[MIPS] Support for building tests for MIPS

tests/syscall_return_sandboxing/sandboxed_mips.S

+/*
+ * Copyright (c) 2013 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.
+ */
+
+/*
+ * Assume 32-byte trampoline slots, 16-byte bundles.
+ */
+#define GETTIMEOFDAY_ADDR 0x10500
+#define STORE_MASK $t7
+#define JUMP_MASK  $t6
+
+        .data
+timebuf:
+        .space 0x200 /* enough for a timeval */
+        .text
+        .globl SyscallReturnIsSandboxed
+SyscallReturnIsSandboxed:
+        .set noreorder
+
+        addiu $sp, $sp, -24
+        and   $sp, $sp, STORE_MASK
+        sw    $ra, 20($sp)
+        lui   $a0, %hi(timebuf)
+
+        addiu $a0, $a0, %lo(timebuf)
+        addu  $a1, $zero, $zero
+        lui   $ra, %hi(bad)
+        addiu $ra, $ra, %lo(bad)
+
+        lui   $a2, %hi(GETTIMEOFDAY_ADDR)
+        addiu $a2, $a2, %lo(GETTIMEOFDAY_ADDR)
+        and   $a2, $a2, JUMP_MASK
+        jr    $a2
+
+        nop                 /* sandboxed (bundle aligned) return address */

[Mark Seaborn, 2013/03/12 00:39:29]

This would be more robust as:

.p2align 4
  /* Expected return address */
  jmp done
  /*
   * We don't expect the syscall to return to the following unaligned
   * address or to align up to the bundle boundary after it.
   */
bad:
.p2align 4
  break /* Abort */

done:
  move $v0, 1 /* Indicate success */

With whatever tweaks you need to make this real MIPS assembly code. :-)

Otherwise, the test can accidentally pass if the syscall returns a non-zero
result, and it doesn't check whether NaCl aligns the address up or down.

+        xor   $v0, $v0, 1   /* expected return value */
+bad:    lw    $ra, 20($sp)  /* non-sandboxed return address */
+        nop
+
+        addiu $sp, $sp, 24
+        and   $sp, $sp, STORE_MASK
+        and   $ra, $ra, JUMP_MASK
+        jr    $ra
+
+        nop
+