[Devel] Re: [PATCH] Send quota messages via netlink

[Serge E. Hallyn]

Quoting Jan Kara (jack at suse.cz):
> On Tue 04-09-07 18:48:52, Serge E. Hallyn wrote:
> > Quoting Jan Kara (jack at suse.cz):
> > > On Tue 04-09-07 16:32:10, Serge E. Hallyn wrote:
> > > > Quoting Jan Kara (jack at suse.cz):
> > > > > On Thu 30-08-07 17:14:47, Serge E. Hallyn wrote:
> > > > > > Quoting Jan Kara (jack at suse.cz):
> > > > > > >   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?
> > >   But what is the problem with using the filesystem ids? All virtual
> > > machines in my example should have a notion of those...
> > 
> > I don't know what you mean by filesystem ids.  Do you mean the uid
> > stored on the fs?  I imagine a network fs could get fancy and store
> > something more detailed than the unix uid, based on the user's keys.
> > 
> > Do you mean the inode->i_uid?  Nothing wrong with that.  Then we just
> > assume that either you are in the superblock or mount's user namespace
> > (depending on how we implement it, probably superblock), or can figure
> > out what that is.
>   I meant the identity the process uses to access the filesystem (to
> identify the user who caused the limit excess) and also the identity stored
> in the quota file (to identify whose quota was exceeded).
>   Anyway, any identity more complicated than just a number needs changes in
> both quota file format and filesystems so at that moment, we can also
> change the netlink interface...
> 
> > Sure, and in many ways.  But if working with NFS, as far as I know the
> > most common way to solve it is to enforce a common /etc/passwd across
> > all the valid NFS clients  :)
>   Then one wonders whether user namespaces are really what users want ;).

Absolutely.

You use nfs to share filesystems among separate machines that you want
to have look similar.

You use user namespaces to pretend one machine is a bunch of separate
machines.  So if you're just going to split up your machine into 5
vms and then have them all share disk over nfs, you may just want to
keep it as one machine :)

Ideally each vm would have completely separate disk space, so file
access across user namespaces wouldn't happen.  More realistically,
file trees will be shared read-only - i.e. /lib, /usr, etc.  Some of
that can be handled simply using read-only bind mounts.  We'd like
to allow users to create vm's as well, so then we want uid 500 in
the initial user namespace to be uid 0 in a newly created user
namespace.

So what Eric and I are worried about are corner cases and admin
mistakes, not regular function.

(And again I really do think we'll want to tie netlink sockets to a user
namespace, not a network namespace, so there may be no issue at all
so long as proper filesystem access checks are implemented so that every
action on some filesystem is done with credentials valid in that
filesystems' user namespace)

-serge
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