Re: [PATCH -mm 5/7] add user namespace

From: Al Boldi
Date: Mon Jul 17 2006 - 07:22:39 EST

Kyle Moffett wrote:
> On Jul 15, 2006, at 13:39:50, Al Boldi wrote:
> > Trond Myklebust wrote:
> >> On Sat, 2006-07-15 at 06:35 -0600, Eric W. Biederman wrote:
> >>> I hope the confusion has passed for Trond. My impression was he
> >>> figured this was per process data so it didn't make sense any
> >>> where near a filesystem, and the superblock was the last place it
> >>> should be.
> >>
> >> You are still using the wrong abstraction. Data that is not global
> >> to the entire machine has absolutely _no_ place being put into the
> >> superblock. It doesn't matter if it is process-specific, container-
> >> specific or whatever-else-specific, it will still be vetoed.
> >>
> >> If your real problem is uid/gid mapping on top of generic
> >> filesystems, then have you looked into the *BSD solution of using
> >> a stackable filesystem (i.e. umapfs)?
> >
> > A stackable FS is really overkill here, when all that is needed is
> > a simple mapping. An easy solution would be, to allow for perMount
> > Handlers via hooks into the VFS, as was suggested in the '[RFC]
> > VFS: FS CoW using redirection' thread.
> IMHO a UID mapping is completely the wrong solution for this.

Sure, maybe it is, but if it were needed, why would I implement it using an
extra VFS below the VFS?

> The problem is the subject (the process) and object (the filesystem)
> place different meanings on different UIDs, in other words their UIDs
> are in different namespaces. The result is that you should tag that
> filesystem (vfsmount, really) with a different namespace tag and fix
> the namespace system to properly handle cross-namespace permissions,
> not forcibly graft on some fragile mapping system.

Grafting is probably the wrong way to implement this too, so what I suggest
is to have the VFS provide hooks (aka plug-ins) for perMount Handlers, which
would relieve developers from the need to either merge their requirements
with the VFS, or introduce a subVFS. This would imply more freedom, while
at the same time reducing overhead.

> By using the
> keyring system for foreign-namespace UID permissions the actual
> permissions fall out quite nicely.

Maybe it is, for this specific situation.



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