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-[[!meta copyright="Copyright © 2009, 2010 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="Porting Valgrind to the Hurd"]]
-
-[Valgrind](http://valgrind.org/) is an extremely useful debugging tool for memory errors.
-(And some other kinds of hard-to-find errors too.)
-Aside from being useful for program development in general,
-a Hurd port will help finding out why certain programs segfault on the Hurd,
-although they work on Linux.
-Even more importantly, it will help finding bugs in the Hurd servers themselfs.
-
-To keep track of memory use,
-Valgrind however needs to know how each [[system call]] affects the validity of memory regions.
-This knowledge is highly kernel-specific,
-and thus Valgrind needs to be explicitely ported for every system.
-
-Such a port involves two major steps:
-making Valgrind understand how kernel traps work in general on the system in question;
-and how all the individual kernel calls affect memory.
-The latter step is where most of the work is,
-as the behaviour of each single [[system call]] needs to be described.
-
-Compared to Linux,
-[[microkernel/Mach]] (the microkernel used by the Hurd) has very few kernel traps.
-Almost all [[system call]]s are implemented as [[RPC]]s instead --
-either handled by Mach itself, or by the various [[Hurd servers|hurd/translator]].
-All RPCs use a pair of `mach_msg` invocations:
-one to send a request message, and one to receive a reply.
-However, while all RPCs use the same `mach_msg` trap,
-the actual effect of the call varies greatly depending on which RPC is invoked --
-similar to the `ioctl` call on Linux.
-Each request thus must be handled individually.
-
-Unlike `ioctl`,
-the RPC invocations have explicit type information for the parameters though,
-which can be retrieved from the message header.
-By analyzing the parameters of the RPC reply message,
-Valgrind can know exactly which memory regions are affected by that call,
-even without specific knowledge of the RPC in question.
-Thus implementing a general parser for the reply messages
-will already give Valgrind a fairly good approximation of memory validity --
-without having to specify the exact semantic of each RPC by hand.
-
-While this should make Valgrind quite usable on the Hurd already, it's not perfect:
-some RPCs might return a buffer that is only partially filled with valid data;
-or some reply parameters might be optional,
-and only contain valid data under certain conditions.
-Such specific semantics can't be deduced from the message headers alone.
-Thus for a complete port,
-it will still be necessary to go through the list of all known RPCs,
-and implement special handling in Valgrind for those RPCs that need it.
-
-The goal of this task is at minimum to make Valgrind grok Mach traps,
-and to implement the generic RPC handler.
-Ideally, specific handling for RPCs needing it should also be implemented.
-
-Completing this project will require digging into Valgrind's handling of [[system call]]s,
-and into Hurd RPCs.
-It is not an easy task, but a fairly predictable one --
-there shouldn't be any unexpected difficulties,
-and no major design work is necessary.
-It doesn't require any specific previous knowledge:
-only good programming skills in general.
-On the other hand,
-the student will obtain a good understanding of Hurd RPCs while working on this task,
-and thus perfect qualifications for Hurd development in general :-)
-
-Possible mentors: Samuel Thibault (youpi)
-
-Exercise: As a starter,
-students can try to teach valgrind a couple of Linux ioctls,
-as this will make them learn how to use the read/write primitives of valgrind.