Re: [PATCH] Send quota messages via netlink

["Serge E. Hallyn" <serue@us.ibm.com>]

Quoting Jan Kara (jack@suse.cz):
> On Thu 30-08-07 17:14:47, Serge E. Hallyn wrote:
> > Quoting Jan Kara (jack@suse.cz):
> > >   Maybe before proceeding further with the discussion I'd like to
> > > understand following: What are these user namespaces supposed to be good
> > > for?
> > 
> > (Please skip to the message end first, as I think you may not care about
> > the next bit of my blathering)
> > 
> > Right now they are only good for providing some separate accounting for
> > uid 1000 in one user namespace versus uid 1000 in another namespace.
> > All security enforcement must be done by actually providing separate
> > filesystems and separate pid namespaces and, hopefully, with a selinux
> > policy.
> > 
> > Eventually the idea will be that uid 1000 in one user namespace and uid
> > 1000 in another namespace will be completely separate entities.  A
> > mounted filesystem will be tied to a particuler user namespace, and
> > the kernel will provide any cross-userns access perhaps the way I
> > described, with uid equivalence implemented through the keyring.
>   I see. Thanks for explanation.
> 
> > But note that this isn't really relevant when we get to NFS.  Two user
> > namespaces on one machine should have different network namespaces and
> > network addresses as well, and so should look to the NFS server like two
> > separate machines.
> > 
> > So the user namespaces are only really relevant when talking about local
> > filesystems.
> > 
> > >   I imagine it so that you have a machine and on it several virtual
> > > machines which are sharing a filesystem (or it could be a cluster). Now you
> > > want UIDs to be independent between these virtual machines. That's it,
> > > right?
> > >   Now to continue the example: Alice has UID 100 on machineA, Bob has
> > >  UID 100 on machineB. These translate to UIDs 1000 and 1001 on the common
> > > filesystem. Process of Alice writes to a file and Bob becomes to be over
> > > quota. In this situation, there would be probably two processes (from
> > > machineA and machineB) listening on the netlink socket. We want to send a
> > > message so that on Alice's desktop we can show a message: "You caused
> > > Bob to exceed his quotas" and of Bob's desktop: "Alice has caused that you
> > > are over quota.".
> > 
> > Since this is over NFS, you handle it the way you would any other time
> > that user Alice on some other machine managed to do this.
>   I meant this would actually happen over a local filesystem (imagine
> something like "hostfs" from UML).

Ok, then that is where I was previously suggesting that we use an api to
report a uid meaningful in bob's context, where we currently (in the
absense of meaningful mount uids and uid equivalence) tell Bob that root
was the one who brought him over quota.  From a user pov 'nobody' would
make more sense, but I don't think we want the kernel to know about user
nobody, right?

So if the msg weren't broadcast, or netlink sockets were tied to one
user namespace, we could call a
	int uid_in_user_ns(struct user *, struct user_ns *)
sending in Alice's user struct and Bob's userns, and use the result in
the netlink message.  Otherwise I'm not sure what is the right answer.
We just might need the equivalent of 'struct pid' to struct user, or
persistant global user namespace ids (persistant after user namespace
destruction, not across reboot) so we can safely send the user_ns * in a
netlink msg.

> > >   Because there may be is not a notion of Bob on machineA or of Alice on
> > > machineB, we are in trouble, right? What I like the most is to use the
> > > filesystem identities (as you suggested in some other email). I. e. because
> > > both Alice and Bob share a filesystem, identities of both have to make sense
> > > to it (for example for purposes of permission checking). So we can probably
> > 
> > Right, so long as we're talking about local filesystems that's the way
> > to go.  If a file write was allowed which brought bob over quota,
> > clearly the person responsible had some uid valid on the filesystem to
> > allow him to do so.
>   Fine. So I'll keep UID in the quota netlink protocol with the meaning
> "the identity of the user for filesystem operations".

I think that's ok.

Hopefully when that changes to accomodate user namespaces, we can use
netlink field versioning to make that transition pretty seamless?

If not, then we probably should in fact make some decision now so as not
to change the api.

> > > send via netlink these (in our example ids 1000 and 1001) and hope that
> > > inside machineA and machineB there will be a way to translate these
> > > identities to names "Alice" and "Bob". So that user can understand what
> > > is happenning. Does this sound plausible?
> > >   If we go this route, then we only need a kernel function, that will
> > > for a pair ($filesystem, $task) return indentity of that $task used
> > > for operations on $filesystem...
> > 
> > Ok, now I see.  This is again unrelated to user namespaces, it's an
> > issue regardless.
> > 
> > Is there no way to just report Alice as the guilty party to Bob on his
> > machine as (host=nfsserver,uid=1000)?
>   You know, in fact this contains all the information but it is quite useless
> for an ordinary user. The message should be understandable to average desktop

What is the ordinary user going to do about it?  If the user didn't set
up the nfsserver and/or the second client, the only thing he can do is
report the guilty user to an admin.  In which case the tuple
(host=nfsserver,uid=1000) is exactly the data he needs to report.

> user so it should contain some name rather than UID - but resolving the
> "filesystem" UID to some meaningful name is completely different issue
> and I'd probably leave that for the moment when the kernel infrastructure
> and use cases would be clearer...
> 
> 								Honza
> -- 
> Jan Kara <jack@suse.cz>
> SuSE CR Labs

thanks,
-serge
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