I don't know. It's something to think about in the future, but not
essential. We know that without the above check the user can do bad
things: propagate mounts back into the source, and we don't want that.
We could allow binding a shared mount if
a) the owners of the source and destination match
b) the destination is made a slave of the source
But the current patchset doesn't allow _any_ changes to propagation
without CAP_SYS_ADMIN, so why should bind be an exception?
And yes, this is something to think about, but I think it's a rather
uncommon corner case, and so the patchset very much makes sense
without having to deal with unprivileged mount propagation changes.
No, we'd be back with the original problem.