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

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Date:	Thu, 20 Dec 2007 21:55:55 +0100 (CET)
From:	Bodo Eggert <7eggert@....de>
To:	Al Viro <viro@....linux.org.uk>
cc:	Bodo Eggert <7eggert@....de>, Mark Lord <lkml@....ca>,
	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 ?

On Wed, 19 Dec 2007, Al Viro wrote:
> On Wed, Dec 19, 2007 at 02:43:26PM +0100, Bodo Eggert wrote:

> > Since nobody knows about this "security boundary" and everybody knows about
> > the annoying "can't link across bind-mountpoints bug",
> 
> ... how about teaching people to RTFM?  Starting, perhaps, with man 2 link?

What about reading POSIX which says 

1264 [EXDEV]
1265 Improper link. A link to a file on another file system was attempted.

So if the link creates a file on NOT another filesystem (which is the point 
of bind mounts), it should NOT return EXDEV.

Having an artificial boundary between different views to a fs may happen to 
be a security feature if used with care, but most users do expect the 
opposite and wonder why mv is needlessly slow. I'm not even sure if 
defaulting to having a barrier is sane at all, but if people confuse 
filesystems and mountpoints^W^W^W^Wuse this feature, they will depend on 
this feature not changing:-)

-- 
"It is generally inadvisable to eject directly over the area you just
bombed."
-U.S. Air Force Manual
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