[MIPS] Add seccomp bpf support

sandbox/linux/seccomp-bpf/kernel_return_value_helpers.cc

Index: sandbox/linux/seccomp-bpf/kernel_return_value_helpers.cc
diff --git a/sandbox/linux/seccomp-bpf/kernel_return_value_helpers.cc b/sandbox/linux/seccomp-bpf/kernel_return_value_helpers.cc
new file mode 100644
index 0000000000000000000000000000000000000000..888bec58b5f169f35efa419bf18c8e66c1c48680
--- /dev/null
+++ b/sandbox/linux/seccomp-bpf/kernel_return_value_helpers.cc
@@ -0,0 +1,39 @@
+// Copyright 2014 The Chromium 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 "sandbox/linux/seccomp-bpf/kernel_return_value_helpers.h"
+#include "sandbox/linux/seccomp-bpf/linux_seccomp.h"
+
+namespace sandbox {
+
+// In order for errno to be converted to the value that kernel returns to
+// indicate an error in syscall, errno needs to be negated on Intel/Arm. On
+// Mips this is not the case since Mips has different mechanism for indicating
+// error.
+int ErrnoToKernelRet(int kernel_ret) {
+#if defined(__mips__)
+  return kernel_ret;
+#else
+  return -kernel_ret;
+#endif
+}
+
+void PutValueInUcontext(intptr_t ret_val, ucontext_t* ctx) {
+#if defined(__mips__)
+  // Mips ABI states that on error a3 CPU register has non zero value and if
+  // there is no error, it should be zero.
+  if (ret_val < 0) {

[jln (very slow on Chromium), 2014/06/13 02:47:36]

This should be if (retval <= -1 && ret_val >= -4095) (or we could define
MAX_ERRNO in syscall.h instead) and use that.

[nedeljko, 2014/06/18 13:41:00]

Done.

+    // The sign of errno that is returned from the kernel was changed in
+    // SandboxSyscall() in case of Mips in order for SandboxSyscall() to

[jln (very slow on Chromium), 2014/06/13 02:47:36]

This code must not have knowledge of the codepath that called it.

Instead, it should reference the interface documentation, which should say that
|ret_val| is -errno on error.

[nedeljko, 2014/06/18 13:41:00]

Done.

+    // behave the same for all supported architectures. In order to write
+    // correct value to return register this sign needs to be changed back.
+    ret_val = -ret_val;
+    SECCOMP_PARM4(ctx) = 1;
+  } else
+    SECCOMP_PARM4(ctx) = 0;
+#endif
+  SECCOMP_RESULT(ctx) = static_cast<greg_t>(ret_val);
+}
+
+} // namespace sandbox