linux-kernel - Re: [PATCH 00/17] [RFC] AFS: Implement OpenAFS pioctls(version)s

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Date:	Wed, 17 Jun 2009 09:09:47 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Andreas Dilger <adilger@....com>
cc:	David Howells <dhowells@...hat.com>,
	Alan Cox <alan@...rguk.ukuu.org.uk>, akpm@...ux-foundation.org,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-afs@...ts.infradead.org
Subject: Re: [PATCH 00/17] [RFC] AFS: Implement OpenAFS pioctls(version)s

On Wed, 17 Jun 2009, Andreas Dilger wrote:
> 
> What about opening the mountpoint (which HAS to be available) and then
> calling an ioctl() on that?

It's very hard to "open the mountpoint" in user space. How would you even 
do it? Remember: we're not living in the 1980's any more, and disco is 
dead. ABBA may have made a comeback, but static mountpoints are long gone, 
and won't be coming back.

These days, you can mount individual files, you can have per-process 
mounts, and automounters have been a fact for a long time. 

So I _agree_ that pioctl's are problematic, but please don't argue against 
them using _stupid_ arguments. And "open the mountpoint" really is a 
stupid argument. It not only isn't possible to do in user space, but you 
may well want to do operations on a particular path, not just the mount.

So you'd need to open the file itself. Which might be a symlink or a 
device node, depending on the exact semantics of pioctl.

We've traditionally had that magic "open with flag=3" to do a magic open 
of device files without waiting, and we have O_NOFOLLOW to open symlinks 
without following them (sadly, it just errors out, rather than opening the 
symlink, but that's another detail). 

So I think it should be solvable some way, but not by trying to find the 
mount point.

			Linus
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