Capability: passing fds over IPC

chrome/common/file_descriptor_posix.h

+// Copyright (c) 2006-2009 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 CHROME_COMMON_FILE_DESCRIPTOR_POSIX_H_
+#define CHROME_COMMON_FILE_DESCRIPTOR_POSIX_H_
+
+#include <vector>
+
+// -----------------------------------------------------------------------------
+// A FileDescriptor is a structure for use in IPC messages. It allows one to
+// send descriptors over an IPC channel.
+//
+// In the Windows world, processes can peek and poke the HANDLE table of other
+// processes. On POSIX, in order to transmit descriptors we need to include
+// them in a control-message (a side-channel on the UNIX domain socket).
+// Serialising this type adds descriptors to a vector in the IPC Message, from
+// which the IPC channel can package them up for the kernel.
+// -----------------------------------------------------------------------------
+struct FileDescriptor {
+  FileDescriptor()
+      : fd(-1),
+        auto_close(false) { }
+
+  int fd;
+  // If true, close this descriptor after it has been sent.
+  bool auto_close;
+};
+
+// -----------------------------------------------------------------------------
+// A DescriptorSet is an ordered set of POSIX file descriptors. These are
+// associated with IPC messages so that descriptors can be transmitted over a
+// UNIX domain socket.
+// -----------------------------------------------------------------------------
+class DescriptorSet {
+ public:
+  DescriptorSet();
+  ~DescriptorSet();
+
+  // This is the maximum number of descriptors per message. We need to know this
+  // because the control message kernel interface has to be given a buffer which
+  // is large enough to store all the descriptor numbers. Otherwise the kernel
+  // tells us that it truncated the control data and the extra descriptors are
+  // lost.
+  //
+  // In debugging mode, it's a fatal error to try and add more than this number
+  // of descriptors to a DescriptorSet.
+  enum {
+    MAX_DESCRIPTORS_PER_MESSAGE = 4,
+  };
+
+  // ---------------------------------------------------------------------------
+  // Interfaces for building during message serialisation...
+
+  // Add a descriptor to the end of the set
+  void Add(int fd);
+  // Add a descriptor to the end of the set and automatically close it after
+  // transmission.
+  void AddAndAutoClose(int fd);
+
+  // ---------------------------------------------------------------------------
+
+
+  // ---------------------------------------------------------------------------
+  // Interfaces for accessing during message deserialisation...
+
+  // Return the number of descriptors remaining
+  unsigned size() const { return descriptors_.size() - next_descriptor_; }
+  // Return true if no unconsumed descriptors remain
+  bool empty() const { return descriptors_.size() == next_descriptor_; }
+  // Fetch the next descriptor from the beginning of the set. This interface is
+  // designed for the deserialising code as it doesn't support close flags.
+  //   returns: file descriptor, or -1 on error
+  int NextDescriptor();
+
+  // ---------------------------------------------------------------------------
+
+
+  // ---------------------------------------------------------------------------
+  // Interfaces for transmission...
+
+  // Fill an array with file descriptors without 'consuming' them. CommitAll
+  // must be called after these descriptors have been transmitted.
+  //   buffer: (output) a buffer of, at least, size() integers.
+  void GetDescriptors(int* buffer) const;
+  // This must be called after transmitting the descriptors returned by
+  // GetDescriptors. It marks all the descriptors as consumed and closes those
+  // which are auto-close.
+  void CommitAll();
+
+  // ---------------------------------------------------------------------------
+
+
+  // ---------------------------------------------------------------------------
+  // Interfaces for receiving...
+
+  // Set the contents of the set from the given buffer. This set must be empty
+  // before calling. The auto-close flag is set on all the descriptors so that
+  // unconsumed descriptors are closed on destruction.
+  void SetDescriptors(const int* buffer, unsigned count);
+
+  // ---------------------------------------------------------------------------
+
+ private:
+  // A vector of descriptors and close flags. If this message is sent, then
+  // these descriptors are sent as control data. After sending, any descriptors
+  // with a true flag are closed. If this message has been received, then these
+  // are the descriptors which were received and all close flags are true.
+  std::vector<FileDescriptor> descriptors_;
+  // When deserialising the message, the descriptors will be extracted
+  // one-by-one. This contains the index of the next unused descriptor.
+  unsigned next_descriptor_;
+};
+
+#endif  // CHROME_COMMON_FILE_DESCRIPTOR_POSIX_H_