[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
+#define BREAK_INST .word 0xd
+
+        .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

[Mark Seaborn, 2013/03/12 21:55:24]

FWIW, adding in ".p2align 4" as well makes this more robust against someone
changing this and accidentally getting the bundling wrong (e.g. by miscounting
the instructions or by not understanding your convention of grouping into
bundles using empty lines).

+        nop
+        nop
+        nop
+
+        /* Expected return address */
+        b done
+        nop
+        /*
+         * We don't expect the syscall to return to the following unaligned
+         * address or to align up to the bundle boundary after it.
+         */
+bad:
+        BREAK_INST  /* Abort */
+        BREAK_INST
+
+        BREAK_INST
+        BREAK_INST
+        BREAK_INST
+        BREAK_INST
+
+done:
+        ori   $v0, $zero, 1 /* Indicate success */
+        lw    $ra, 20($sp)
+        addiu $sp, $sp, 24
+        and   $sp, $sp, STORE_MASK
+
+        and   $ra, $ra, JUMP_MASK
+        jr    $ra
+        nop
+