:47:41]

Does this mean untrusted code is allowed to modify $gp?  It's not clear to me
from the sandbox design doc on
https://code.google.com/p/nativeclient/issues/detail?id=2275.  Do you need to
restore $gp too?

[petarj, 2012/11/01 17:06:40]

Untrusted code can modify $gp, yet saving $gp is a cruft I have just removed
from the source along with some other $gp related code.

   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) */

[Mark Seaborn, 2012/11/01 01:47:41]

Should you change NaClStartThreadInApp() to explicitly reserve these 16 bytes? 
We used to reserve 32 bytes that way for the Windows x86-64 ABI (see before
http://src.chromium.org/viewvc/native_client?view=rev&revision=7168).

Currently it does this for MIPS:

  natp->user.trusted_stack_ptr = (NaClGetStackPtr() & ~0xf) + 4;

The "+ 4" there is inconsistent with NACL_STACK_PAD_BELOW_ALIGN in
nacl_config.h, BTW.

[petarj, 2012/11/01 17:06:40]

The 16 bytes are already reserved at that point. No need to explicitly reserve
them.
The inconsistency has been corrected.

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

OK, good point.

-  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