Extending Operating Systems at the User-Level:
                     How to Build Your Own Ufo


                         Klaus E. Schauser,
         Albert Alexandrov, Maximilian Ibel, Chris Scheiman

                 Department of Computer Science
              University of California at Santa Barbara


                             Abstract

In this talk we present a novel way of extending the functionality of the
operating system completely at the user-level.  We demonstrate our
technique in the context of Ufo, a personal file system useful for global
computing environments.  We also discuss how our methodology can be applied
to other system extensions desirable for a network of workstation setting,
such as co-scheduling, sandboxed execution of untrusted applications,
parallel debugging, and process migration.  Our approach works by
intercepting selected system calls and signals 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
modified to implement new functionality.

We used this approach to implement a global file system prototype, called
Ufo, which allows users to treat remote files exactly as if they were
local.  With the advent of global clusters of workstations and the rapid
growth of the Internet and World-Wide Web, it becomes increasingly
important to provide users and applications with this kind of transparent
access to files located on remote servers.  Currently, Ufo understands the
FTP and HTTP protocols and allows new protocols to be plugged in.  While
several other projects have implemented global file system abstractions,
all of them require either changes to the operating system or modifications
to standard libraries.  Ufo, on the other hand, is based on user-level
extensions to the operating system and installs without system
administrator assistance.  Ufo does not require any re-linking or
re-compilation of existing applications.  In fact, it can even be
dynamically ``installed'' into already running processes.  While our
approach results in a substantial increase in the cost of the individual
system calls being intercepted, the remaining system calls are not
affected.  Measurements of our prototype implementation under Solaris show
that, while running under Ufo, the user sees an acceptable overall overhead
for many applications.