Re: [patch] unprivileged mounts update

["Serge E. Hallyn" <serge@hallyn.com>]

Quoting Miklos Szeredi (miklos@szeredi.hu):
> > Quoting Miklos Szeredi (miklos@szeredi.hu):
> > > > So then as far as you're concerned, the patches which were in -mm will
> > > > remain unchanged?
> > > 
> > > Basically yes. I've merged the update patch, which was not yet added
> > > to -mm, did some cosmetic code changes, and updated the patch headers.
> > > 
> > > There's one open point, that I think we haven't really explored, and
> > > that is the propagation semantics.  I think you had the idea, that a
> > > propagated mount should inherit ownership from the parent into which
> > > it was propagated.
> > 
> > Don't think that was me.  I stayed out of those early discussions
> > because I wasn't comfortable guessing at the proper semantics yet.
> 
> Yes, sorry, it was Eric's suggestion.
> 
> > But really, I, as admin, have to set up both propagation and user mounts
> > for a particular subtree, so why would I *not* want user mounts to be
> > propagated?
> > 
> > So, in my own situation, I have done
> > 
> > 	make / rshared
> > 	mount --bind /share /share
> > 	make /share unbindable
> > 	for u in $users; do
> > 		mount --rbind / /share/$u/root
> > 		make /share/$u/root rslave
> > 		make /share/$u/root rshared
> > 		mount --bind -o user=$u /share/$u/root/home/$u /share/$u/root/home/$u
> > 	done
> > 
> > All users get chrooted into /share/$USER/root, some also get their own
> > namespace.  Clearly if a user in a new namespace does
> > 
> > 	mount --bind -o user=me ~/somedir ~/otherdir
> > 
> > then logs out, and logs back in, I want the ~/otherdir in the new
> > namespace (and the one in the 'init' namespace) to also be owned by
> > 'me'.
> > 
> > > That sounds good if everyone agrees?
> > 
> > I've shown where I think propagating the mount owner is useful.  Can you
> > detail a scenario where doing so would be bad?  Then we can work toward
> > semantics that make sense...
> 
> But in your example, the "propagated mount inherits ownership from
> parent mount" would also work, since in all namespaces the owner of
> the parent would necessary be "me".

true.

> The "inherits parent" semantics would work better for example in the
> "all nosuid" namespace, where the user is free to modify it's mount
> namespace. 
> 
> If for example propagation is set up from the initial namespace to
> this user's namespace and a new mount is added to the initial
> namespace, it would be nice if the propagated new mount would also be
> owned by the user (and be "nosuid" of course).

ok, so in the example i gave, this would be the admin in the
initial namespace mounting something under /home/$USER/, which
gets propagated to slave /share/$USER/root/home/$USER, where
we would want a different mount owner.

> Does the above make sense?  I'm not sure I've explained clearly
> enough.

I think I see.  Sounds like inherit from parent does the right thing
all around, at least in cases we've thought of so far.

thanks,
-serge