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diff --git a/community/gsoc/project_ideas/valgrind.mdwn b/community/gsoc/project_ideas/valgrind.mdwn new file mode 100644 index 00000000..79c8bd1d --- /dev/null +++ b/community/gsoc/project_ideas/valgrind.mdwn @@ -0,0 +1,81 @@ +[[!meta copyright="Copyright © 2009, 2010, 2011 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. |