NaCl: Add sanity check for number of open FDs at startup

sandbox/linux/services/credentials.h

 // Copyright (c) 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_SERVICES_CREDENTIALS_H_
 #define SANDBOX_LINUX_SERVICES_CREDENTIALS_H_
 
 #include "build/build_config.h"
 // Link errors are tedious to track, raise a compile-time error instead.
 #if defined(OS_ANDROID)
 #error "Android is not supported."
 #endif  // defined(OS_ANDROID).
 
 #include <string>
 
 #include "base/basictypes.h"
 #include "base/memory/scoped_ptr.h"
 #include "sandbox/linux/sandbox_export.h"
 
 namespace sandbox {
 
 // This class should be used to manipulate the current process' credentials.
 // It is currently a stub used to manipulate POSIX.1e capabilities as
 // implemented by the Linux kernel.
 class SANDBOX_EXPORT Credentials {
  public:
   Credentials();
   ~Credentials();
 
+  // Returns the number of file descriptors in the current process's FD
+  // table, excluding |proc_fd|, which should be a file descriptor for
+  // /proc.
+  int CountOpenFds(int proc_fd);
+
   // Checks whether the current process has any directory file descriptor open.
   // Directory file descriptors are "capabilities" that would let a process use
   // system calls such as openat() to bypass restrictions such as
   // DropFileSystemAccess().
   // Sometimes it's useful to call HasOpenDirectory() after file system access
   // has been dropped. In this case, |proc_fd| should be a file descriptor to
   // /proc. The file descriptor in |proc_fd| will be ignored by
   // HasOpenDirectory() and remains owned by the caller. It is very important
   // for the caller to close it.
   // If /proc is available, |proc_fd| can be passed as -1.
@@ skipping 38 matching lines @@
   // process has any directory file descriptor open.
   bool DropFileSystemAccess();
 
  private:
   DISALLOW_COPY_AND_ASSIGN(Credentials);
 };
 
 }  // namespace sandbox.
 
 #endif  // SANDBOX_LINUX_SERVICES_CREDENTIALS_H_