[PATCH RFC 0/1] mount: universally disallow mounting over symlinks

[Ian Kent]

On Tue, 2020-01-14 at 13:01 +0800, Ian Kent wrote:
> On Tue, 2020-01-14 at 04:39 +0000, Al Viro wrote:
> > On Tue, Jan 14, 2020 at 08:25:19AM +0800, Ian Kent wrote:
> > 
> > > This isn't right.
> > > 
> > > There's actually nothing stopping a user from using a direct map
> > > entry that's a multi-mount without an actual mount at its root.
> > > So there could be directories created under these, it's just not
> > > usually done.
> > > 
> > > I'm pretty sure I don't check and disallow this.
> > 
> > IDGI...  How the hell will that work in v5?  Who will set _any_
> > traps outside the one in root in that scenario?  autofs_lookup()
> > won't (there it's conditional upon indirect mount).  Neither
> > will autofs_dir_mkdir() (conditional upon version being less
> > than 5).  Who will, then?
> > 
> > Confused...
> 
> It's easy to miss.
> 
> For autofs type direct and offset mounts the flags are set at fill
> super time.
> 
> They have to be set then because they are direct mounts and offset
> mounts behave the same as direct mounts so they need to be set then
> too. So, like direct mounts, offset mounts are each distinct autofs
> (trigger) mounts.
> 
> I could check for this construct and refuse it if that's really
> needed. I'm pretty sure this map construct isn't much used by
> people using direct mounts.

Ok, once again I'm not exactly accurate is some of what I said.

It turns out that the autofs connectathon tests, one of the tests
that I use, does test direct mounts with offsets both with and
without a real mount at the base of the mount.

Based on that, I have to say this map construct is meant to be
supported with Sun format maps of autofs (even though I think it's
probably not used much).

So not allowing it is probably the wrong thing to do.

OTOH initial testing with the #work.namei branch shows these are
functioning as required.

Ian