Extending the Operating System at the User Level: The UFO Global File System

In this paper we show how to extend the functionality of standard operating systems completely at the user level. Our approach works by intercepting selected system calls at the user level, using tracing facilities such as the /proc file system provided by many Unix operating systems. The behavior of some intercepted system calls is then modified to implement new functionality. This approach does not require any re-linking or re-compilation of existing applications. In fact, the extensions can even be dynamically "installed'' into already running processes. The extensions work completely at the user level and install without system administrator assistance.

We used this approach to implement a global file system, called Ufo, which allows users to treat remote files exactly as if they were local. Currently, Ufo supports file access through the FTP and HTTP protocols and allows new protocols to be plugged in. While several other projects have implemented global file system abstractions, they all require either changes to the operating system or modifications to standard libraries. The paper gives a detailed performance analysis of our approach to extending the OS and establishes that Ufo introduces acceptable overhead for common applications even though intercepting system calls incurs a high cost.