linux-kernel - Re: new ...at() flag: AT_NO

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Date:   Fri, 5 May 2017 02:06:54 +0100
From:   Al Viro <viro@...IV.linux.org.uk>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Jann Horn <jannh@...gle.com>,
        Linux API <linux-api@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: new ...at() flag: AT_NO_JUMPS

On Thu, May 04, 2017 at 05:44:19PM -0700, Andy Lutomirski wrote:
> > It's not quite O_BENEATH, and IMO it's saner that way - a/b/c/../d is
> > bloody well allowed, and so are relative symlinks that do not lead out of
> > the subtree.  If somebody has a good argument in favour of flat-out
> > ban on .. (_other_ than "other guys do it that way, and it doesn't need
> > to make sense 'cuz security!!1!!!", please), I'd be glad to hear it.
> 
> I don't have an argument for allowing '..'.  I think it would be okay
> to disallow it, but I don't think it matters all that much either way.

Relative symlinks as argument in favour of allowing .. _when_ _it_ _stays_
_in_ _subtree_.

> > For the latter I would prefer -EXDEV, for obvious reasons.  For the former...
> > not sure.  I'm not too happy about -ELOOP, but -EPERM (as with O_BENEATH)
> > is an atrocity - it's even more overloaded.
> >
> > Suggestions?
> 
> -EDOTDOT would be amusing.

For ln -s /tmp foo/bar, lookup for foo/bar/baz?  Seriously?  Hell, even
-EXDEV would make more sense...