1. INTRODUCTION

When we talk about free software, we usually refer to the free

software licenses. We also need relief from software patents, so our freedom

is not restricted by them. But there is a third type of freedom we need, and

that's user freedom.

Expert users don't take a system as it is. They like to change the

configuration, and they want to run the software that works best for them.

That includes window managers as well as your favourite text editor. But

even on a GNU/Linux system consisting only of free software, you can not

easily use the filesystem format, network protocol or binary format you want

without special privileges. In traditional Unix systems, user freedom is

severly restricted by the system administrator.

The Hurd is built on top of CMU's Mach 3.0 kernel and uses Mach's

virtual memory management and message-passing facilities. The GNU C

Library will provide the Unix system call interface, and will call the Hurd for

needed services it can't provide itself. The design and implementation of the

Hurd is being lead by Michael Bushnell, with assistance from Richard

Stallman, Roland McGrath, Jan Brittenson, and others.

2. A MORE USABLE APPROACH TO OS DESIGN

The fundamental purpose of an operating system (OS) is to enable a

variety of programs to share a single computer efficiently and productively.

This demands memory protection, preemptively scheduled timesharing,

coordinated access to I/O peripherals, and other services. In addition, an OS

can allow several users to share a computer. In this case, efficiency demands

services that protect users from harming each other, enable them to share

without prior arrangement, and mediate access to physical devices.

On today's computer systems, programmers usually implement these

goals through a large program called the kernel. Since this program must be

accessible to all user programs, it is the natural place to add functionality to

the system. Since the only model for process interaction is that of specific,

individual services provided by the kernel, no one creates other places to add

functionality. As time goes by, more and more is added to the kernel.

A traditional system allows users to add components to a kernel only

if they both understand most of it and have a privileged status within the

system. Testing new components requires a much more painful edit-compile-

debug cycle than testing other programs. It cannot be done while others are

using the system. Bugs usually cause fatal system crashes, further disrupting

others' use of the system. The entire kernel is usually non-pageable. (There

are systems with pageable kernels, but deciding what can be paged is

difficult and error prone. Usually the mechanisms are complex, making them

difficult to use even when adding simple extensions.)

Because of these restrictions, functionality which properly belongs

behind the wall of a traditional kernel is usually left out of systems unless it

is absolutely mandatory. Many good ideas, best done with an open/read/write

interface cannot be implemented because of the problems inherent in the

monolithic nature of a traditional system. Further, even among those with the

endurance to implement new ideas, only those who are privileged users of

their computers can do so. The software copyright system darkens the mire

by preventing unlicensed people from even reading the kernel source The

Hurd removes these restrictions from the user. It provides an user extensible

system framework without giving up POSIX compatibility and the unix

security model.

When Richard Stallman founded the GNU project in 1983, he

wanted to write an operating system consisting only of free software. Very

soon, a lot of the essential tools were implemented, and released under the

GPL. However, one critical piece was missing: The kernel. After considering

several alternatives, it was decided not to write a new kernel from scratch,

but to start with the Mach microkernel. This was in 1988, and it was not

before 1991 that Mach was released under a license allowing the GNU

project to distribute it as a part of the system.

2.1 Kernel Architectures

Microkernels were very popular in the scientific world around that

time. They don't implement a full operating system, but only the

infrastructure needed to enable other tasks to implement most features. In

contrast, monolithical kernels like Linux contain program code of device

drivers, network protocols, process management, authentication, file systems,

POSIX compatible interfaces and much more.

So what are the basic facilities a microkernel provides? In general,

this is resource management and message passing. Resource management,

because the kernel task needs to run in a special privileged mode of the

processor, to be able to manipulate the memory management unit and

perform context switches (also to manage interrupts). Message passing,

because without a basic communication facility the other tasks could not

interact to provide the system services. Some rudimentary hardware device

support is often necessary to bootstrap the system. So the basic jobs of a

microkernel are enforcing the paging policy (the actual paging can be done

by an external pager task), scheduling, message passing and probably basic

hardware device support.

Mach was the obvious choice back then, as it provides a rich set of

interfaces to get the job done. Beside a rather brain-dead device interface, it

provides tasks and threads, a messaging system allowing synchronous and

asynchronous operation and a complex interface for external pagers. The

GNU project maintains its own version of Mach, called GNU Mach, which is

based on Mach 4.0. In addition to the features contained in Mach 4.0, the

GNU version contains many of the Linux 2.0 block device and network card

drivers.

2.2 Micro vs Monolithic

Microkernel

Clear cut responsibilities

Flexibility in operating system design, easier debugging

More stability (less code to break)

New features are not added to the kernel

Monolithic kernel

Intolerance or creeping featuritis

Danger of spaghetti code

Small changes can have far reaching side effects Because the system is split up into several components, clean interfaces have

to be developed, and the responsibilities of each part of the system must be

clear.

Once a microkernel is written, it can be used as the base for several

different operating systems. Those can even run in parallel which makes

debugging easier. When porting, most of the hardware dependant code is in

the kernel.

Much of the code that doesn't need to run in the special kernel mode

of the processor is not part of the kernel, so stability increases because there

is simply less code to break. New features are not added to the kernel, so

there is no need to hold the barrier high for new operating system features.

Compare this to a monolithical kernel, where you either suffer from creeping

featuritis or you are intolerant of new features (we see both in the Linux

kernel).

Because in a monolithical kernel, all parts of the kernel can access

all data structures in other parts, it is more likely that short cuts are used to

avoid the overhead of a clean interface. This leads to a simple speed up of the

kernel, but also makes it less comprehensible and more error prone. A small

change in one part of the kernel can break remote other parts.

2.3 Single Server vs Multi Server

Single Server

A single task implements the functionality of the operating system.

Multi Server

Many tasks cooperate to provide the system's functionality.

One server provides only a small but well-defined part of the whole

system.

The responsibilities are distributed logically among the servers.

A single-server system is comparable to a monolithic kernel system.

It has similar advantages and disadvantages.

There exist a couple of operating systems based on Mach, but they

all have the same disadvantages as a monolithical kernel, because those

operating systems are implemented in one single process running on top of

the kernel. This process provides all the services a monolithical kernel would

provide. This doesn't make a whole lot of sense (the only advantage is that

you can probably run several of such isolated single servers on the same

machine). Those systems are also called single-server systems. The Hurd is

the only usable multi-server system on top of Mach. In the Hurd, there are

many server programs, each one responsible for a unique service provided by

the operating system. These servers run as Mach tasks, and communicate

using the Mach message passing facilities. One of them does only provide a

small part of the functionality of the system, but together they build up a

complete and functional POSIX compatible operating system.

3. MACH MEMORY MANAGEMENT

The Mach virtual memory architecture has some unique features

such as the ability to provide much of the functionality through user level

tasks. The second is the issue of translation lookaside buffer consistency on

multiprocessors. The third is the problem of using virtually addressed caches

correctly and efficiently.

3.1 Design Goals

Mach provide a rich set of features including the following:

Copy-on-write and read-write sharing of memory between related and

unrelated tasks

Memory-mapped file access

Large,sparsely populated address space

Memory sharing between processes on different machines

User control over page replacement policies

Mach separates all machine-dependent code into a small pmap

layer.This makes it easy to port Mach to a new hardware related architecture,

since only pmap layer needs to be rewritten. The rest of the code is machine-

independent and not modeled after any specific MMU architecture.

An important objective in the Mach VM design is to push much of

the VM functionality out of the kernel. From its conception, Mach’s

microkernel architecture allowed the traditional kernel level functionality

provided by user-level server tasks. Hence Mach VM relegates functions

such as paging to user level tasks.

Finally Mach integrates the memory level management and IPC

subsystems to gain two advantages. The location-independence of Mach IPC

allows virtual memory facilities to be transparently extended to a distributed

environment. Conversely, the copy-on-write sharing supported by the VM

subsystem allows the faster tranfer of large messages. There are however

some important drawbacks. The VM system is larger, slower, and more

complex than the BSD design. It uses more and larger data structures. Hence

it consumes more phyical memory for itself, leaving less available for the

processes. Since the design keeps machine dependent code to a minimum, it

cannot be properly optimized for any particular MMU architecture.

In addition, the se of message passing adds considerable overhead.

The cost is reduced in some cases by optimizing kernel-to kernel message

transfers. Overall, though, message passing is still a lot more expensive than

simple function calls. Except for the network shared memory manager,

external pagers are not used commonly.

3.2 Multiprocessor issues for TLB

Maintaining TLB consistency on a shared-memory multiprocessor is

a much more complex problem. Although all processors share the main

memory, each has its own TLB. Problems arise when one processor changes

an entry in a page table that may be active on another processor. The latter

may have a copy of that entry in its TLB and hence may continue to use

obsolete mapping. It is essential to propagate the change to the TLBs of any

processor that may be using the page table.

There are many situations in which a change to one page affects

TLBs on several processors:

The page is a kernel page

The page is shared by multiple processes, each running on a different

processor.

On a multi threaded systems, different threads of the same process may

be running concurrently on different processors. If one thread modifies a

mapping, all threads must see the change.

In the absence of hardware support, the kernel must solve the

problem in software using a notification mechanism based on cross-processor

interrupts.

3.2.1 TLB shootdown in mach

FIG 1 :Mach TLB shootdown algorithm

ResponderInitiator

Disable all interruptsClear own active flag

Lock pmapPost flush requests

Send CP interrupts

Unlock pmap

Set active flag Enable interrupts

Receive CP interruptsDisable all interrupts

Clear own active flag

Process flush requests Invalidate TLB entries

Set own active flagEnable interrupts

Change pmapFlush own TLB

Wait till active flags cleared

Wait till pmap unlocked

The Mach TLB shootdown algorithm involves a set of interactions

between the initiator and responder. The term shootdown refers to an

invalidating a TLB on another processor. To implement it, Mach uses a set of

per-process data structures:

An active flag, which shows whether the processor is actively using

some page table. If this flag is clear, the processor is participating in

shootdown and will not access and modifiable pmap entry.

A queue of invalidation requests. Each request specifies a mapping that

must be flushed from the TLB.

A set of currently active pmaps. Each processor usually has two active

pmaps-the kernel pmap and that of the current task.

Each pmap is protected by a spin lock, which serializes operations

on it. Each pmap also has a list of processors on which the pmap is currently

active.

3.3 Synchronization and deadlock avoidance

The shootdown algorithm uses several synchronization mechanisms,

and the precise order of the operations is important. It is important to disable

all interrupts, otherwise a device interrupt can idle multiple processors for a

long time. The lock on the pmap prevents two processors from

simultaneously initiating the shootdowns for the same pmap. The interrupts

must be disabled before locking the page table, or a processor may deadlock

when it receives a cross-processor interrupt while holding a lock.

The initiator clears its own active flag before locking the pmap, to

avoid some deadlock conditions. Suppose two processors, P1 and P2, attempt

to modify the same pmap. P1 disables interrupts, locks the map, and sends an

interrupt to P2. Meanwhile, P2 disables interrupt and blocks on the same

lock. Now we have a deadlock, since P1 is waiting for P2 to acknowledge the

interrupt, and P2 is waiting for P1 to release the pmap.

Clearing the active flag effectively acknowledges interrupts before

they arrive. In the above example, P1 will not block since P2 clears its flag

before trying to lock the pmap. When P1 unlocks the pmap, P2 will not

resume and process the flush request posted by P1.

The shootdown algorithm has a subtle effect on all resource locking.

It requires a consistent policy about whether interrupts are disabled before

acquiring a lock. Suppose P1 holds a resource with interrupts enabled, P2

tries to acquire it with interrupts disabled, and P3 initiates a shootdown with

P1 and P2 as responders. P3 sends a cross processor image to P1 and P2, and

blocks till they are acknowledged. P1 acknowledges its interrupt and blocks

until the pmap is released. P2 is blocked on the lock with interrupts disabled

and hence does not see or respond to the interrupt As a result we have a three

way deadlock. To prevent this, the system must enforce a fixed interrupt state

for each lock: Either a lock should always be acquired with interrupts

disabled or with interrupts enabled.

3.4 Page Replacement

The Mach page replacement uses three FIFO lists-active, inactive,

and free Pages migrate from one list to another in the following ways:

2. The first reference to a page results in a page fault. The fault handler

removes a page from the head of the free list and initializes it with

appropriate data. It then puts the page at the tail of the active list.

Eventually, the page migrates to the head of this list, and pages ahead of

it become inactive.

3. Whenever free memory falls below a threshold value, the pagedaemon is

awakened. It removes some pages from the head of the active list to the

tail of the inactive list. It turns off the reference bit in the hardware

address translation mappings for these pages.

4. The pagedaemon also examines the number of pages at the head of the

inactive list. Pages whose reference bits are set are returned to the tail of

the active list.

5. If the pagedaemon finds a page number whose reference bit is still clear,

the page has not been referenced while on the inactive queue and can be

moved to the tail of the free list. If the page is dirty, it is first written back

to its memory object.

6. If the page is referenced while on the free list, it may still be reclaimed. In

this case, it goes back to the tail of the active list. Otherwise, it will

migrate to the head of the free list and eventually be reused.

Fig 2 Page replacement

4. MACH INTER PROCESS COMMUNICATION

Ports are message queues, which can be used as one-way

communication channels.

Port rights are receive, send or send-once

Free list

Inactive list

Active list

1

23

5

4

Exactly one receiver

Potentially many senders

MiG provides remote procedure calls on top of Mach IPC. RPCs

look like function calls to the user.

Inter-process communication in Mach is based on the ports concept.

A port is a message queue, used as a one-way communication channel. In

addition to a port, you need a port right, which can be a send right, receive

right, or send-once right. Depending on the port right, you are allowed to

send messages to the server, receive messages from it, or send just one single

message.

For every port, there exists exactly one task holding the receive right,

but there can be no or many senders. The send-once right is useful for clients

expecting a response message. They can give a send-once right to the reply

port along with the message. The kernel guarantees that at some point, a

message will be received on the reply port (this can be a notification that the

server destroyed the send-once right).

You don't need to know much about the format a message takes to

be able to use the Mach IPC. The Mach interface generator mig hides the

details of composing and sending a message, as well as receiving the reply

message. To the user, it just looks like a function call, but in truth the

message could be sent over a network to a server running on a different

computer. The set of remote procedure calls a server provides is the public

interface of this server.

Ports also represent kernel objects. Hence each object, such as task,

thread or process is represented by a port. Rights to these ports represent

object references and allow the holder to perform operations on that object.

The kernel holds the receive rights to such ports.

Each port has a finite-size message queue. The size of this queue

provides a simple flow-control mechanism. Senders are blocked when the

queue is full, and the receivers when the queue is empty.

Each task and thread has a set of default ports. For instance each task

has send rights to a task_self_port that represents itself and receive rights to a

task_notify_port. Tasks also have send rights to a bootstrap port that provides

access to a name server. Each thread has send rights to a thread_self port, and

receive rights to a reply port, used to receive replies from system calls and

remote procedure calls to other tasks. There is an exception port associated

with each task and each thread. The rights to the per-thread ports are owned

by the task in which the thread runs; hence these ports can be accessed by all

threads within the task.

Tasks also inherit other port rights from their parents. Each task has

a list of registered ports. These allow the task to access various system-wide

services. These ports are inherited by new tasks during task creation.

4.1 Messages

Mach is a message-passing kernel, and most system services are

accessed by exchanging messages. Mach IPC provides communication

between user tasks, between user and kernel and between different kernel

subsystems. A user-level program called the netmsgserver transparently

extends Mach IPC across network, so tasks can exchange messages with

remote tasks as easily as with local ones. The fundamental abstractions of

Mach IPC are the messages and the port.

4.2 How to get a port?

The Hurd:

The filesystem is used as the server namespace.

Root directory port is inserted into each task.

The C library finds other ports with hurd_file_name_lookup, performing a

pathname resolution. Like a tree of phone books.

You need something like a phone book for server ports, or otherwise

you can only talk to yourself. In the original Mach system, a special

nameserver is dedicated to that job. A task could get a port to the nameserver

from the Mach kernel and ask it for a port (with send right) to a server that

registered itself with the nameserver at some earlier time.

In the Hurd, there is no nameserver. Instead, the filesystem is used as

the server namespace. This works because there is always a root filesystem in

the Hurd (remember that the Hurd is a POSIX compatible system); this is an

assumption the people who developed Mach couldn't make, so they had to

choose a different strategy. You can use the function

hurd_file_name_lookup, which is part of the C library, to get a port to the

server belonging to a filename. Then you can start to send messages to the

server in the usual way.

Example of hurd_file_name_lookup

mach_port_t identity;

mach_port_t pwserver;

kern_return_t err;

pwserver = hurd_file_name_lookup

("/servers/password");

err = password_check_user (pwserver,

0 /* root */, "supass",

&identity);

As a concrete example, the special filename /servers/password can

be used to request a port to the Hurd password server, which is responsible to

check user provided passwords.

(explanation of the example)

Pathname resolution example

Task: Lookup /mnt/readme.txt where /mnt has a mounted filesystem.

The C library asks the root filesystem server about /mnt/readme.txt.

The root filesystem returns a port to the mnt filesystem server

(matching /mnt) and the retry name /readme.txt.

The C library asks the mnt filesystem server about /readme.txt.

The mnt filesystem server returns a port to itself and records that this

port refers to the regular file /readme.txt.

The C library itself does not have a full list of all available servers. Instead

pathname resolution is used to traverse through a tree of servers. In fact,

filesystems themselves are implemented by servers (let us ignore the chicken

and egg problem here). So all the C library can do is to ask the root

filesystem server about the filename provided by the user (assuming that the

user wants to resolve an absolute path), using the dir_lookup RPC. If the

filename refers to a regular file or directory on the filesystem, the root

filesystem server just returns a port to itself and records that this port

corresponds to the file or directory in question. But if a prefix of the full path

matches the path of a server the root filesystem knows about, it returns to the

C library a port to this server and the remaining part of the pathname that

couldn't be resolved. The C library than has to retry and query the other

server about the remaining path component. Eventually, the C library will

either know that the remaining path can't be resolved by the last server in the

list, or get a valid port to the server in question.

Mapping the POSIX Interface

Filedescriptor Port to server providing the file

fd = open(name,...) dir_lookup(..,name,..,&port)

[pathname resolution]

Read(fd, ...) io_read(port, ...)

write(fd, ...) io_write(port, ...)

Fstat(fd, ...) io_stat(port, ...)

...

It should by now be obvious that the port returned by the server can be used

to query the files status, content and other information from the server, if

good remote procedure calls to do that are defined and implemented by it.

This is exactly what happens. Whenever a file is opened using the C libraries

open() call, the C library uses the above pathname resolution to get a port to

a server providing the file. Then it wraps a file descriptor around it. So in the

Hurd, for every open file descriptor there is a port to a server providing this

file. Many other C library calls like read() and write() just call a

corresponding RPC using the port associated with the file descriptor.

File System Servers

Provide file and directory services for ports (and more).

These ports are returned by a directory lookup.

Translate filesystem accesses through their root path (hence the name

translator).

The C library maps the POSIX file and directory interface (and more)

to RPCs to the filesystem servers ports, but also does work on its own.

Any user can install file system servers on inodes they own.

So we don't have a single phone book listing all servers, but rather a tree of

servers keeping track of each other. That's really like calling your friend and

asking for the phone number of the blond girl at the party yesterday. He

might refer you to a friend who hopefully knows more about it. Then you

have to retry.

This mechanism has huge advantages over a single nameserver. First, note

that standard unix permissions on directories can be used to restrict access to

a server (this requires that the filesystems providing those directories

behave). You just have to set the permissions of a parent directory

accordingly and provide no other way to get a server port.

But there are much deeper implications. Most of all, a pathname never

directly refers to a file, it refers to a port of a server. That means that

providing a regular file with static data is just one of the many options the

server has to service requests on the file port. A server can also create the

data dynamically. For example, a server associated with /dev/random can

provide new random data on every io_read() on the port to it. A server

associated with /dev/fortune can provide a new fortune cookie on every

open().

While a regular filesystem server will just serve the data as stored in a

filesystem on disk, there are servers providing purely virtual information, or

a mixture of both. It is up to the server to behave and provide consistent and

useful data on each remote procedure call. If it does not, the results may not

match the expectations of the user and confuse him.

A footnote from the Hurd info manual:

(1) You are lost in a maze of twisty little filesystems, all alike....

Because a server installed in the filesystem namespace translates all

filesystem operations that go through its root path, such a server is also called

"active translator". You can install translators using the settrans command

with the -a option.

Passive Transla

Active Translators:

"settrans -a /cdrom /hurd/isofs /dev/hd2"

Are running filesystem servers.

Are attached to the root node they translate.

Run as a normal process.

Go away with every reboot, or even time out.

Many translator settings remain constant for a long time. It would be very

lame to always repeat the same couple of dozens settrans calls manually or at

boot time. So the Hurd provides a filesystem extension that allows to store

translator settings inside the filesystem and let the filesystem servers do the

work to start those servers on demand. Such translator settings are called

"passive translators". A passive translator is really just a command line string

stored in an inode of the filesystem. If during a pathname resolution a server

encounters such a passive translator, and no active translator does exist

already (for this node), it will use this string to start up a new translator for

this inode, and then let the C library continue with the path resolution as

described above. Passive translators are installed with settrans using the -p

option (which is already the default).

Passive Translators:

"settrans /mnt /hurd/ext2fs /dev/hd1s1"

Are stored as command strings into an inode.

Are used to start a new active translator if there isn't one.

Startup is transparent to the user.

Startup happens the first time the server is needed.

Are permanent across reboots (like file data).

So passive translators also serve as a sort of automounting feature, because

no manual interaction is required. The server start up is deferred until the

service is need, and it is transparent to the user.

When starting up a passive translator, it will run as a normal process with the

same user and group id as those of the underlying inode. Any user is allowed

to install passive and active translators on inodes that he owns. This way the

user can install new servers into the global namespace (for example, in his

home or tmp directory) and thus extend the functionality of the system (recall

that servers can implement other remote procedure calls beside those used for

files and directories). A careful design of the trusted system servers makes

sure that no permissions leak out.

In addition, users can provide their own implementations of some of the

system servers instead the system default. For example, they can use their

own exec server to start processes. The user specific exec server could for

example start java programs transparently (without invoking the interpreter

manually). This is done by setting the environment variable

EXECSERVERS. The systems default exec server will evaluate this

environment variable and forward the RPC to each of the servers listed in

turn, until some server accepts it and takes over. The system default exec

server will only do this if there are no security implications. (XXX There are

other ways to start new programs than by using the system exec server.

Those are still available.)

Let's take a closer look at some of the Hurd servers. It was already mentioned

that only few system servers are mandatory for users. To establish your

identity within the Hurd system, you have to communicate with the trusted

systems authentication server auth. To put the system administrator into

control over the system components, the process server does some global

bookkeeping.

But even these servers can be ignored. However, registration with the

authentication server is the only way to establish your identity towards other

system servers. Likewise, only tasks registered as processes with the process

server can make use of its services.

Authentication

A user identity is just a port to an auth server. The auth server stores four

set of ids for it:

effective user ids

effective group ids

available user ids

available group ids

Basic properties:

Any of these can be empty.

A 0 among the user ids identifies the super user.

Effective ids are used to check if the user has the permission.

Available ids can be turned into effective ids on user request.

The Hurd auth server is used to establish the identity of a user for a server.

Such an identity (which is just a port to the auth server) consists of a set of

effective user ids, a set of effective group ids, a set of available user ids and a

set of available group ids. Any of these sets can be empty.

Operations on authentication ports

The auth server provides the following operations on ports:

Merge the ids of two ports into a new one.

Return a new port containing a subset of the ids in a port.

Create a new port with arbitrary ids (super user only).

Establish a trusted connection between users and servers.

If you have two identities, you can merge them and request an identity

consisting of the unions of the sets from the auth server. You can also create

a new identity consisting only of subsets of an identity you already have.

What you can't do is extending your sets, unless you are the super user,

which is denoted by having the user id 0.

Establishing trusted connections

User provides a rendezvous port to the server (with io_reauthenticate).

User calls auth_user_authenticate on the authentication port (his

identity), passing the rendezvous port.

Server calls auth_server_authenticate on its authentication port (to a

trusted auth server), passing the rendezvous port and the server port.

If both authentication servers are the same, it can match the

rendezvous ports and return the server port to the user and the user ids to

the server.

Finally, the auth server can establish the identity of a user for a server. This is

done by exchanging a server port and a user identity if both match the same

rendezvous port. The server port will be returned to the user, while the server

is informed about the id sets of the user. The server can then serve or reject

subsequent RPCs by the user on the server port, based on the identity it

received from the auth server.

Anyone can write a server conforming to the auth protocol, but of course all

system servers use a trusted system auth server to establish the identity of a

user. If the user is not using the system auth server, matching the rendezvous

port will fail and no server port will be returned to the user. Because this

practically requires all programs to use the same auth server, the system auth

server is minimal in every respect, and additional functionality is moved

elsewhere, so user freedom is not unnecessarily restricted.

Password Server

The password server /servers/password runs as root and returns a new

authentication port in exchange for a unix password.

The ids corresponding to the authentication port match the unix user and

group ids.

Support for shadow passwords is implemented here.

The password server sits at /servers/password and runs as root. It can hand

out ports to the auth server in exchange for a unix password, matching it

against the password or shadow file. Several utilities make use of this server,

so they don't need to be setuid root.

Process Server

The superuser must remain control over user tasks, so:

All mach tasks are associated with a PID in the system default proc

server.

Optionally, user tasks can store:

Their environment variables.

Their argument vector.

A port, which others can request based on the PID (like a name

server).

Also implemented in the proc server:

Sessions and process groups.

Global configuration not in Mach, like hostname, hostid, system

version.

The process server is responsible for some global bookkeeping. As such it

has to be trusted and is not replaceable by the user. However, a user is not

required to use any of its service. In that case the user will not be able to take

advantage of the POSIXish appearance of the Hurd.

The Mach Tasks are not as heavy as POSIX processes. For example, there is

no concept of process groups or sessions in Mach. The proc server fills in the

gap. It provides a PID for all Mach tasks, and also stores the argument line,

environment variables and other information about a process (if the mach

tasks provide them, which is usually the case if you start a process with the

default fork()/exec()). A process can also register a message port with the

proc server, which can then be requested by anyone. So the proc server also

functions as a name server using the process id as the name.

The proc server also stores some other miscellaneous information not

provided by Mach, like the hostname, hostid and system version. Finally, it

provides facilities to group processes and their ports together, as well as to

convert between pids, process server ports and mach task ports.

User tasks not registering themselves with proc only have a PID assigned.

Users can run their own proc server in addition to the system default, at

least for those parts of the interface that don't require super user privileges.

Although the system default proc server can't be avoided (all Mach tasks

spawned by users will get a pid assigned, so the system administrator can

control them), users can run their own additional process servers if they

want, implementing the features not requiring super user privileges.

File systems

Store based file systems

ext2fs

ufs

isofs (iso9660, RockRidge, GNU extensions)

fatfs (under development)

Network file systems

nfs

ftpfs

Miscellaneous

hostmux

usermux

tmpfs (under development)

We already talked about translators and the file system service they provide.

Currently, we have translators for the ext2, ufs and iso9660 file systems. We

also have an nfs client and an ftp file system. Especially the latter is

intriguing, as it provides transparent access to ftp servers in the file system.

Programs can start to move away from implementing a plethora of network

protocols, as the files are directly available in the file system through the

standard POSIX file interface.

Part 2: A Look at Some of the Hurd's Disadvantages

The Authentication Server

One of the Hurd's more central servers is the authentication server. Each port

to this server identifies a user and is associated by this server with an id

block. Each id block contains sets of user and group ids. Either set may be

empty. This server is not the same as the password server referred to above.

The authentication server exports three services. First, it provides simple

boolean operations on authentication ports: given two authentication ports,

this server will provide a third port representing the union of the two sets of

uids and gids. Second, this server allows any user with a uid of zero to create

an arbitrary authentication port. Finally, this server provides RPCs (Remote

Procedure Calls between different programs and possibly different hosts)

which allow mutually untrusting clients and servers to establish their

identities and pass initial information on each other. This is crucial to the

security of the filesystem and I/O protocols.

Any user could write a program which implements the authentication

protocol; this does not violate the system's security. When a 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 there is a separate password

server.)

The Process Server

The process server acts as an information categorization repository. There are

four main services supported by this server. First, the process server keeps

track of generic host-level information not handled by the Mach kernel. For

example, the hostname, the hostid, and the system version are maintained by

the process server. Second, this server maintains the Posix notions of

sessions and process groups, to help out programs that wish to use Posix

features.

Third, the process server maintains a one-to-one mapping between Mach

tasks and Hurd processes. Every task is assigned a pid. Processes can register

a message port with this server, which can then be given out to any program

which requests it. This server makes no attempt to keep these message ports

private, so user programs are expected to implement whatever security they

need themselves. (The GNU C Library provides convenient functions for all

this.) Processes can tell the process server their current `argv' and `envp'

values; this server will then provide, on request, these vectors of arguments

and environment. This is useful for writing ps-like programs and also makes

it easier to hide or change this information. None of these features are

mandatory. Programs are free to disregard all of this and never register

themselves with the process server at all. They will, however, still have a pid

assigned.

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, while ensuring the security of the ports managed.

It is important to stress 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. 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 remain to be fleshed out, and will

doubtless change with experience.

In a BSD kernel, a transparent FTP file system would be no harder to write

than 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 systems to place such

things 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. (Recent BSD kernels make it possible to write a user program that

provides transparent FTP. An example is alex, but it needs to run with full

root privileges.)

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 probably not be

directly list able, though further subdirectories will be. There will be a

variety of possible formats. For example, to access files on UUNet, one could

cd /ftp/ftp.uu.net:anonymous:mib@gnu. Or to access files on a remote

account, one might cd /ftp/gnu.org:mib:passwd. Parts of this command could

be left out and the transparent FTP program would read them from a user's

.netrc file. In the last case, one might just cd /ftp/gnu.org; when the rest of the

data is already in .netrc.

There is no need to do a cd first--use any file command. To find out about

RFC 1097 (the Telnet Subliminal Message Option), just type more

/ftp/ftp.uu.net/inet/rfc/rfc1097. A copy command to a local disk could be

used if the RFC would be read frequently.

File systems

Ordinary filesystems are also being implemented. The initial release of the

Hurd will contain a file system upwardly compatible with the BSD 4.4 Fast

File System. In addition to the ordinary semantics, it will provide means to

record translators, offer thirty-two bit user ids and group ids, and supply a

new id per file, called the author of the file, which can be set by the owner

arbitrarily. 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 existing world category of file permissions).

The Network File System protocol will be implemented using 4.4 BSD as a

starting point. A log-structured file system will also be implemented using

the same ideas as in Sprite, but probably not the same format. A GNU

network file protocol may be designed in time, or NFS may be extended to

remove its deficiencies. There will also be various ``little'' file systems, such

as the MS-DOS file system, to help people move files between GNU and

other OSs.

Terminals

An I/O server will provide the terminal semantics of Posix. The GNU C

Library has features for keeping track of the controlling terminal and for

arranging to have proper job control signals sent at the proper times, as well

as features for obeying keyboard and hang-up signals.

Programs will be able to insert a terminal driver into communications

channels in a variety of ways. Servers like rlogind will be able to insert the

terminal protocol onto their network communication port. Pseudo-terminals

will not be necessary, though they will be provided for backward

compatibility with older programs. No programs in GNU will depend on

them.

Nothing about a terminal driver is forced upon users. A terminal driver

allows a user to get at the underlying communications channel easily, to

bypass itself on an as-needed basis or altogether, or to substitute a different

terminal driver-like program. In the last 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. In time, there will be a readline-based terminal driver, which will

provide complex line-editing features for those users who want them.

The terminal driver will probably not provide good support for the high-

volume, rapid data transmission required by UUCP or SLIP. Those programs

do not need any of its features. Instead they will be use the underlying Mach

device ports for terminals, which support moving large amounts of data

efficiently.

Executing Programs

The implementation of the execve call is spread across three programs. The

library marshals the argument and environment vectors. It then sends a

message to the file server that holds the file to be executed. The file server

checks execute permissions and makes 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 of the new image. 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, then the exec needs to take place in a new

Mach task to maintain security.

After deciding the policy associated with the new image, the file system calls

the exec server to load the task. This server, using the BFD (Binary File

Descriptor) library, loads the image. BFD supports a large number of object

file formats; almost any supported format will be executable. This server also

handles scripts starting with #!, running them through the indicated program.

The standard exec server also looks at the environment of the new image; if

it contains a variable EXECSERVERS 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 GNU C Library, which sends a message

to the exec server to get the arguments, environment, umask, current

directory, etc. None of this additional state is special to the file or exec

servers; if programs wish, they can use it in a different manner than the

Library.

New Processes

The fork call is implemented almost entirely in the GNU 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. (These features should not be used to

replace the normal fork-calling sequence; it is intended for libraries which

need to close ports or clean up before a fork occurs.) The C library will

implement both fork calls specified by the draft Posix.4a (the proposed

standard dealing with 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, or the function can even be

completely replaced. None of this is possible in a traditional 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 GNU C Library lists a

variety of ports in a table, each of which identifies a set of signals that can be

sent by anyone who possesses that port. For example, if the user possesses

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 hang-up

signals. Users can add arbitrary new entries into the C library's 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 C library also has 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 messages with all the I/O servers it knows

about, proving to them its new authorization. Similarly, a message can delete

uids. 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.) The Hurd will provide user programs to send these

messages to processes. For example, the su command will be able to cause

all the programs in your current login session, to gain a new uid, rather than

spawn a subshell.

The C 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 of 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.

In some environments binary compatibility will also be supported. This

works by building a special version of the library which is then loaded

somewhere in the address space of the process. (For example, on a VAX, 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 in Mach. On a

386 it's about two dozen instructions. This is little 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 features 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

The Hurd will have 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 optimal performance some work would be necessary to take advantage of

Hurd features that provide for very high speed I/O. For most protocols this

will require some thought, but not too much time. The Hurd will run the

TCP/IP protocols 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 and avoiding

copies for page-aligned data is irrelevant.

Who Should Use the Hurd?

Firstly, a note for end-users: if you do not consider yourself "computer

literate," then you definitely will not want to use the Hurd. No official release

of the Hurd has yet been made, and the system is currently unstable. If you

run the Hurd, you will encounter many bugs. For those people who use their

computers for web-surfing, email, word processing, etc., and just want the

infernal machine to work, the Hurd's bugs would prove extremely annoying.

For such people, a more stable system is preferable. Fortunately, the GNU

system currently exists in a very stable state with Linux substituted for the

Hurd as the kernel, so it is possible for end-users to use a powerful, stable,

and Free Unix-like operating system. Debian GNU/Linux, a very high-

quality GNU/Linux distribution, is available; in addition, many commercial

companies sell boxed GNU/Linux distributions with printed documentation,

and, to various extents, take measures to hide the complexity of the system

from the user. Keep in mind that the GNU/Hurd is not the system you use for

web-surfing, email, word processing, and other such tasks now ... it is the

system that you will use for these tasks in the future.

Those who consider themselves computer-literate and are interested in the

Hurd, but do not have experience with Unix-like systems, may also wish to

learn the ropes using a more stable GNU/Linux system. Unix-like systems

are quite different from other systems you may have used, and they take

some getting used to. If you plan to use the GNU/Hurd in the future, we

recommend you use Debian GNU/Linux.

If you consider yourself computer-literate, but are not a programmer, you can

still contribute to the Hurd project. Tasks for non-programmers include

running GNU/Hurd systems and testing them for bugs, writing

documentation, and translating existing documentation into other languages.

At present, most GNU/Hurd documentation is available only in English

and/or French.

Anyone who might be interested in the Hurd: a student studying the system,

a programmer helping to develop the Hurd servers, or an end-user finding

bugs or writing documentation, will be interested in how GNU/Hurd is

similar to, and different from, Unix-like kernels.

For all intents and purposes, the Hurd is a modern Unix-like kernel, like

Linux and the BSDs. GNU/Hurd uses the GNU C Library, whose

development closely tracks standards such as ANSI/ISO, BSD, POSIX,

Single Unix, SVID, and X/Open. Hence, most programs available on

GNU/Linux and BSD systems will eventually be ported to run on GNU/Hurd

systems.

An advantage of all these systems - GNU/Hurd, GNU/Linux, and the BSDs -

is that, unlike many popular operating systems, is that they are Free

Software. Anybody can use, modify, and redistribute these systems under the

terms of the GNU General Public License (see GNU General Public License)

in the case of GNU/Hurd and GNU/Linux, and the BSD license in the case of

the BSDs. In fact, the entire GNU System is a complete Unix-like operating

system licensed under the GNU GPL.

Although it is similar to other Free Unix-like kernel projects, the Hurd has

the potential to be much more. Unlike these other projects, the Hurd has an

object-oriented structure that allows it to evolve without compromising its

design. This structure will help the Hurd undergo major redesign and

modifications without having to be entirely rewritten. This extensibility

makes the Hurd an attractive platform for learning how to become a kernel

hacker or for implementing new ideas in kernel technology, as every part of

the system is designed to be modified and extended. For example, the MS-

DOS FAT filesysetem was not supported by GNU/Hurd until a developer

wrote a translator that allows us to access this filesystem. In a standard Unix-

like environment, such a feature would be put into the kernel. In GNU/Hurd,

this is done in a different manner and recompiling the kernel is not necessary,

since the filesystem is implemented as a user-space program.

Scalability has traditionally been very difficult to achieve in Unix-like

systems. Many computer applications in both science and business require

support for symmetric multiprocessing (SMP). At the time of this writing,

Linux could scale to a maximum of 8 processors. By contrast, the Hurd

implementation is aggressively multi-threaded so that it runs efficiently on

both single processors and symmetric multiprocessors. The Hurd interfaces

are designed to allow transparent network clusters (collectives), although this

feature has not yet been implemented.

Of course, the Hurd (currently) has its limitations. Many of these limitations

are due to GNU Mach, the microkernel on which the Hurd runs. For

example, although the Hurd has the potential to be a great platform for SMP,

no such multiprocessing is currently possible, since GNU Mach has no

support for SMP. The Hurd has supports less hardware than current versions

of Linux, since GNU Mach uses the hardware drivers from version 2.0 of the

Linux kernel. Finally, GNU Mach is a very slow microkernel, and

contributes to the overall slowness of GNU/Hurd systems.

Mach is known as a 1st-generation microkernel - much work has gone into

the much-improved 2nd-generation microkernels currently being developed.

A long-term goal is to port the Hurd to L4, a very fast 2nd-generation

microkernel. This port will offer substantial improvements to the system. In

the short term, the Hurd developers plan to move the Hurd to OSKit Mach,

an improved version of Mach being developed at the University of Utah.

The Hurd is still under active development, and no stable release has been

made. This means that the Hurd's code base is much less mature than that of

Linux or the BSDs. There are bugs in system that are still being found and

fixed. Also, many features, such as a DHCP client, and support for several

filesystem types, is currently missing.

The deficiencies in the Hurd are constantly being addressed; for example,

until recently, pthreads (POSIX threads), were missing. This meant that

several major applications, including GNOME, KDE, and Mozilla, could not

run on GNU/Hurd. Now that the Hurd has a preliminary pthreads

implementation, we may soon see these applications running on GNU/Hurd

systems.

The Hurd is a very modern design. It is more modern than Linux or the

BSDs, because it uses a microkernel instead of a monolithic kernel. It is also

more modern than Apple's Darwin or Microsoft's NT microkernel, since the

it has a multi-sever design, as opposed to the single-server design of Darwin

and NT. This makes the Hurd an ideal platform for students interested in

operating systems, since its design closely matches the recommendations of

current operating system theory. In addition, the Hurd's modular nature

makes it much easier to understand than many other kernel projects.

GNU/Hurd is also an excellent system for developers interested in kernel

hacking. Whereas Linux and the BSDs are quite stable, there is still much

work to be done on the Hurd; for example, among all the filesystem types in

use today, the Hurd only supports ext2fs (the Linux filesystem), ufs (the BSD

filesystem), and iso9660fs (the CD filesystem). GNU/Hurd offers a

developer the opportunity to make a substantial contribution to a system at a

relatively-early point in its development.

Hurd Today

The GNU operating system is alive and well today. In 1998 Marcus

Brinkmann brought the Hurd to the Debian Project. This allowed GNU/Hurd

to have a software management system(apt and dpkg), world wide access to

the latest free software, and a extreme increase of popularity.

The original architect, Thomas Bushnell, BSG, is involved on the mailing

lists helping the young hackers design and help with Hurd development.

There is now a team of individuals from around the world. They are

constantly giving talks at conferences about the importance of design and

implementation of the Hurd's unique multiserver microkernel environment.

There is also work on porting the Hurd to two other micro-kernels and

another architecture.

Debian GNU/Hurd has been Debian's most active non-Linux port. The user

applications that are ported to GNU/Hurd are very current and are actively

being improved. Development of GNU/Hurd within the Debian project has

also contributed to incremental development releases on sets of CDs. Also,

new developers are allowed to get the source code to the Hurd from

anywhere on the planet via Debian's packaging tool apt, and can get an even-

more recent snapshot release from the the GNU project's alpha.gnu.org

server.

As far as a multi-server microkernel based operating systems are concerned:

GNU/Hurd has had it's share of critisim. Many people have criticized the

Hurd for its lack of a stable release, its design, which is unfamiliar to those

used to monolithic kernels such as Linux and the BSD kernels, and its

connection to the strong ideals of the GNU Project; however, GNU/Hurd is a

cutting-edge operating system. Other operating systems with designs similar

to GNU/Hurd are QNX and Sawmill.

QNX is meant for embedded systems and many of the GNU Projects most

famous programs are available for it . Sawmill was a research project set up

by IBM. It was never released outside the academic community. Sawmill is a

version of the Linux kernel running as multi-server on top of the L4

microkernel. It used some of the same ideas as GNU/Hurd but with members

of the L4 Project influencing some of there research in microkernel based

technology.