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Subject[ANNOUNCE] FUSD v1.00: Framework for User-Space Devices
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On behalf of Sensoria Corporation, I'm happy to announce the first
public release of FUSD, a Linux Framework for User-Space Devices.

Briefly, FUSD lets you write user-space daemons that can respond to
device-file callbacks on files in /dev. These device files look and
act just like any other device file from the point of view of a
process trying to use them. When the FUSD kernel module receives a
file callback on a device being managed from user-space, it marshals
the arguments into a message (including data copied from the caller,
if necessary), blocks the caller, and sends the message to the daemon
managing the device. When the daemon generates a reply, the process
happens in reverse, and the caller is unblocked.

More information can be found at the official FUSD home page:
http://www.circlemud.org/~jelson/software/fusd

I've pasted a portion of the README file below, which has a somewhat
more detailed description of what FUSD is and does. A much more
comprehensive user manual is available on the web page (above).

Best regards,
Jeremy

--------


WHAT IS FUSD?
=============

FUSD (pronounced "fused") is a Linux framework for proxying device
file callbacks into user-space, allowing device files to be
implemented by daemons instead of kernel code. Despite being
implemented in user-space, FUSD devices can look and act just like any
other file under /dev which is implemented by kernel callbacks.

A user-space device driver can do many of the things that kernel
drivers can't, such as perform a long-running computation, block while
waiting for an event, or read files from the file system. Unlike
kernel drivers, a user-space device driver can use other device
drivers--that is, access the network, talk to a serial port, get
interactive input from the user, pop up GUI windows, or read from
disks. User-space drivers implemented using FUSD can be much easier to
debug; it is impossible for them to crash the machine, are easily
traceable using tools such as gdb, and can be killed and restarted
without rebooting. FUSD drivers don't have to be in C--Perl, Python,
or any other language that knows how to read from and write to a file
descriptor can work with FUSD. User-space drivers can be swapped out,
whereas kernel drivers lock physical memory.

FUSD drivers are conceptually similar to kernel drivers: a set of
callback functions called in response to system calls made on file
descriptors by user programs. FUSD's C library provides a device
registration function, similar to the kernel's devfs_register_chrdev()
function, to create new devices. fusd_register() accepts the device
name and a structure full of pointers. Those pointers are callback
functions which are called in response to certain user system
calls--for example, when a process tries to open, close, read from, or
write to the device file. The callback functions should conform to
the standard definitions of POSIX system call behavior. In many ways,
the user-space FUSD callback functions are identical to their kernel
counterparts.

The proxying of kernel system calls that makes this kind of program
possible is implemented by FUSD, using a combination of a kernel
module and cooperating user-space library. The kernel module
implements a character device, /dev/fusd, which is used as a control
channel between the two. fusd_register() uses this channel to send a
message to the FUSD kernel module, telling the name of the device the
user wants to register. The kernel module, in turn, registers that
device with the kernel proper using devfs. devfs and the kernel don't
know anything unusual is happening; it appears from their point of
view that the registered devices are simply being implemented by the
FUSD module.

Later, when kernel makes a callback due to a system call (e.g. when
the character device file is opened or read), the FUSD kernel module's
callback blocks the calling process, marshals the arguments of the
callback into a message and sends it to user-space. Once there, the
library half of FUSD unmarshals it and calls whatever user-space
callback the FUSD driver passed to fusd_register(). When that
user-space callback returns a value, the process happens in reverse:
the return value and its side-effects are marshaled by the library
and sent to the kernel. The FUSD kernel module unmarshals this
message, matches it up with a corresponding outstanding request, and
completes the system call. The calling process is completely unaware
of this trickery; it simply enters the kernel once, blocks, unblocks,
and returns from the system call---just as it would for any other
blocking call.

One of the primary design goals of FUSD is stability. It should
not be possible for a FUSD driver to corrupt or crash the kernel,
either due to error or malice. Of course, a buggy driver itself may
corrupt itself (e.g., due to a buffer overrun). However, strict error
checking is implemented at the user-kernel boundary which should
prevent drivers from corrupting the kernel or any other user-space
process---including the errant driver's own clients, and other FUSD
drivers.

For more information, please see the comprehensive documentation in
the 'doc' directory.

Jeremy Elson <jelson@circlemud.org>
Sensoria Corporation
September 28, 2001
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