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

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Date:	Fri, 20 Nov 2015 09:59:05 +0000
From:	David Howells <dhowells@...hat.com>
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	dhowells@...hat.com, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-security-module@...r.kernel.org, miklos@...redi.hu
Subject: Re: [RFC] readlink()-related oddities

Al Viro <viro@...IV.linux.org.uk> wrote:

> 3) normally, readlink(2) fails for non-symlinks.  Moreover, according to
> POSIX it should do so (with -EINVAL).  There is a pathological case when
> it succeeds for a directory, though.  Namely, one of the kinds of AFS
> "mountpoints".

All AFS mountpoints are magic symlinks that are specially interpreted by the
client as far as I'm aware.  I'm not sure why the designers didn't just select
a different file type for them, but they didn't.

Unfortunately, it means that iget has to read the contents of the symlinks :-/

> stat(2) reports those as directories, stepping into them leads to
> automounting a directory there (why do we have ->open() for them, BTW?).

I think I put that in to make sure the open() syscall returned EREMOTE rather
than another error if you tried to open it.  It can probably be removed
because with the d_automount code you can't ever get there I think - unless
you can pass AT_NO_AUTOMOUNT to openat().

> How the hell is userland supposed to guess to call readlink(2) on those
> suckers to get the information of what'll get automounted there if we step
> upon them?

There's an AFS userspace command that could be used to query a mountpoint that
was going to use it.  However, I suspect readlink() will now always trigger
the automount.  This is one of the things OpenAFS uses pioctl() for - but
since I'm not allowed to add that to the kernel, I have to find some other way
of doing it.

> And could we please get rid of that kludge?  David?

Sure.

David
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