Discussion of GNU Hurd, an advanced oper-
ating system based on the CMU Mach micro-
kernel. Hurd is being developed by the
Free Software Foundation. No release date
available as of yet.

From: [email protected] (Michael I Bushnell)
Newsgroups: gnu.misc.discuss
Subject: Re: Hurd?
Date: 27 Jan 93 12:59:30
Organization: MIT Artificial Intelligence Lab
Lines: 592
Distribution: usa

In article <[email protected]>
[email protected] (David D Kaas) writes:

> Is there still a gnu project to produce a unix like
> operating system? If so is there any information
> available on it?

The GNU Hurd, in combination with Mach, will provide a unix like
operating system.

Here is some information that might whet people's curiosity. Please
don't ask me lots of questions about this; I don't have enough time.
If people can understand the extant code reasonably well with only
about two questions, and they have time to do work, there are a number
of projects that can be done by other people to help out with the
Hurd.

-mib


TOWARDS A NEW STRATEGY OF OPERATING SYSTEM DESIGN

PART 1 -- What we are doing

The fundamental purpose of an operating system is to enable a variety
of programs to share a single computer efficiently. Efficiency
demands things such as memory protection, preemptively scheduled
timesharing, and coordinated access to I/O peripherals. In addition,
operating systems can allow a variety of users to share a computer
efficiently. The same general areas apply here as well: preventing
users from harming each other, enabling them to share without prior
arrangement, and mediating access to physical devices.

On today's computer systems, implementing these goals usually involves
a large program, called the kernel, which mediates all these
exchanges. Since this program must be accessible to all user
programs, it becomes a natural place to add functionality to the
system. As time goes by, more and more gets added to the kernel,
because the only model for interaction between processes is that of
specific, individual services provided by the kernel.

The traditional system allows users to add components to the mammoth
shared library "vmunix" only if they understand most of that library,
and only if they have a privileged status within the system. Testing
these new components requires a much more painful edit-compile-debug
cycle than other programs, and cannot be done while others are using
the system. Bugs usually cause fatal system crashes, further
disrupting others' use of the system. All of the kernel is usually
non-pageable, [fn: There are systems with pageable kernels. However,
deciding what can be pageable is difficult and error prone. As well,
the mechanisms are usually quite complex, making them difficult to use
by people wanting to add simple extensions.] further adding to the
cost of adding functions to it.

Because of these restrictions, functionality which properly belongs
"behind" the wall of the traditional kernel is usually left out of
systems unless it is absolutely mandatory. Many good ideas, best
implemented with an open/read/write interface cannot be implemented
because of the problems inherent in the monolithic nature of
traditional systems. Further, even for those with the endurance to
implement such ideas, only those who are privileged users of their
computers can do so. The software copyright system, preventing
unlicensed people from even reading the kernel source, only compounds
the problem still more.

Some systems have addressed these difficulties. Smalltalk-80 and the
Lisp Machine both represent one method of getting around the problem.
System code is not distinguished from user code; all of the system is
accessible to the user, and can be changed as need be. Both systems
were built around languages that facilitated such easy replacement and
extension, and were moderately successful. These systems, however,
were fairly poor at insulating users and programs from each other,
thus failing one of the principle goals of operating system design.

Most projects using the Mach 3.0 kernel today carry on the same
tradition of operating system design. The internal structure of the
system has changed, but the same heavy barrier between user and system
remains. The single-servers, while fairly easy to construct, inherit
quite naturally all the deficiencies of the monolithic kernels from
which they came.

Most multi-server projects do somewhat better. Much more of the
system is pageable. The system is more easily debugged, and testing
of new system components need not interfere with other users. But
there is still a wall between the user and the system, and no user can
cross that wall without special privilege.

The GNU Hurd, by contrast, is designed to make the area of "system"
code as limited as possible. Users are only required to communicate
with the kernel; the rest of the system is replacable dynamically.
Users can use whatever parts of the remainder of the system they want,
and can add components themselves, easily, for other users to take
advantage of. No mutual trust need exist in advance for users to use
each other's services, nor does the system become vulnerable by
trusting the services of arbitrary users.

We have done this by identifying those components of the system which
users must use in order to communicate with each other. One of these
is a mechanism responsible for identifying users' identities. This
program is called the authentication server. Programs must
communicate, each with an authentication server they trust, in order
to establish each other's identities. The other is a mechanism for
establishing control over system components by the superuser, and
global bookkeeping operations. This server is called the process
server.

No user program needs to communicate with the process server at all;
it is only necessary for users which require its services. The
authentication server is only necessary for programs which wish to
establish their identity to another agent. All remaining services in
the system carry no special status. This includes the network
implementation, the filesystems, the exec mechanism (including
setuid), and so forth.



THE TRANSLATOR MECHANISM

The Hurd uses Mach ports primarily as methods for communicating
between users and servers. Each port implements a particular set of
protocols, representing operations that can be undertaken on the
underlying object represented by the port. Some of the protocols
specified by the Hurd are the I/O protocol, used for generic I/O
operations; the file protocol, used for filesystem operations; the
socket protocol, used for network operations; the process protocol,
used for manipulating processes; and so forth.

Most servers are accessed by opening files. Normally, opening a file
results in a port associated with that file, owned by the same server
as the directory containing the file. For example, a disk based
filesystem would normally be serving a large number of ports, each
representing an open file or directory. When a new file is opened the
server creates a new port, associates it with the file, and returns a
send right to the user.

However, a file can have a "translator" associated with it. In this
case, rather than returning its own port referring to the contents of
the file, the server executes a program, called the translator. This
program is provided a port to the actual contents of the file, and is
then asked to return a port to the original user to complete the open
operation.

This mechanism is used quite obviously for mount. A mount point would
have a translator associated with it. When a user opens the mount
point, the translator, in this case, a program which understands the
disk format of the mounted filesystem, is executed, and returns a port
to the user. Once the translator is started, it need not be run
again, unless it dies; the parent filesystem retains a port to the
translator to use in further requests. Translators are guaranteed
that new programs will not be started for further open operations.

Translators can be associated with files by the owner of the file,
needing no additional permission. As a result, any program can be
specified as a translator. Obviously the system will not work
properly if the translator does not implement the file protocol
correctly, or at all. The system is so constructed, however, to cause
an interruptible hang to be the worst possible result.

One way to use translators, then, is to access hierarchically
structured data using the file protocol. All the complexity of the
user interface to the ftp program, for example, is then removed.
Users need only know that a particular directory represents FTP, and
can use all the standard file manipulation commands, ls, cp, mv, and
so forth, to access the remote system, rather than learning a new set.
Similarly, the complexity of tar can be eased by a simple translator.
[fn: Such transparent access to tar would not necessarily be cheap,
though it would be convenient.]


GENERIC SERVICES

Another way to use translators is to use the filesystem as a
rendezvous for interfaces which are not similar to files. Consider a
service which implements some version of the X protocol using Mach
messages as an underlying transport. For each X display, a file can
be created with the appropriate program as its translator. X clients
would open that file. At that point, most file operations (read and
write, for example) would not be useful, but new operations
(XCreateWindow or XDrawText) might become meaningful. In this kind of
case, the filesystem protocol is used further only to manipulate
characteristics of the node used for the rendezvous. The node need
not support I/O operations, though it should reply to any such
messages with a "message not understood" return code. (If MiG stubs
are used to demultiplex messages, this will happen automatically.)

This technique is used to contact most of the services in the Hurd
which are not structured like hierarchical filesystems. For example,
the password server, which hands out authorization tags in exchange
for passwords, is contacted this way. Network protocol servers as
well are contacted in this fashion. Roland McGrath thought up this
use of translators.


CLEVER FILESYSTEM PICTURES

The third common method of using translators in the Hurd is to present
a filesystem-like view of another part of the filesystem, with some
semantics changed. For example, it might be nice to have a filesystem
which cannot be changed, but records changed versions of its files
elsewhere. (This might be useful for source code management.) A
translator is available which presents "that directory" with all
changes going "over there instead".

Similarly, a translator is available which creates a directory which
is a conceptual union of a number of other directories, with collision
resolution rules of various sorts. A variety of other ideas have been
presented which do similar things.


WHAT THE USER CAN DO

None of these translators gain extra privilege by virtue of being
hooked into the filesystem. Translators run with the uid of the owner
of the file being translated, and can only be set or changed by that
owner. The I/O and filesystem protocols are carefully designed to
allow their use by mutually untrusting clients and servers.

Translators are just ordinary programs. There are a variety of
facilities in the GNU C library to make common sorts of translators
easier to write. Some translators which might appear to need special
privilege are those which allow setuid exec or the password server
referred to above. In fact, these translators could be run by anyone.
Only if they are set on a root-owned node, however, would they be able
to successfully provide all their services. This is analogous to
letting any user call the reboot system call, but only honoring it if
that user is root.


WHY THIS IS SO DIFFERENT

What this system organization lets users do is completely novel in the
Unixoid world. To this point, operating systems have kept huge
portions of this functionality in the realm of system code, thus
preventing its modification and extension, except in cases of extreme
need. Individual users cannot replace parts of the system in their
programs no matter how much easier that would make their task, and
system managers are loath to install random tweaks off the net into
their kernels.

In our system, users can change almost all of the things which are
decided for them in advance by traditional systems. In combination
with the tremendous control given by the Mach kernel over task address
spaces and so forth, for the first time we have a system in which
users will be able to replace parts of the system they dislike,
without disrupting other users of the same system.

Most Mach-based operating systems to date have concentrated, instead,
on implementing a wider set of the "same old" Unix semantics in a new
environment. By contrast, we are extending those semantics in ways
that allow users to bypass or replace them, virtually arbitrarily.


PART 2 -- What we are doing, in detail


THE AUTHENTICATION SERVER

One of the most central servers in the Hurd is the authentication
server. Each port to the authentication server identifies a user, and
is associated by the authentication server with an "id block". Each
id block contains a set of user ids and a set of group ids. Either
set may be empty. This server is not the same as the "password
server" referred to above.

There are three services exported by the authentication server.
First, the authentication server provides simple boolean operations on
authentication ports. Given a two authentication ports, the
authentication server will provide a third port representing the union
of the two sets of uids and gids.

Second, the authentication server allows any user with a uid of zero
to create an arbitrary authentication port.

Finally, the authentication server provides RPC's which allow mutually
untrusting clients and servers to establish identity and pass initial
information on each other. This is crucial to the security of the
filesystem and I/O protocols, for example.

Any user could write a program which implements the authentication
protocol. This does not, however, violate the security of the system.
When a given service needs to authenticate a user, it communicates
with its trusted authentication server. If that user is using a
different authentication server, the transaction will fail, and the
server can refuse to communicate further. Because, in effect, this
forces all programs on the system to use the same authentication
server, we have designed its interface to make any safe operation
possible, and to include no extraneous operations. (This is why
passwords are implemented in a different server.)


THE PROCESS SERVER

The process server has undergone much change in the system design.
Originally it was to be responsible for much of the mechanics of
signal delivery, process creation and destruction, and so forth. We
realized, however, that virtually all of these features did not need
to be in a global system server.

The process server, in the final design, acts as an information
categorization repository. There are four main services supported by
the process server.

First, the process server keeps track of generic host-level
information not handled by the Mach kernel. The hostname, the hostid,
the system version, and so forth are maintained by the process server.

Second, the process server maintains the Posix notions of sessions and
process groups, for the convenience of programs which wish to use
Posix features.

Third, the process server maintains a one-to-one mapping between tasks
and processes. Every task is assigned a pid. Processes can register
a message port with the process server, which will then be given out
to any other program which requests it. The process server makes no
attempt to keep these message ports private, so user programs need to
implement whatever security they need themselves. (The C Library,
which normally implements reception of messages on the message port,
provides convenient functions for doing so.) Processes can tell the
process server their current argv and envp values; the process server
will then provide to those requesting it a vector of the arguments and
environment for any process. This is useful in writing ps-like
programs, as well as making it easier to hide or change this
information. None of these features is mandatory. Programs are free
to disregard all of this if they wish, and never register themselves
with the process server at all. They will still have a pid assigned,
however.

Finally, the process server implements "process collections", which
are used to collect a number of process message ports at the same
time. Also, facilities are provided for converting between pids,
process server ports, and Mach task ports, all the while ensuring the
security of the ports managed.

It is important to stress that that the process server is optional.
Because of restrictions in Mach, programs must run as root in order to
identify all the tasks in the system, but given that, multiple process
servers could co-exist, each with their own clients, giving their own
model of the universe. Those process server features which do not
require root privileges to be implemented could be done as per-user
servers, for those user programs which wish to do recordkeeping in
that fashion. The user's hands are not tied.


TRANSPARENT FTP

Transparent FTP is an intriguing idea whose time has come. The
popular ange-ftp package available for GNU Emacs makes access to FTP
files virtually transparent to all the emacs file manipulation
functions. Transparent FTP does the same thing, but in a system wide
fashion. This server is not yet written; the details of its access
remain to be fleshed out, and will doubtless change when we have
experience using the server.

In a BSD kernel, a transparent FTP filesystem would be no harder to
write than it is in the Hurd. But mention the idea to a BSD kernel
hacker, and the response is that "such a thing doesn't belong in the
kernel". In a sense, this is correct. It violates all the layering
principles of such a system to place such a thing in the kernel. The
unfortunate side effect, however, is that the design methodology
(which is based on preventing users from changing things they don't
like) is being used to prevent system designers from making things
better.

In the Hurd, there are no obstacles to doing transparent FTP. A
translator will be provided for the node /ftp. The contents of /ftp
will not be directly listable, though further subdirectories will.
The will have a variety of possible formats. If I want to access some
files on uunet, for example, I might do
`cd /ftp/ftp.uu.net:anonymous:mib@gnu'. If I want to access some
files on an account I have elsewhere, I might do
`cd /ftp/unmvax.cs.unm.edu:mike:my-password-here'. Parts of this
could be left out, and the transparent FTP program would read them
from my .netrc file. In the last case, this would allow me to do
simply `cd /ftp/unmvax.cs.unm.edu'; the rest of the information is
present in my .netrc file already.

There is no need to do a `cd' first--I can use any file commands I
want. If I want to implement RFC 1097 (the Telnet Subliminal Message
Option), I can just type `more /ftp/ftp.uu.net/inet/rfc/rfc1097'. I
can use a copy command if I will need it frequently, or just directly
load the file into my emacs.


FILESYSTEMS

We are implementing ordinary filesystems as well. The initial release
of the system will contain a filesystem upward compatible with the
Fast File System as found in BSD 4.4. In addition to the ordinary
semantics, we will provide the recording of translators, thirty-two
bit user ids and group ids, and a new id per file, called the "author"
of the file, which can be arbitrarily set by the owner. In addition,
because users in the Hurd can have multiple uids, or even none, there
is an additional set of permission bits providing access control for
"unknown user" (no uids) as distinct from "known but arbitrary user"
(some uids). The latter category is the existing "world" category of
file permissions.

We plan to implement the Network File System protocol, using the 4.4
BSD implementation as a starting point. We will also implement a
log-structured filesystem, using the same ideas as the work at Sprite,
but probably not the same format. We may design our own network file
protocol as well, or we may just extend NFS to rid it of deficiencies.
We will also implement various "little" filesystems, such as the MSDOS
filesystem, to help people move files between GNU and other operating
systems on the same hardware.


TERMINALS

We will have an I/O server to implement the terminal semantics of
Posix. The C Library has features for keeping track of the
controlling terminal and arranging to have the proper job control
signals sent at the proper times, as well as obeying keyboard and
hangup signals from the terminal driver.

Programs will be able to insert the terminal driver into
communications channels in a variety of ways. Servers like rlogind,
for example, will be able to insert the terminal protocol onto their
network communication port. Pseudo-terminals will not be necessary,
though the will be provided for backward compatibility with older
programs--no programs in GNU will depend on them.

Nothing about the terminal driver is forced upon users. The terminal
driver allows a user to get at the underlying communications channel
easily, and to either bypass the terminal driver on an as-needed
basis, or entirely, or even substitute a different terminal
driver-like program. In the latter case, provided the alternate
program implements the necessary interfaces, it will be used by the C
Library exactly as if it were the ordinary terminal driver.

Because of this flexibility, the original terminal driver will not
provide complex line editing features, restricting itself to the
behavior found in Posix and BSD. We plan, eventually, to have a
readline-like terminal driver, which will provide complex line-editing
features for those users who wish to use it.

The terminal driver will probably not support high-volume rapid data
transmission (such as is required by UUCP or slip) very well. Those
programs, however, do not need any of its features, and will be
modified to use the underlying Mach device ports for terminals, which
do efficiently support moving large amounts of data.


EXECUTING PROGRAMS

The mechanics of implementing the execve call are distributed between
three programs. The library handles marshalling the argument and
environment vectors. It then sends a message to the file server
holding the file to be executed. The file server checks execute
permissions, making whatever changes it desires in the exec call. For
example, if the file is marked setuid and the fileserver has the
ability, it will change the user identification going to the new
image appropriately. The file server also decides if programs which
had access to the old task should continue to have access to the new
task. If the file server is augmenting permissions, or executing an
unreadable image, for example, then the exec needs to take place in a
new Mach task to maintain security. (The process server contains some
special features to allow the new task to keep the pid of the old task
in this case, thus preserving Posix semantics.)

After deciding the policy associated with the new image, the
filesystem calls the exec server to load the task. The exec server,
written using the GNU BFD (Binary File Descriptor) library, loads the
image. BFD supports a large number of object file formats; almost any
object file format it supports will be executable. The exec server
also recognizes scripts starting with `#!' and does the right thing
for them.

It was thought that it would be nice to make it easy for users to have
their own exec servers. Roland McGrath thought of the following
technique. The standard exec server also looks at the environment of
the new image; if it contains a variable EXEC_SERVERS then it uses the
programs specified there as exec servers instead of the system
default. (This is, of course, not done for execs that the file server
has requested be kept secure.)

The new image starts running in the C Library, which sends a message
to the exec server to get the arguments, environment, and other state
(umask, current directory, etc.). None of this additional state is
special to the file server or the exec server; if programs wish, they
can use it in a different manner than the C Library.


NEW PROCESSES

The fork call is implemented almost entirely in the C Library. The
new task is created by Mach kernel calls. The C Library arranges to
have its image inherited properly. The new task is registered with
the process server (though this is not mandatory). The C Library
provides vectors of functions to be called at fork time: one vector to
be called before the fork, one after in the parent, and one after in
the child. This should not be used to replace the normal sequence of
calling fork; it is intended for libraries which need to close ports
or clean up lock state before a fork occurs. The library will
implement both fork calls specified by the draft Posix.4a (the threads
extension to the real-time extension).

Nothing forces the user to create new tasks this way. If a program
wants to use almost the normal fork, but with some special
characteristics, then it can do so. Hooks will be provided by the C
Library; in addition, the function can even be completely replaced if
desired. None of this is possible in a Unix system.


ASYNCHRONOUS MESSAGES

As mentioned above, the process server maintains a "message port" for
each task registered with it. These ports are public, and are used to
send asynchronous messages to the task. Signals, for example, are
sent to the message port. The signal message also provides a port as
an indication that the sender should be trusted to send the signal.
The C Library maintains a variety of ports in a table, each of which
identifies a set of signals that can be sent by anyone who posesses
that port. For example, if the user possess the task's kernel port,
it is allowed to send any signal. If the user possesses a special
"terminal id" port, it is allowed to send the keyboard and hangup
signals. Users can add arbitrary new entries into the C Libraries
signal permissions table.

When a process's process group changes, the process server will send
it a message indicating the new process group. In this case, the
process server proves its authority by providing the task's kernel
port.

The library also implements messages to add and delete uids currently
used by the process. If new uids are sent to the program, the library
adds them to its current set, and then exchanges message with all the
I/O servers it knows about, proving to them its new authorization.
Similarly, uids can be deleted with a message. In the latter case,
the caller must provide the process's task port. (You can't harm a
process by giving it extra permission, but you can harm it by taking
permission away.) We will provide user programs to send these
messages to processes. This will enable the `su' command to cause all
the programs in your current login session to gain a new uid, rather
than spawn a subshell.

The library will allow programs to add asynchronous messages they wish
to recognize, as well as prevent recognition of the standard set.


MAKING IT LOOK LIKE UNIX

The C Library will implement all the calls from BSD and Posix, as well
as some obvious extensions to them. This enables users to replace
those calls they dislike, or bypass them entirely, whereas in Unix,
the calls must be used "as they come", with no alternatives possible.

On some environments, we will support binary compatibility as well.
This works by building a special version of the library. This is then
loaded somewhere in the address space of the process. (On, a Vax, for
example, it would be tucked in above the stack.) A feature of Mach,
called system call redirection is then used to trap Unix system calls
and turn them into jumps into this special version of the library.
(On almost all machines, the cost of such a redirection is very small;
this is a highly optimized path of Mach. On a 386 it's about two
dozen instructions.) This is only slightly worse than a simple
procedure call.

Many features of Unix, such as signal masks and vectors, are handled
completely by the library. This makes such calls significantly
cheaper than in Unix. It is now reasonable to use sigblock
extensively to protect critical sections, rather than seeking out some
other, less expensive method.


NETWORK PROTOCOLS

We are writing a library that will make it very easy to port 4.4 BSD
protocol stacks into the Hurd. This will enable operation, virtually
for free, of all the protocols supported by BSD. Currently, this
includes the CCITT protocols, the TCP/IP protocols, the Xerox NS
protocols, and the ISO protocols.

For maximal performace, some work would be necessary to take advantage
of Hurd features that provide for very high speed I/O. Generally, for
most protocols, this would require some thought, but not too much
time. We intend to spend effort making the TCP/IP protocols run as
efficiently as possible.

As an interesting example of the flexibility of the Hurd design,
consider the case of IP trailers, used extensively in BSD for
performance. While the Hurd will be willing to send and receive
trailers, it will gain fairly little advantage in doing so, because
there is no requirement that data be copied *ever*, so avoiding copies
for page-aligned data is not important.


--
Michael I. Bushnell | Watch, for you do not know when the master of
+1 617 625 4518 (H) -+- the house will come, in the evening, or at
+1 617 253 8568 (W) | midnight, or at cockcrow, or in the morning,
[email protected] | lest he come suddenly and find you asleep.