linux-kernel - Re: [RFC] readlink()-related oddities

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Message-ID: <20151120025749.GJ22011@ZenIV.linux.org.uk>
Date:	Fri, 20 Nov 2015 02:57:49 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	LSM List <linux-security-module@...r.kernel.org>,
	Miklos Szeredi <miklos@...redi.hu>,
	David Howells <dhowells@...hat.com>
Subject: Re: [RFC] readlink()-related oddities

On Thu, Nov 19, 2015 at 06:13:53PM -0800, Linus Torvalds wrote:

> > 3) normally, readlink(2) fails for non-symlinks.  Moreover, according to
> > POSIX it should do so (with -EINVAL).
> 
> I don't think POSIX is necessarily relevant here.
> 
> We have had magic file behavior outside the scope of POSIX before, and
> we will have it in the future. It makes perfect sense to use
> readlink() for management tools for automounting, even if the normal
> oepration is to treat the thing as a directory.
> 
> Not everything is within the domain of POSIX.

How would those tools know that this particular pathname _is_ a magical
symlink?  Sure, if you see a symlink with body that starts with % or #,
you could figure out that it's not a regular one and go parse the body,
but for stat(2) it looks like a directory.  Do those tools call readlink()
on every directory they spot on AFS volume?  David?

And what's the story with magical ->open() for those?  How could one get
to ->open() on those sucker and avoid triggering the automount instead?
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