Re: [PATCH] namespace.c: fix bind mount from foreign namespace

[Jamie Lokier <jamie@shareable.org>]

Ram wrote:
> > I'd rather not speculate on what Al Viro was thinking, it may have
> > been just a misunderstanding.
> 
> Can somebody who know internals of Al Viro's thinking help here?

Presumably he wrote this line:

	if (check_mnt(nd->mnt) && (!recurse || check_mnt(old_nd.mnt))) {

Which /explicitly/ permits bind mounts between namespaces if it's not
recursive.  It's not accidental: that !recurse is blatantly making a
point of allowing it.

I take that to mean that /at least at one time/ Al chose to allow it.

Then again, he also wrote this:

> > Bind mount from a foreign namespace results in
> 
> ... -EINVAL

Which means that /at another time/ Al thought he'd disallowed it.

This is a bit like arguing over what the Founding Fathers of the US
Constitution meant.  Does it matter?  We really should ask what
behaviour makes sense now.  Should we add more explicit restrictions
to the code, making the concept of namespaces more restrictive?  Or
remove the restrictions, on the grounds that they don't really add any
security, it'd be useful to relax them, and the code would be simpler?

-- Jamie
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