Re: The Perils of Pluggability

[Bas Wijnen]

On Tue, Oct 11, 2005 at 12:08:26PM -0600, Christopher Nelson wrote:
> > 
> > On Tue, Oct 11, 2005 at 10:30:54AM -0600, Christopher Nelson wrote:
> > > I suppose that you really must decide who your target audience is.  
> > > Who are YOU aiming the Hurd at?
> > 
> > We aren't aiming at anyone in particular.  How the system 
> > handles is policy, it's done in user space.  The GNU system 
> > may have a target audience, the Hurd does not.
> 
> I wasn't aware there was a difference.  Perhaps the web page for the
> Hurd should be updated from "The GNU Hurd is the GNU project's
> replacement for the Unix kernel. The Hurd is a collection of servers
> that run on the Mach microkernel to implement file systems, network
> protocols, file access control, and other features that are implemented
> by the Unix kernel or similar kernels (such as Linux)."  to something
> that makes it clear that GNU system and the Hurd are two very different
> projects.

Oh, they aren't very different.  They have the same goals.  Indeed, the Hurd
is designed as the kernel of the GNU system.  What I meant to say is that we
try to make it a very good kernel.  And we believe that a good kernel does not
impose policy (well, as little as possible) on the user.  So in theory, any
sytem could be run on top of the Hurd.  However, we don't expect this to
happen (at least not in the near future).

> In any case, you must be targeting SOMEONE.  Do you simply mean that the
> Hurd is simply a set of servers that provide Unix-like behavior on top
> of some other microkernel?  Are not the servers that implement Hurd
> running in user space?

They are, but they should leave as much of the policy as they can to other
parts, at least the ones which cannot be replaced (physmem, task, that sort of
servers.  They can be wrapped, but not replaced).

> Do they not implement policy?

As little as possible in the low-level servers.  There is policy in
higher-level servers, for example to match the policy mandated by POSIX.

> If not, what value do you add, then, to L4? Or Mach?

The microkernel itself is not very usable for programs.  It needs a layer
around it so it can be handled.  The Hurd provides management of scarce
resources (memory, processor time, devices) so that a multi-user multi-tasking
system is possible in a secure way.  It should impose policy where this is
needed, but nowhere else.

> I am now confused about what the Hurd is supposed to be.

I hope this helps.  Note though that I'm not officially connected with the
Hurd, and this is purely my personal view.  Alfred (who is officially
connected) seems to disagree.  If there exists a right and wrong in this
matter, then most likely he is right and I am wrong.

Thanks,
Bas

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