From be6ec0cbbf094924d989d80ddde40d359e7f8ffd Mon Sep 17 00:00:00 2001 From: Thomas Schwinge Date: Thu, 18 Jun 2009 15:32:59 +0200 Subject: Move back some pages to their long-year locations, add some redirections, add stable_URL tags. --- hurd-talk.html | 1062 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 1062 insertions(+) create mode 100644 hurd-talk.html (limited to 'hurd-talk.html') diff --git a/hurd-talk.html b/hurd-talk.html new file mode 100644 index 00000000..ec9eb777 --- /dev/null +++ b/hurd-talk.html @@ -0,0 +1,1062 @@ +[[!meta copyright="Copyright © 2001 Marcus Brinkmann"]] + +[[!meta license="Verbatim copying and distribution of this entire article is +permitted in any medium, provided this notice is preserved."]] + +[[!meta title="The Hurd, a presentation by Marcus Brinkmann"]] + +[[!tag stable_URL]] + +

Table of Contents

+ +
+

Talk about the Hurd

+

+This talk about the Hurd was written by Marcus Brinkmann for +

+ +

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 removes these restrictions from the user. It provides an +user extensible system framework without giving up POSIX compatibility +and the unix security model. Throughout this talk, we will see that +this brings further advantages beside freedom. + +

Overview

+
+ +

+The Hurd is a POSIX compatible multi-server +system operating on top of the GNU Mach microkernel. + +

+Topics: +

    +
  • GNU Mach
    • +
  • The Hurd
    • +
  • Development
    • +
  • Debian GNU/Hurd
    • +
+
+ +

+The Hurd is a POSIX compatible multi-server system operating on top of +the GNU Mach Microkernel. + +

+I will have to explain what GNU Mach is, so we start with that. Then +I will talk about the Hurd's architecture. After that, I will give a +short overview on the Hurd libraries. Finally, I will tell you how +the Debian project is related to the Hurd. + +

Historicals

+ +
+
    +
  • 1983: Richard Stallman founds the GNU project.
    • +
  • 1988: Decision is made to use Mach 3.0 as the kernel.
    • +
  • 1991: Mach 3.0 is released under compatible license.
    • +
  • 1991: Thomas Bushnell, BSG, founds the Hurd project.
    • +
  • 1994: The Hurd boots the first time.
    • +
  • 1997: Version 0.2 of the Hurd is released.

    • +
  • 1998: Debian hurd-i386 archive is created.
    • +
  • 2001: Debian GNU/Hurd snapshot fills three CD images.
    • +
+
+ +

+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. +

+In 1998, I started the Debian GNU/Hurd project, and in 2001 the number +of available GNU/Hurd packages fills three CD images. + +

Kernel Architectures

+
+

+Microkernel: +

    +
  • Enforces resource management (paging, scheduling)
    • +
  • Manages tasks
    • +
  • Implements message passing for IPC
    • +
  • Provides basic hardware support
    • +
+

+Monolithic kernel: +

    +
  • No message passing necessary
    • +
  • Rich set of features (filesystems, authentication, network + sockets, POSIX interface, ...)
    • +
+
+

+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. It's certainly not one of the sexiest microkernels +that exist today, but more like a big old mama. 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. +

+A complete treatment of the differences between a microkernel and +monolithical kernel design can not be provided here. But a couple of +advantages of a microkernel design are fairly obvious. + +

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. + +

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. + +

Multi Server is superior, ...

+
+

+Any multi-server has advantages over single-server: +

    +
  • Clear cut responsibilities
    • +
  • More stability: If one server dies, all others remain
    • +
  • Easier development cycle: Testing without reboot (or replacing + running servers), debugging with gdb
    • +
  • Easier to make changes and add new features +
+
+

+Using several servers has many advantages, if done right. If a file +system server for a mounted partition crashes, it doesn't take down +the whole system. Instead the partition is "unmounted", and +you can try to start the server again, probably debugging it this time +with gdb. The system is less prone to errors in individual +components, and over-all stability increases. The functionality of +the system can be extended by writing and starting new servers +dynamically. (Developing these new servers is easier for the reasons +just mentioned.) +

+But even in a multi-server system the barrier between the system and +the users remains, and special privileges are needed to cross it. We +have not achieved user freedom yet. + +

The Hurd even more so.

+
+

+The Hurd goes beyond all this, and allows users to write and run their +servers, too! +

    +
  • Users can replace system servers dynamically with their own + implementations.
    • +
  • Users can decide what parts of the remainder of the system they + want to use.
    • +
  • Users can extend the functionality of the system.
    • +
  • No mutual trust necessary to make use of other users + services.
    • +
  • Security of the system is not harmed by trusting users + services.
    • +
+
+

+To quote Thomas Bushnell, BSG, from his paper +[[``Towards_a_New_Strategy_of_OS_design''_(1996)|hurd-paper]]: +

+The GNU Hurd, by contrast, is designed to make the area of system code +as limited as possible. Programs are required to communicate only +with a few essential parts of the kernel; the rest of the system is +replaceable dynamically. Users can use whatever parts of the +remainder of the system they want, and can easily add components +themselves 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. +
+ +

+So the Hurd is a set of servers running on top of the Mach +micro-kernel, providing a POSIX compatible and extensible operating +system. What servers are there? What functionality do they provide, +and how do they cooperate? + +

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
    • +
  • 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. + + +

How to get a port?

+
+

+Traditional Mach: +

    +
  • Nameserver provides ports to all registered servers.
    • +
  • The nameserver port itself is provided by Mach.
    • +
  • Like a phone book: One list.
    • +
+

+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.
    • +
+
+

+So how does one get a port to a server? 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

+
+ + + + + + + + + + + + + + + + + + + +
FiledescriptorPort 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. + +

Active vs Passive

+
+

+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 authserver. 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 superuser.
    • +
  • 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 (superuser 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 superuser 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 + nameserver).
    • +
+

+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 nameserver 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 themselve 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 superuser +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 +superuser privileges. + +

Filesystems

+
+

+Store based filesystems +

    +
  • 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 +filesystems. We also have an nfs client and an ftp filesystem. +Especially the latter is intriguing, as it provides transparent access +to ftp servers in the filesystem. Programs can start to move away +from implementing a plethora of network protocols, as the files are +directly available in the filesystem through the standard POSIX file +interface. + + +

Developing the Hurd

+
+

+Over a dozen libraries support the development of new servers. +

+For special server types highly specialized +libraries require only the implementation of a +number of callback functions. +

    +
  • Use libdiskfs for store based filesystems.
    • +
  • Use libnetfs for network filesystems, also for + virtual filesystems.
    • +
  • Use libtrivfs for simple filesystems providing only + a single file or directory.
    • +
+
+

+The Hurd server protocols are complex enough to allow for the +implementation of a POSIX compatible system with GNU extensions. +However, a lot of code can be shared by all or at least similar +servers. For example, all storage based filesystems need to be able to +read and write to a store medium splitted in blocks. The Hurd comes +with several libraries which make it easy to implement new servers. +Also, there are already a lot of examples of different server types in +the Hurd. This makes writing a new server easier. +

+libdiskfs is a library that supports writing store based +filesystems like ext2fs or ufs. It is not very useful for filesystems +which are purely virtual, like /proc or files in +/dev. +

+libnetfs is intended for filesystems which provide a rich +directory hierarchy, but don't use a backing store (for example ftpfs, +nfs). +

+libtrivfs is intended for filesystems which just provide +a single inode or directory. Most servers which are not intended to +provide a filesystem but other services (like +/servers/password) use it to provide a dummy file, so +that file operations on the servers node will not return errors. But +it can also be used to provide meaningful data in a single file, like +a device store or a character device. + +

Store Abstraction

+
+

+Another very useful library is libstore, which is used by all store +based filesystems. It provides a store media abstraction. A store +consists of a store class and a name (which itself can sometimes +contain stores). +

+Primitive store classes: +

    +
  • device store like device:hd2, device:hd0s1, device:fd0
    • +
  • file store like file:/tmp/disk_image
    • +
  • task store like task:PID
    • +
  • zero store like zero:4m (like /dev/zero, of size 4 MB)
    • +
+
+
+

+Composed store classes: +

    +
  • copy store like copy:zero:4m
    • +
  • gunzip/bunzip2 store like gunzip:device:fd0
    • +
  • concat store like concat:device:hd0s2:device:hd1s5
    • +
  • ileave store (RAID-0(2))
    • +
  • remap store like remap:10+20,50+:file:/tmp/blocks
    • +
  • ...
    • +
+

+Wanted: A similar abstraction for streams (based on channels), which +can be used by network and character device servers. +

+

+libstore provides a store abstraction, which is used by +all store based filesystems. The store is determined by a type and a +name, but some store types modify another store rather than providing +a new store, and thus stores can be stacked. For example, the device +store type expects a Mach device, but the remap store expects a list +of blocks to pick from another store, like remap:1+:device:hd2, which +would pick all blocks from hd2 but the first one, which skipped. +Because this functionality is provided in a library, all libstore +using filesystems support many different store kinds, and adding a new +store type is enough to make all store based filesystems support it. + +

Debian GNU/Hurd

+
+

+Goal: +

    +
  • Provide a binary distribution of the Hurd that is easy to + install.
    • +
+

+Constraints: +

    +
  • Use the same source packages as Debian GNU/Linux.
    • +
  • Use the same infrastructure: +
      +
    • Policy
      • +
    • Archive
      • +
    • Bug tracking system
      • +
    • Release process
      • +
    • +
+

+Side Goal: +

    +
  • Prepare Debian for the future: +
      +
    • More flexibility in the base system
      • +
    • Identify dependencies on the Linux kernel
      • +
    • +
+
+

+The Debian distribution of the GNU Hurd that I started in 1998 is +supposed to become a complete binary distribution of the Hurd that is +easy to install. + +

Status of the Debian GNU/Hurd binary archive

+

+See +http://buildd.debian.org/stats/graph.png +for the most current version of the statistic. + +

Status of the Debian infrastructure

+
+

+Plus: +

    +
  • Source packages can identify build and host OS using + dpkg-architecture.
    • +
+

+Minus: +

    +
  • The binary architecture field is insufficient.
    • +
  • The BTS has no architecture tag.
    • +
  • The policy/FHS need (small) Hurd specific extensions.
    • +
+
+

+While good compatibiity can be achieved at the source level, +the binary packages can not always express their relationship +to the available architectures sufficiently. +

+For example, the Linux version of makedev is binary-all, where +a binary-all-linux relationship would be more appropriate. +

+More work has to be done here to fix the tools. + +

Status of the Debian Source archive

+
+
    +
  • Most packages just work.
    • +
  • Maintainers are usually responsive and cooperative.
    • +
  • Turtle, the autobuilder, crunches through the whole list right + now.
    • +
+

+Common pitfalls are POSIX incompatibilities: +

    +
  • Upstream: +
      +
    • Unconditional use of PATH_MAX + (MAXPATHLEN), MAXHOSTNAMELEN.
      • +
    • Unguarded use of Linux kernel features.
      • +
    • Use of legacy interfaces (sys_errlist, + termio).
      • +
    • +
  • Debian: +
      +
    • Unguarded activation of extensions available with Linux.
      • +
    • Low quality patches.
      • +
    • Assuming GNU/Linux in package scripts.
      • +
    • +
+
+

+Most packages are POSIX compatible and can be compiled without +changes on the Hurd. The maintainers of the Debian source packages +are usually very kind, responsiver and helpful. +

+The Turtle autobuilder software (http://turtle.sourceforge.net) +builds the Debian packages on the Hurd automatically. + +

Debian GNU/Hurd: Good idea, bad idea?

+
+

+Upstream benefits: +

    +
  • Software packages become more portable.
    • +
+

+Debian benefits: +

    +
  • Debian becomes more portable.
    • +
  • Maintainers learn about portability and other systems.
    • +
  • Debian gets a lot of public recognition.
    • +
+

+GNU/Hurd benefits: +

    +
  • Large software base.
    • +
  • Great infrastructure.
    • +
  • Nice community to partner with.
    • +
+
+

+The sheet lists the advantages of all groups involved. + +

End

+
+

+Join us at +

+
+

+List of contacts. -- cgit v1.2.3