From: Al Viro <viro@ftp.linux.org.uk>
Newsgroups: fa.linux.kernel
Subject: Re: RFC: permit link(2)  to work across --bind mounts ?
Date: Tue, 18 Dec 2007 23:14:35 UTC
Message-ID: <fa.YISucUXfTfca+ucujUQfuv71dBI@ifi.uio.no>

On Tue, Dec 18, 2007 at 11:00:16PM +0000, Al Viro wrote:
> On Tue, Dec 18, 2007 at 05:46:21PM -0500, Mark Lord wrote:
> > Why does link(2) not support hard-linking across bind mount points
> > of the same underlying filesystem ?
>
> Because it gives you a security boundary around a subtree.

PS: that had been discussed quite a few times, but to avoid searches:
consider e.g. mount --bind /tmp /tmp; now you've got a situation when
users can't create links to elsewhere on root fs, even though they
have /tmp writable to them.  Similar technics works for other isolation
needs - basically, you can confine rename/link to given subtree.  IOW,
it's a deliberate feature.  Note that you can bind a bunch of trees
into chroot and get predictable restrictions regardless of how the
stuff might get rearranged a year later in the main tree, etc.