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Re: some other memory considerations.

From: Bas Wijnen
Subject: Re: some other memory considerations.
Date: Tue, 26 Oct 2004 14:08:03 +0200
User-agent: Mozilla Thunderbird 0.8 (X11/20040926)

Espen Skoglund wrote:
[Rian Hunter]

By the way, is it even possible for two arbitrary tasks in L4 to map
memory to each other, assuming not one of them are pagers to the
other?  I'm not entirely sure but I don't think so.

Yes.  It is possible.

L4 allows it, indeed. However, our current ideas of the Hurd don't. In fact, they don't allow it even if you are a pager.

Mapping memory to an other task costs kernel memory. Therefore, it can be used for a denial of service attack against the system. Because of that, it must be a restricted operation, which can only be performed by trusted tasks (and will in practice only be performed by physmem, I think.) Note also that pagers are not trusted tasks. So for a pager to map memory to a task, it has to ask physmem to do it. Granting memory is not a problem, and will be allowed. I'm not sure if it would be used much, though.


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