diff options
Diffstat (limited to 'community/gsoc/project_ideas/server_overriding.mdwn')
| -rw-r--r-- | community/gsoc/project_ideas/server_overriding.mdwn | 74 |
1 files changed, 74 insertions, 0 deletions
diff --git a/community/gsoc/project_ideas/server_overriding.mdwn b/community/gsoc/project_ideas/server_overriding.mdwn new file mode 100644 index 00000000..c35d88de --- /dev/null +++ b/community/gsoc/project_ideas/server_overriding.mdwn @@ -0,0 +1,74 @@ +[[!meta copyright="Copyright © 2008, 2009 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]]."]]"""]] + +[[!meta title="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 customized 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. It is closely related to the +[[virtualization]] task. + +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. Take a look at the [socket server overriding +patch](http://www.assembla.com/spaces/VNetHurd/documents/aJidqKp6ur3z-Nab7jnrAJ/download/A%20patch%20of%20glibc) +for an example. + +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 themselves 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 previous year's GSoC application -- see +<http://lists.gnu.org/archive/html/bug-hurd/2007-03/msg00050.html>, +<http://lists.gnu.org/archive/html/bug-hurd/2007-03/msg00114.html>, +<http://lists.gnu.org/archive/html/bug-hurd/2007-06/msg00082.html>, +<http://lists.gnu.org/archive/html/bug-hurd/2008-03/msg00039.html>. + +Possible mentors: Olaf Buddenhagen (antrik), Carl Fredrik Hammar (cfhammar) + +Exercise: Come up with a glibc patch that allows overriding one specific +standard server using method (1). |