Re: how to show propagation state for mounts

[Al Viro <viro@ZenIV.linux.org.uk>]

On Wed, Feb 20, 2008 at 11:29:13AM -0800, Ram Pai wrote:

> I wonder, what is wrong in reporting mounts in other namespaces that
> either receive and send propagation to mounts in our namespace?

A plenty.  E.g. if foo trusts control over /var/blah to bar, it's not
obvious that foo has any business knowing if bar gets it from somebody
else in turn.  And I'm not sure that bar has any business knowing that
foo has the damn thing attached in five places instead of just one,
let alone _where_ it has been attached.

If you get down to it, the thing is about delegating control over part
of namespace to somebody, without letting them control, see, etc. the
rest of it.  So I'd rather be very conservative about extra information
we allow to piggyback on that.  I don't know... perhaps with stable peer
group IDs it would be OK to show peer group ID by (our) vfsmount + peer
group ID of master + peer group ID of nearest dominating group that has
intersection with our namespace.  Then we don't leak information (AFAICS),
get full propagation information between our vfsmounts and cooperating
tasks in different namespaces can figure the things out as much as possible
without leaking 3rd-party information to either.