linux-kernel - Re: RFC: permit link(2) to work across --bind mounts ?

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Message-Id: <E1J4zCp-0003fj-5I@be1.7eggert.dyndns.org>
Date:	Wed, 19 Dec 2007 14:43:26 +0100
From:	Bodo Eggert <7eggert@....de>
To:	Al Viro <viro@....linux.org.uk>, Mark Lord <lkml@....ca>,
	Al Viro <viro@....linux.org.uk>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: RFC: permit link(2)  to work across --bind mounts ?

Al Viro <viro@....linux.org.uk> wrote:
> 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 no 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.

Since nobody knows about this "security boundary" and everybody knows about
the annoying "can't link across bind-mountpoints bug", what about introducing
a mount option to allow link()ing?

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