Add SandboxBPFPolicy::InvalidSyscall() to simplify writing policies

sandbox/linux/seccomp-bpf/sandbox_bpf_policy.h

 // Copyright 2013 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.
 
 #ifndef SANDBOX_LINUX_SECCOMP_BPF_SANDBOX_BPF_POLICY_H_
 #define SANDBOX_LINUX_SECCOMP_BPF_SANDBOX_BPF_POLICY_H_
 
 #include "base/basictypes.h"
+#include "sandbox/sandbox_export.h"
 
 namespace sandbox {
 
 class ErrorCode;
 class SandboxBPF;
 
 // This is the interface to implement to define a BPF sandbox policy.
-class SandboxBPFPolicy {
+class SANDBOX_EXPORT SandboxBPFPolicy {
  public:
   SandboxBPFPolicy() {}
   virtual ~SandboxBPFPolicy() {}
 
   // The EvaluateSyscall method is called with the system call number. It can
   // decide to allow the system call unconditionally by returning ERR_ALLOWED;
   // it can deny the system call unconditionally by returning an appropriate
   // "errno" value; or it can request inspection of system call argument(s) by
   // returning a suitable ErrorCode.
+  // Will only be called for valid system call numbers.
   virtual ErrorCode EvaluateSyscall(SandboxBPF* sandbox_compiler,
                                     int system_call_number) const = 0;
 
+  // The InvalidSyscall method specifies the behavior used for invalid
+  // system calls.  The default implementation is to return ENOSYS.
+  virtual ErrorCode InvalidSyscall(SandboxBPF* sandbox_compiler) const;
+
  private:
   DISALLOW_COPY_AND_ASSIGN(SandboxBPFPolicy);
 };
 
 }  // namespace sandbox
 
 #endif  // SANDBOX_LINUX_SECCOMP_BPF_SANDBOX_BPF_POLICY_H_