Re: some other memory considerations.

[Neal H. Walfield]

> 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.

No.  The design does not disallow; it just doesn't use it.

> Mapping memory to an other task costs kernel memory.  Therefore, it can 
> be used for a denial of service attack against the system.

This is a potential problem which Marcus and I discussed a while ago.
The solution that we came up with was to use a redirector for all
non-privileged threads which would filter out non-sanctioned mappings
thereby eliminating this vulnerability.  That is extreme and induces a
huge amount of overhead.  The alternative is to ignore it: if the
kernel allows it (and mapping and granting are not privileged
operations) then the L4 people likely do not see this as a serious
flaw perhaps because in the next design revision they will add a
mechanism to page kernel memory.