[[meta copyright="Copyright © 2008 Free Software Foundation, Inc."]] [[meta license="""[[toggle id="license" text="GFDL 1.2+"]][[toggleable id="license" text="Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license is included in the section entitled [[GNU_Free_Documentation_License|/fdl]]."]]"""]] This list offers a wide range of tasks: From rather straightforward to pretty involved ones; from low-level kernel hacking (drivers, VM) to high-level translators; from fixing bugs, implementing new functionality based on existing interfaces, to creating totally new mechanisms. If you have questions regarding the projects, or if there is more than one that you are interested in and you are unsure which to choose, don't hesitate to [[contact_us|communication]]. ## Bindings to Other Programming Languages The main idea of the Hurd design is giving users the ability to easily modify/extend the system's functionality ([[extensible_system|extensibility]]). This is done by creating [[filesystem_translators|hurd/translator]] and other kinds of Hurd servers. However, in practice this is not as easy as it should, because creating translators and other servers is quite involved -- the interfaces for doing that are not exactly simple, and available only for C programs. Being able to easily create simple translators in RAD languages is highly desirable, to really be able to reap the advantages of the Hurd architecture. Originally Lisp was meant to be the second system language besides C in the GNU system; but that doesn't mean we are bound to Lisp. Bindings for any popular high-level language, that helps quickly creating simple programs, are highly welcome. Several approaches are possible when creating such bindings. One way is simply to provide wrappers to all the available C libraries ([[hurd/libtrivfs]], [[hurd/libnetfs]] etc.). While this is easy (it requires relatively little consideration), it may not be the optimal solution. It is preferable to hook in at a lower level, thus being able te create interfaces that are specially adapted to make good use of the features available in the respective language. These more specialised bindings could hook in at some of the lower level library interfaces ([[hurd/libports]], [[hurd/glibc]], etc.); use the [[microkernel/mach/MIG]]-provided [[microkernel/mach/RPC]] stubs directly; or even create native stubs directly from the interface definitions. The task is to create easy to use Hurd bindings for a language of the student's choice, and some example servers to prove that it works well in practice. This project will require gaining a very good understanding of the various Hurd interfaces. Skills in designing nice programming interfaces are a must. There has already been some [earlier work on Python bindings](http://www.sigill.org/files/pytrivfs-20060724-ro-test1.tar.bz2), that perhaps can be re-used. Also some work on [Perl bindings](http://www.nongnu.org/hurdextras/#pith) is availabled. ### Lisp Most Lisp implementations provide a Foreign Function Interface (FFI) that enables the Lisp code to call functions written in another language. Specifically, most implementations provide an FFI to the C ABI (hence giving access to C, Fortran and possibly C++). Common Lisp has even a portability layer for such FFI, [CFFI](http://common-lisp.net/project/cffi/), so that you can write bindings purely in Lisp and use the same binding code on any implementation supported by CFFI. Many Scheme implementation also provide an FFI. [Scheme48](http://www.s48.org/) is even the implementation used to run scsh, a Scheme shell designed to provide instant access to POSIX functions. [Guile](http://www.gnu.org/software/guile/guile.html) is the GNU project's Scheme implementation, meant to be embeddable and provide access to C. At least [Gambit](http://dynamo.iro.umontreal.ca/~gambit/), [Chicken](http://www.call-with-current-continuation.org/), [Bigloo](http://www-sop.inria.fr/mimosa/fp/Bigloo/) and [PLT](http://www.plt-scheme.org/) are known to provide an FFI too. With respect to the packaging and dependencies, the good news is that Debian comes handy: 5 Common Lisp implementations are packaged, one of which has already been ported to Hurd (ECL), and CFFI is also packaged. As far as Scheme is concerned, 14 [R5RS](http://www.schemers.org/Documents/Standards/R5RS/) implementations are provided and 1 [R6RS](http://www.r6rs.org/). Pierre THIERRY would mentor creation of bindings for either Common Lisp or Scheme with their FFI (and welcome any attempt). ## Virtualization Using Hurd Mechanisms The main idea behind the Hurd design is to allow users to replace almost any system functionality ([[extensible_system|extensibility]]). Any user can easily create a subenvironment using some custom [[servers|hurd/translator]] instead of the default system servers. This can be seen as an [[advanced_lightweight_virtualization|hurd/virtualization]] mechanism, which allows implementing all kinds of standard and nonstandard virtualization scenarios. However, though the basic mechanisms are there, currently it's not easy to make use of these possibilities, because we lack tools to automatically launch the desired constellations. The goal is to create a set of powerful tools for managing at least one desirable virtualization scenario. One possible starting point could be the [[hurd/subhurd]]/[[hurd/neighborhurd]] mechanism, which allows a second almost totally independant instance of the Hurd in parallel to the main one. The current implementation has serious limitations though. A subhurd can only be started by root. There are no communication channels between the subhurd and the main one. There is no mechanism for safe sharing of hardware devices. Fixing this issues could turn subhurds into a very powerful solution for lightweight virtualization using so-called logical partitions. (Similar to Linux-vserver, OpenVZ etc.) While subhurd allow creating a complete second system instance, with an own set of Hurd servers and UNIX daemons and all, there are also situations where it is desirable to have a smaller subenvironment, living withing the main system and using most of its facilities -- similar to a chroot environment. A simple way to create such a subenvironment with a single command would be very helpful. It might be possible to implement (perhaps as a prototype) a wrapper using existing tools (chroot and [[hurd/translator/unionfs]]); or it might require more specific tools, like some kind of unionfs-like filesytem proxy that mirrors other parts of the filesystem, but allows overriding individual locations, in conjuction with either chroot or some similar mechanism to create a subenvironment with a different root filesystem. It's also desirable to have a mechanism allowing a user to set up such a custom environment in a way that it will automatically get launched on login -- practically allowing the user to run a customized operating system in his own account. Yet another interesting scenario would be a subenvironment -- using some kind of special filesystem proxy again -- in which the user serves as root, being able to create local sub-users and/or sub-groups. This would allow the user to run "dangerous" applications (webbrowser, chat client etc.) in a confined fashion, allowing it access to only a subset of the user's files and other resources. (This could be done either using a lot of groups for individual resources, and lots of users for individual applications; adding a user to a group would give the corresponding application access to the corresponding resource -- an advanced [[ACL]] mechanism. Or leave out the groups, assigning the resources to users instead, and use the Hurd's ability for a process to have multiple user IDs, to equip individual applications with sets of user IDs giving them access to the necessary resources -- basically a [[capability]] mechanism.) The student will have to pick (at least) one of the described scenarios -- or come up with some other one in a similar spirit -- and implement all the tools (scripts, translators) necessary to make it available to users in an easy-to-use fashion. While the Hurd by default already offers the necessary mechanisms for that, these are not perfect and could be further refined for even better virtualization capabilities. Should need or desire for specific improvements in that regard come up in the course of this project, implementing these improvements can be considered part of the task. Completing this project will require gaining a very good understanding of the Hurd architecture and spirit. Previous experience with other virtualization solutions would be very helpful. ## Namspace-based Translator Selection The main idea behind the Hurd is to make (almost) all system functionality user-modifiable ([[extensible_system|extensibility]]). This includes a user-modifiable filesystem: the whole filesystem is implemented decentrally, by a set of filesystem servers forming the directory tree together, a [[hurd/virtual_file_system]]. These filesystem servers are called [[translators|hurd/translator]], and are the most visible feature of the Hurd. The reason they are called translators is because when you set a translator on a filesystem node, the underlying node(s) are hidden by the translator, but the translator itself can access them, and present their contents in a different format -- translate them. A simple example is a [[gunzip_translator|hurd/translator/storeio]], which can be set on a gzipped file, and presents a virtual file with the uncompressed contents. Or the other way around. Or a translator that presents an [[XML_file_as_a_directory_tree|hurd/translator/xmlfs]]. Or an mbox as a set of individual files for each mail ([[hurd/translator/mboxfs]]); or ever further breaking it down into headers, body, attachements... This gets even more powerful when translators are used as building blocks for larger applications: A mail reader for example doesn't need backends for understanding various mailbox formats anymore. All formats can be parsed by special translators, and the mail reader gets the data as a uniform, directly usable filesystem structure. Translators can also be stacked: If you have a compressed mailbox for example, first apply a gunzip translator, and then an mbox translator on top of that. There are a few problems with the way translators are set, though. For one, once a translator is set on a node, you always see the translated content. If you need the untranslated contents again, to do a backup for example, you first need to remove the translator again. Also, having to set a translator explicitely before accessing the contents is pretty cumbersome, making this feature almost useless. A possible solution is implementing a mechanism for selecting translators through special filename attributes. For example you could use `index.html.gz,,+` and `index.html.gz,,-` to choose between translated and untranslated versions of a file. Or you could use `index.html.gz,,u` to get the contents of the file with a gunzip translator applied automatically. You could also use attributes on whole directory trees: `.,,0/` would give you a directory tree corresponding to the current directory, but with any translators disabled, for doing a backup. And `site,,u/*.html.gz` would present a whole directory tree of compressed HTML files as uncompressed files. One benefit of the Hurd's flexibility is that it should be possible to implement such a mechanism without touching the existing Hurd components: Rather, just implement a special proxy, that mirrors the normal filesystem, but is able to interpret the special extensions and present transformed files in place of the original ones. In the long run it's probably desirable to have the mechanism implemented in the standard name lookup mechanism, so it will be available globally, and avoid the overhead of a proxy; but for the beginnig the proxy solution is much more flexible. The goal of this project is implementing a prototype proxy; perhaps also a first version of the global variant as proof of concept, if time permits. It requires good understanding of the name lookup mechanism, and translator programming; but the implementation should not be too hard. Perhaps the hardest part is finding a convenient, flexible, elegant, hurdish method for mapping the special extensions to actual translators... ## Fix File Locking Over the years, UNIX has aquired a host of different file locking mechanisms. Some of them work on the Hurd, while others are buggy or only partially implemented. This breaks many applications. The goal is to make all file locking mechanisms work properly. This requires finding all existing shortcomings (through systematic testing and/or checking for known issues in the bug tracker and mailing list archives), and fixing them. This task will require digging into parts of the code to understand how file locking works on the Hurd. Only general programming skills are required. ## `procfs` Although there is no standard (POSIX or other) for the layout of the `/proc` pseudo-filesystem, it turned out a very useful facility in GNU/Linux and other systems, and many tools concerned with process management use it. (`ps`, `top`, `htop`, `gtop`, `killall`, `pkill`, ...) Instead of porting all these tools to use [[hurd/libps]] (Hurd's official method for accessing process information), they could be made to run out of the box, by implementing a Linux-compatible `/proc` filesystem for the Hurd. The goal is to implement all `/proc` functionality needed for the various process management tools to work. (On Linux, the `/proc` filesystem is used also for debugging purposes; but this is highly system-specific anyways, so there is probably no point in trying to duplicate this functionality as well...) The [[existing_partially_working_procfs_implementation|hurd/translator/procfs]] can serve as a starting point, but needs to be largely rewritten. (It should use [[hurd/libnetfs]] rather than [[hurd/libtrivfs]]; the data format needs to change to be more Linux-compatible; and it needs adaptation to newer system interfaces.) This project requires learning [[hurd/translator]] programming, and understanding some of the internals of process management in the Hurd. It should not be too hard coding-wise; and the task is very nicely defined by the exising Linux `/proc` interface -- no design considerations necessary. **Note**: We already have several applications for this task. ## New Driver Glue Code Although a driver framework in userspace would be desirable, presently the Hurd uses kernel drivers in the microkernel, [[GNU_Mach|microkernel/mach/gnumach]]. (And changing this would be far beyond a GSoC project...) The problem is that the drivers in GNU Mach are presently old Linux drivers (mostly from 2.0.x) accessed through a glue code layer. This is not an ideal solution, but works quite OK, except that the drivers are very old. The goal of this project is to redo the glue code, so we can use drivers from current Linux versions, or from one of the free BSD variants. This is a doable, but pretty involved project. Experience with driver programming under Linux (or BSD) is a must. (No Hurd-specific knowledge is required, though.) This is [[GNU_Savannah_task 5488]]. ## Server Overriding Mechanism The main idea of the Hurd is that every user can influence almost all system functionality ([[extensible_system|extensibility]]), by running private Hurd servers that replace or proxy the global default implementations. However, running such a cumstomized subenvironment presently is not easy, because there is no standard mechanism to easily replace an individual standard server, keeping everything else. (Presently there is only the [[hurd/subhurd]] method, which creates a completely new system instance with a completely independent set of servers.) The goal of this project is to provide a simple method for overriding individual standard servers, using environment variables, or a special subshell, or something like that. Various approaches for such a mechanism has been discussed before. Probably the easiest (1) would be to modify the Hurd-specific parts of [[hurd/glibc]], which are contacting various standard servers to implement certain system calls, so that instead of always looking for the servers in default locations, they first check for overrides in environment variables, and use these instead if present. A somewhat more generic solution (2) could use some mechanism for arbitrary client-side namespace overrides. The client-side part of the filename lookup mechanism would have to check an override table on each lookup, and apply the desired replacement whenever a match is found. Another approach would be server-side overrides. Again there are various variants. The actual servers themself could provide a mechanism to redirect to other servers on request. (3) Or we could use some more generic server-side namespace overrides: Either all filesystem servers could provide a mechanism to modify the namespace they export to certain clients (4), or proxies could be used that mirror the default namespace but override certain locations. (5) Variants (4) and (5) are the most powerful. They are intimately related to chroots: (4) is like the current chroot implementation works in the Hurd, and (5) has been proposed as an alternative. The generic overriding mechanism could be implemented on top of chroot, or chroot could be implemented on top of the generic overriding mechanism. But this is out of scope for this project... In practice, probably a mix of the different approaches would prove most useful for various servers and use cases. It is strongly recommended that the student starts with (1) as the simplest approach, perhaps augmenting it with (3) for certain servers that don't work with (1) because of indirect invocation. This tasks requires some understanding of the Hurd internals, especially a good understanding of the file name lookup mechanism. It's probably not too heavy on the coding side. This is [[GNU_Savannah_task 6612]]. Also there are quite a bit of emails discussing this topic, from a last year's GSoC application -- see , , , . ## `dtrace` Support One of the main problems of the current Hurd implementation is very poor performance. While we have a bunch of ideas what could cause the performance problems, these are mostly just guesses. Better understanding what really causes bad performance is necessary to improve the situation. For that, we need tools for performance measurements. While all kinds of more or less specific profiling tools could be convieved, the most promising and generic approach seems to be a framework for logging certain events in the running system (both in the microkernel and in the Hurd servers). This would allow checking how much time is spent in certain modules, how often certain situations occur, how things interact, etc. It could also prove helpful in debugging some issues that are otherwise hard to find because of complex interactions. The most popular framework for that is Sun's dtrace; but there might be others. The student has to evaluate the existing options, deciding which makes most sense for the Hurd; and implement that one. (Apple's implementation of dtrace in their Mach-based kernel might be helpful here...) This project requires ability to evaluate possible solutions, and experience with integrating existing components as well as low-level programming. ## Hurdish TCP/IP Stack The Hurd presently uses a [[TCP/IP_stack|hurd/translator/pfinet]] based on code from an old Linux version. This works, but lacks some rather important features (like PPP/PPPoE), and the design is not hurdish at all. A true hurdish network stack will use a set of stack of [[hurd/translator]] processes, each implementing a different protocol layer. This way not only the implementation gets more modular, but also the network stack can be used way more flexibly. Rather than just having the standard socket interface, plus some lower-level hooks for special needs, there are explicit (perhaps filesystem-based) interfaces at all the individual levels; special application can just directly access the desired layer. All kinds of packet filtering, routing, tunneling etc. can be easily achieved by stacking compononts in the desired constellation. While the general architecture is pretty much given by the various network layers, it's up to the student to design and implement the various interfaces at each layer. This task requires understanding the Hurd philosophy and translator programming, as well as good knowledge of TCP/IP. This is [[GNU_Savannah_task 5469]]. ## Improved NFS Implementation The Hurd has both NFS server and client implementations, which work, but not very well: File locking doesn't work properly (at least in conjuction with a GNU/Linux server), and performance is extremely poor. Part of the problems could be owed to the fact that only NFSv2 is supported so far. This project encompasses implementing NFSv3 support, fixing bugs and performance problems -- the goal is to have good NFS support. The work done in a previous unfinished GSoC project can serve as a starting point. Both client and server parts need work, though the client is probably much more important for now, and shall be the major focus of this project. This task, [[GNU_Savannah_task 5497]], has no special prerequisites besides general programming skills, and an interest in file systems and network protocols. ## Fix `libdiskfs` Locking Issues Nowadays the most often encountered cause of Hurd crashes seems to be lockups in the [[hurd/translator/ext2fs]] server. One of these could be traced recently, and turned out to be a lock inside [[hurd/libdiskfs]] that was taken and not released in some cases. There is reason to believe that there are more faulty paths causing these lockups. The task is systematically checking the [[hurd/libdiskfs]] code for this kind of locking issues. To achieve this, some kind of test harness has to be implemented: For exmple instrumenting the code to check locking correctness constantly at runtime. Or implementing a unit testing framework that explicitely checks locking in various code paths. (The latter could serve as a template for implementing unit checks in other parts of the Hurd codebase...) This task requires experience with debugging locking issues in multithreaded applications. ## Convert Hurd Libraries and Servers to pthreads The Hurd was originally created at a time when the [pthreads standard](http://www.opengroup.org/onlinepubs/009695399/basedefs/pthread.h.html) didn't exist yet. Thus all Hurd servers and libraries are using the old [[cthreads|hurd/libcthreads]] package that came with [[microkernel/Mach]], which is not compatible with [[pthreads|hurd/libpthread]]. Not only does that mean that people hacking on Hurd internals have to deal with a non-standard thread package, which nobody is familiar with. Although a pthreads implementation for the Hurd was created in the meantime, it's not possible to use both cthreads and pthreads in the same program. Consequently, pthreads can't presently be used in any Hurd servers -- including translators. Some work already has been done once on converting the Hurd servers and libraries to use pthreads, but that work hasn't been finished. It is available as [[GNU_Savannah_task 5487]] and can of course be used to base the new work upon. The goal of this project is to have all the Hurd code use pthreads. Should any limitations in the existing pthreads implementation turn up that hinder this transition, they will have to be fixed as well. One possible option is creating a wrapper that implements the cthreads interfaces on top of pthreads, to ease the transition -- but it might very well turn out that it's easier to just change all the existing code to use pthreads directly. This is up to the student. Such a wrapper has been proposed as [[GNU_Savannah_task 7895]] and its implementation would be a useful starting-point. This project requires relatively little Hurd-specific knowledge. Experience with multithreaded programming in general and pthreads in particular is required, though. ## Sound Support The Hurd presently has no sound support. Fixing this, [[GNU_Savannah_task 5485]], requires two steps: the first is to port some other kernel's drivers to [[GNU_Mach|microkernel/mach/gnumach]] so we can get access to actual sound hardware. The second is to implement a userspace server ([[hurd/translator]]), that implements an interface on top of the kernel device that can be used by applications -- probably OSS or maybe ALSA. Completing this task requires porting at least one driver (e.g. from Linux) for a popular piece of sound hardware, and the basic userspace server. For the driver part, previous experience with programming kernel drivers is strongly advisable. The userspace part requires some knowledge about programming Hurd translators, but shouldn't be too hard. Once the basic support is working, it's up to the student to use the remaining time for porting more drivers, or implementing a more sophisticated userspace infrastructure. The latter requires good understanding of the Hurd philosophy, to come up with an appropriate design. Another option would be to evaluate whether a driver that is completely running in user-space is feasible. ## Disk I/O Performance Tuning The most obvious reason for the Hurd feeling slow compared to mainstream systems like GNU/Linux, is very slow harddisk access. The reason for this slowness is lack and/or bad implementation of common optimisation techniques, like scheduling reads and writes to minimalize head movement; effective block caching; effective reads/writes to partial blocks; reading/writing multiple blocks at once; and read-ahead. The [[ext2_filesystem_server|hurd/translator/ext2fs]] might also need some optimisations at a higher logical level. The goal of this project is to analyze the current situation, and implement/fix various optimisations, to achieve significantly better disk performance. It requires understanding the data flow through the various layers involved in disk acces on the Hurd ([[filesystem|hurd/virtual_file_system]], [[pager|hurd/libpager]], driver), and general experience with optimising complex systems. That said, the killing feature we are definitely missing is the read-ahead, and even a very simple implementation would bring very big performance speedups. ## VM Tuning Hurd/[[microkernel/Mach]] presently make very bad use of the available physical memory in the system. Some of the problems are inherent to the system design (the kernel can't distinguish between important application data and discardable disk buffers for example), and can't be fixed without fundamental changes. Other problems however are an ordinary lack of optimisation, like extremely crude heuristics when to start paging. (See for example.) Many parameters are based on assumptions from a time when typical machines had like 16 MiB of RAM, or simply have been set to arbitrary values and never tuned for actual use. The goal of this project is to bring the virtual memory management in Hurd/Mach closer to that of modern mainstream kernels (Linux, FreeBSD), by comparing the implementation to other systems, implementing any worthwhile improvements, and general optimisation/tuning. It requires very good understanding of the Mach VM, and virtual memory in general. This project is related to [[GNU_Savannah_task 5489]]. ## `mtab` In traditional monolithic system, the kernel keeps track of all mounts; the information is available through `/proc/mounts` (on Linux at least), and in a very similar form in `/etc/mtab`. The Hurd on the other hand has a totally [[decentralized_file_system|hurd/virtual_file_system]]. There is no single entity involved in all mounts. Rather, only the parent file system to which a mountpoint ([[hurd/translator]]) is attached is involved. As a result, there is no central place keeping track of mounts. As a consequence, there is currently no easy way to obtain a listing of all mounted file systems. This also means that commands like `df` can only work on explicitely specified mountpoints, instead of displaying the usual listing. One possible solution to this would be for the translator startup mechanism to update the `mtab` on any `mount`/`unmount`, like in traditional systems. However, there are same problems with this approach. Most notably: what to do with passive translators, i.e., translators that are not presently running, but set up to be started automatically whenever the node is accessed? Probably these should be counted an among the mounted filesystems; but how to handle the `mtab` updates for a translator that is not started yet? Generally, being centralized and event-based, this is a pretty unelegant, non-hurdish solution. A more promising approach is to have `mtab` exported by a special translator, which gathers the necessary information on demand. This could work by traversing the tree of translators, asking each one for mount points attached to it. (Theoretically, it could also be done by just traversing *all* nodes, checking each one for attached translators. That would be very inefficient, though. Thus a special interface is probably required, that allows asking a translator to list mount points only.) There are also some other issues to keep in mind. Traversing arbitrary translators set by other users can be quite dangerous -- and it's probably not very interesting anyways what private filesystems some other user has mounted. But what about the global `/etc/mtab`? Should it list only root-owned filesystems? Or should it create different listings depending on what user contacts it?... That leads to a more generic question: which translators should be actually listed? There are different kinds of translators: ranging from traditional filesystems ([[disks|hurd/libdiskfs]] and other actual [[stores|hurd/translator/storeio]]), but also purely virtual filesystems like [[hurd/translator/ftpfs]] or [[hurd/translator/unionfs]], and even things that have very little to do with a traditional filesystem, like a [[gzip_translator|hurd/translator/storeio]], [[mbox_translator|hurd/translator/mboxfs]], [[xml_translator|hurd/translator/xmlfs]], or various device file translators... Listing all of these in `/etc/mtab` would be pretty pointless, so some kind of classification mechanism is necessary. By default it probably should list only translators that claim to be real filesystems, though alternative views with other filtering rules might be desirable. After taking decisions on the outstanding design questions, the student will implement both the actual [[mtab_translator|hurd/translator/mtabfs]], and the necessery interface(s) for gathering the data. It requires getting a good understanding of the translator mechanism and Hurd interfaces in general. ## GNU Mach Code Cleanup Although there are some attempts to move to a more modern microkernel alltogether, the current Hurd implementation is based on [[GNU_Mach|microkernel/mach/gnumach]], which is only a slightly modified variant of the original CMU [[microkernel/Mach]]. Unfortunately, Mach was created about two decades ago, and is in turn based on even older BSD code. Parts of the BSD kernel -- file systems, UNIX mechanisms like processes and signals, etc. -- were ripped out (to be implemented in [[userspace_servers|hurd/translator]] instead); while other mechanisms were added to allow implementing stuff in userspace. ([[Pager_interface|microkernel/mach/external_pager_mechanism]], [[microkernel/mach/IPC]], etc.) Also, Mach being a research project, many things were tried, adding lots of optional features not really needed. The result of all this is that the current code base is in a pretty bad shape. It's rather hard to make modifications -- to make better use of modern hardware for example, or even to fix bugs. The goal of this project is to improve the situation. The task starts out easy, with fixing compiler warnings. Later it moves on to more tricky things: removing dead or unneeded code paths; restructuring code for readability and maintainability. This task requires good knowledge of C, and experience with working on a large existing code base. Previous kernel hacking experience is an advantage, but not really necessary. ## `xmlfs` Hurd [[translators|hurd/translator]] allow presenting underlying data in a different format. This is a very powerful ability: it allows using standard tools on all kinds of data, and combining existing components in new ways, once you have the necessary translators. A typical example for such a translator would be xmlfs: a translator that presents the contents of an underlying XML file in the form of a directory tree, so it can be studied and edited with standard filesystem tools, or using a graphical file manager, or to easily extract data from an XML file in a script etc. The exported directory tree should represent the DOM structure of the document, or implement XPath, or both, or some combination thereof (perhaps XPath could be implemented as a second translator working on top of the DOM one) -- whatever works well, while sticking to XML standards as much as possible. Ideally, the translation should be reversible, so that another, complementary translator applied on the expanded directory tree would yield the original XML file again; and also the other way round, applying the complementary translator on top of some directory tree and xmlfs on top of that would yield the original directory again. However, with the different semantics of directory trees and XML files, it might not be possible to create such a universal mapping. Thus it is a desirable goal, but not a strict requirement. The goal of this project is to create a fully usable XML translator, that allows both reading and writing any XML file. Implementing the complementary translator also would be nice if time permits, but is not mandatory part of the task. The [[existing_partial_(read-only)_xmlfs_implementation|hurd/translator/xmlfs]] can serve as a starting point. This task requires pretty good designing skills. Good knowledge of XML is also necessary. Learning translator programming will obviously be necessary to complete the task. ## Allow Using `unionfs` Early at Boot In UNIX systems, traditionally most software is installed in a common directory hierachy, where files from various packages live beside each other, grouped by function: user-invokable executables in `/bin`, system-wide configuration files in `/etc`, architecture specific static files in `/lib`, variable data in `/var`, and so on. To allow clean installation, deinstallation, and upgrade of software packages, GNU/Linux distributions usually come with a package manager, which keeps track of all files upon installation/removal in some kind of central database. An alternative approach is the one implemented by GNU Stow: each package is actually installed in a private directory tree. The actual standard directory structure is then created by collecting the individual files from all the packages, and presenting them in the common `/bin`, `/lib`, etc. locations. While the normal Stow package (for traditional UNIX systems) uses symlinks to the actual files, updated on installation/deinstallation events, the Hurd [[hurd/translator]] mechanism allows a much more elegant solution: [[hurd/translator/stowfs]] (which is actually a special mode of [[hurd/translator/unionfs]]) creates virtual directories on the fly, composed of all the files from the individual package directories. The problem with this approach is that unionfs presently can be launched only once the system is booted up, meaning the virtual directories are not available at boot time. But the boot process itself already needs access to files from various packages. So to make this design actually usable, it is necessary to come up with a way to launch unionfs very early at boot time, along with the root filesystem. Completing this task will require gaining a very good understanding of the Hurd boot process and other parts of the design. It requires some design skills also to come up with a working mechanism. ## Fix `tmpfs` In some situations it is desirable to have a file system that is not backed by actual disk storage, but only by anonymous memory, i.e. lives in the RAM (and possibly swap space). A simplistic way to implement such a memory filesystem is literally creating a ramdisk, i.e. simply allocating a big chunck of RAM (called a memory store in Hurd terminology), and create a normal filesystem like ext2 on that. However, this is not very efficient, and not very convenient either (the filesystem needs to be recreated each time the ramdisk is invoked). A nicer solution is having a real [[hurd/translator/tmpfs]], which creates all filesystem structures directly in RAM, allocating memory on demand. The Hurd has had such a tmpfs for a long time. However, the existing implementation doesn't work anymore -- it got broken by changes in other parts of the Hurd design. There are several issues. The most serious known problem seems to be that for technical reasons it receives [[microkernel/mach/RPC]]s from two different sources on one [[microkernel/mach/port]], and gets mixed up with them. Fixing this is non-trivial, and requires a good understanding of the involved mechanisms. The goal of this project to get a fully working, full featured tmpfs implementation. It requires digging into some parts of the Hurd, incuding the [[pager_interface|hurd/libpager]] and [[hurd/translator]] programming. This task probably doesn't require any design work, only good debugging skills. ## Lexical `..` Resolution For historical reasons, UNIX filesystems have a real (hard) `..` link from each directory pointing to its parent. However, this is problematic, because the meaning of "parent" really depends on context. If you have a symlink for example, you can reach a certain node in the filesystem by a different path. If you go to `..` from there, UNIX will traditionally take you to the hard-coded parent node -- but this is usually not what you want. Usually you want to go back to the logical parent from which you came. That is called "lexical" resolution. Some application already use lexical resolution internally for that reason. It is generally agreed that many problems could be avoided if the standard filesystem lookup calls used lexical resolution as well. The compatibility problems probably would be negligable. The goal of this project is to modify the filename lookup mechanism in the Hurd to use lexical resolution, and to check that the system is still fully functional afterwards. This task requires understanding the filename resolution mechanism. It's probably a relatively easy task. See also [[GNU_Savannah_bug 17133]]. ## Secure `chroot` implementation As the Hurd attempts to be (almost) fully UNIX-compatible, it also implements a `chroot()` system call. However, the current implementation is not really good, as it allows easily escaping the `chroot`, for example by use of [[passive_translators|hurd/translator]]. Many solutions have been suggested for this problem -- ranging from simple workaround changing the behaviour of passive translators in a `chroot`; changing the context in which passive translators are exectuted; changing the interpretation of filenames in a chroot; to reworking the whole passive translator mechanism. Some involving a completely different approch to `chroot` implementation, using a proxy instead of a special system call in the filesystem servers. The task is to pick and implement one approach for fixing chroot. This task is pretty heavy: it requires a very good understanding of file name lookup and the translator mechanism, as well as of security concerns in general -- the student must prove that he really understands security implications of the UNIX namespace approach, and how they are affected by the introduction of new mechanisms. (Translators.) More important than the acualy code is the documentation of what he did: he must be able to defend why he chose a certain approach, and explain why he believes this approach really secure. ## Hurdish Package Manager for the GNU System Most GNU/Linux systems use pretty sophisticated package managers, to ease the management of installed software. These keep track of all installed files, and various kinds of other necessary information, in special databases. On package installation, deinstallation, and upgrade, scripts are used that make all kinds of modifications to other parts of the system, making sure the packages get properly integrated. This approach creates various problems. For one, *all* management has to be done with the distribution package management tools, or otherwise they would loose track of the system state. This is reinforced by the fact that the state information is stored in special databases, that only the special package management tools can work with. Also, as changes to various parts of the system are made on certain events (installation/deinstallation/update), managing the various possible state transitions becomes very complex and bug-prone. For the official (Hurd-based) GNU system, a different approach is intended: making use of Hurd [[translators|hurd/translator]] -- more specifically their ability to present existing data in a different form -- the whole system state will be created on the fly, directly from the information provided by the individual packages. The visible system state is always a reflection of the sum of packages installed at a certain moment; it doesn't matter how this state came about. There are no global databases of any kind. (Some things might require caching for better performance, but this must happen transparently.) The core of this approach is formed by [[hurd/translator/stowfs]], which creates a traditional unix directory structure from all the files in the individual package directories. But this only handles the lowest level of package management. Additional mechanisms are necessary to handle stuff like dependencies on other packages. The goal of this task is to create these mechanisms. ## Port the Debian Installer to the Hurd The primary means of distributing the Hurd is through Debian GNU/Hurd. However, the installation CDs presently use an ancient, non-native installer. The situation could be much improved by making sure that the newer *Debian Installer* works on the Hurd. Some preliminary work has been done, see . The goal is to have the Debian Installer fully working on the Hurd. It requires relatively little Hurd-specific knowledge.