Trond Myklebust <trond.myklebust@fys.uio.no> writes:

> On Wed, 2006-11-08 at 01:52 +0100, Herbert Poetzl wrote:
>> On Mon, Nov 06, 2006 at 10:18:14PM -0600, Serge E. Hallyn wrote:
>> > Cedric has previously sent out a patchset
>> > (http://lists.osdl.org/pipermail/containers/2006-August/000078.html)
>> > impplementing the very basics of a user namespace. It ignores
>> > filesystem access checks, so that uid 502 in one namespace could
>> > access files belonging to uid 502 in another namespace, if the
>> > containers were so set up.
>> >
>> > This isn't necessarily bad, since proper container setup should
>> > prevent problems. However there has been concern, so here is a
>> > patchset which takes one course in addressing the concern.
>> >
>> > It adds a user namespace pointer to every superblock, and to
>> > enhances fsuid equivalence checks with a (inode->i_sb->s_uid_ns ==
>> > current->nsproxy->uid_ns) comparison.
>> 
>> I don't consider that a good idea as it means that a filesystem
>> (or to be precise, a superblock) can only belong to one specific
>> namespace, which is not very useful for shared setups
>> 
>> Linux-VServer provides a mechanism to do per inode (and per
>> nfs mount) tagging for similar 'security' and more important
>> for disk space accounting and limiting, which permits to have
>> different disk limits, quota and access on a shared partition
>> 
>> i.e. I do not like it
>
> Indeed. I discussed this with Eric at the kernel summit this summer and
> explained my reservations. As far as I'm concerned, tagging superblocks
> with a container label is an unacceptable hack since it completely
> breaks NFS caching semantics.

As I recall there are two basic issues.

Putting the default on the mount structure instead of the superblock
for filesystems that are not uid namespaces aware sounded reasonable,
and allowed certain classes of sharing between namespaces where they
agreed on a subset of the uids (especially for read-only data).

The other was to have a mechanism that allows a uid namespace aware
filesystem (like some of the distributed filesystems can be) to perform
the mapping on their own.

Some mostly this is a case of simply not going far enough in the uid
namespace direction.

Eric
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