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

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Message-ID: <alpine.LFD.2.01.0906171027470.16802@localhost.localdomain>
Date:	Wed, 17 Jun 2009 10:33:22 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	David Howells <dhowells@...hat.com>
cc:	Andreas Dilger <adilger@....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, David Howells wrote:

> Linus Torvalds <torvalds@...ux-foundation.org> 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.
> 
> I think what Andreas means is open the directory at the root of the mounted
> tree, i.e. "/afs" for AFS, and then do an ioctl() on that that emulates
> pioctl().

I agree that that is what he means.

What _I_ mean is that THIS IS IMPOSSIBLE TO DO FROM USER SPACE!

Try it. Not doable. User space simply doesn't know enough, and has 
fundamental races with mount/umount.

Sure, you can try to do it by trying to parse the pathname and looking in 
/etc/mtab or /proc/mounts. And I guarantee that the end result will be a 
buggy pile of sh*t.

End result: you do need a new system call. 

I just don't think "pioctl()" is a good one. You'd be better off with some 
modification of open and then use ioctl.

			Linus
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