MIPS: Fix NaClSwitch() to set $t9 to the address of the entry point

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

 /*
  * 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 "native_client/src/trusted/service_runtime/arch/mips/sel_rt.h"
 #include "native_client/src/trusted/service_runtime/nacl_config.h"
 
 .text
 
 /*
  * This trusted code is linked into the service_runtime and
  * executed when switching from the service runtime to a nacl
  * module. This happens when a main nacl thread is created and starting to
  * execute the nacl code, or when nacl module is returning from a system
  * call. This piece of code lives in a service runtime part of address space.
  * The one and only argument is in register a0
  *
  * a0 -- address of thread context (struct NaClThreadContext)
  */
 
 DEFINE_GLOBAL_HIDDEN_IDENTIFIER(NaClSwitch):
 
   .set noreorder
 
-/*
- * We clear registers a1, a2, a3, t0, t1, t2, t3, t4, t5 and ra to avoid
- * information leaks. The remaining registers are overwritten by the code that
- * follows after.
- */
-
+  /*
+   * We clear the following registers to avoid information leaks. The
+   * remaining registers are overwritten by the code that follows
+   * after.
+   */
+  addu $v1, $zero, $zero
   addu $a1, $zero, $zero
   addu $a2, $zero, $zero
   addu $a3, $zero, $zero
   addu $t0, $zero, $zero
   addu $t1, $zero, $zero
   addu $t2, $zero, $zero
   addu $t3, $zero, $zero
   addu $t4, $zero, $zero
   addu $t5, $zero, $zero
   addu $ra, $zero, $zero
@@ skipping 12 matching lines @@
   lw $s5, NACL_THREAD_CONTEXT_OFFSET_S5($a0)
   lw $s6, NACL_THREAD_CONTEXT_OFFSET_S6($a0)
   lw $s7, NACL_THREAD_CONTEXT_OFFSET_S7($a0)
   lw $t8, NACL_THREAD_CONTEXT_OFFSET_T8($a0)
   lw $sp, NACL_THREAD_CONTEXT_OFFSET_STACK_PTR($a0)
   lw $fp, NACL_THREAD_CONTEXT_OFFSET_FRAME_PTR($a0)
 
   /* NACL_CALLEE_SAVE_LIST END*/
 
   lw $v0, NACL_THREAD_CONTEXT_OFFSET_SYSRET($a0)
-  lw $v1, NACL_THREAD_CONTEXT_OFFSET_NEW_PROG_CTR($a0)
+  lw $t9, NACL_THREAD_CONTEXT_OFFSET_NEW_PROG_CTR($a0)
 
   /* At startup, context->sysret contains not the the return value, but the
      first argument. Put it in a0. */
   addu $a0, $v0, $zero
 
-  /* Transfer control to untrusted code */
-  jr  $v1
+  /*
+   * Transfer control to untrusted code.
+   *
+   * We leave $t9 containing the address of the function being called,
+   * as required by the MIPS ABI.
+   */
+  jr  $t9
   nop
 
   .set reorder