From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1760387AbbKTQIs (ORCPT ); Fri, 20 Nov 2015 11:08:48 -0500 Received: from zeniv.linux.org.uk ([195.92.253.2]:49666 "EHLO ZenIV.linux.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752014AbbKTQIq (ORCPT ); Fri, 20 Nov 2015 11:08:46 -0500 Date: Fri, 20 Nov 2015 16:08:42 +0000 From: Al Viro To: David Howells Cc: linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, Linus Torvalds , linux-security-module@vger.kernel.org, miklos@szeredi.hu Subject: Re: [RFC] readlink()-related oddities Message-ID: <20151120160842.GL22011@ZenIV.linux.org.uk> References: <20151119232635.GI22011@ZenIV.linux.org.uk> <22313.1448013545@warthog.procyon.org.uk> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <22313.1448013545@warthog.procyon.org.uk> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Nov 20, 2015 at 09:59:05AM +0000, David Howells wrote: > Al Viro 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. All of them? I see two kinds there - one is magical symlink (recognized by contents in afs_iget()), another is this autocell thing, the latter having no ->readlink(). Both serve as automount points, don't they? > > 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(). Just how would openat() get the AT_... flags? Only statat(2) accepts AT_NO_AUTOMOUNT, sorry. > > 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. Well, pioctl() is a piec^H^Hle of shit interface; let's figure out what we'd actually want to implement and do that. One obvious thing is "here's a pathname, tell me what gets automounted here" (with interesting question of what to do if the automount is being triggered right now). Another thing is locating those guys; if we had a separate file type for them (i.e. could recognize them by st_mode _and_ d_type), we would be fine (the usual tree-walkers would be able to spot such places and query them for prospective automount targets), but without that... a syscall for everything in a tree just to list those suckers?