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

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Message-ID: <25388.1198986232@turing-police.cc.vt.edu>
Date:	Sat, 29 Dec 2007 22:43:52 -0500
From:	Valdis.Kletnieks@...edu
To:	dean gaudet <dean@...tic.org>
Cc:	David Newall <david@...idnewall.com>, 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 ?

On Sat, 29 Dec 2007 12:40:47 PST, dean gaudet said:

> > See, this is where you show that you don't understand the system.  I'll
> > explain it, just once.  /var/home contains  home directories.  /var/log and
> > /var/home are on the same filesystem.  So /var/log/* can be linked to
> > /var/home/malicious, and that's just one of your basic misunderstandings.
> 
> yes you are on crack.
> 
> i told you i understand this exactly.  it's right there in the message 
> sent.

So... You understand that if /var/home and /var/log are on one file system,
you can hard-link, and you set your system up knowing that, and then you're
*surprised* that:

> the main worry i have is some user maliciously hardlinks everything
> under /var/log somewhere else and slowly fills up the file system with
> old rotated logs.

"Doctor, it hurts when I do this.." "Well, don't do that then".

I think the first time I saw the recommendation "Put /home on its own
filesystem and don't give users directly writable directories on /var (except
via set-uid helpers) so they can't play hardlink games" back in 1983 or so.
I know that when SunOS 3.1 came out, that was already well-understood basic
sysadmining.  Sometimes, there's actual good reasons behind 20-year-old
voodoo.. ;)

You sure you don't want to redesign your filesystem layout so you don't
have to worry about your malicious users hardlinking stuff? Might be a lot
easier than trying to get the kernel to do what you want in this case....

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