[MIPS] Add seccomp bpf support

sandbox/linux/seccomp-bpf/kernel_return_value_helpers.cc

+// 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 {
+
+int ErrnoToKernelRet(int kernel_ret) {
+#if defined(__mips__)
+  return kernel_ret;
+#else
+  return -kernel_ret;
+#endif
+}
+
+void PutValueInUcontext(int 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), 2014/06/03 01:00:33]

This can't work for instance with mmap(2). mmap() can return negative values
that are not an error.

We have a fundamental issue here, because this function requires information
that has been lost at this point.

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