linux-kernel - Re: How to tell whether a struct file is held by a process?

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Date:	Thu, 21 May 2009 10:06:00 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Al Viro <viro@...IV.linux.org.uk>
cc:	Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: How to tell whether a struct file is held by a process?

On Thu, 21 May 2009, Al Viro wrote:

> On Tue, May 19, 2009 at 12:57:21PM -0400, Alan Stern wrote:
> > What's the best way to tell whether the current process has a
> > particular struct file among its open files?  Is there any better way
> > to find out than blindly calling fget() for each possible fd?
> > 
> > Is this a totally insane thing to do?
> 
> It is insane.  You might lock fdtable and scan it, but as soon as you
> drop the spinlock your return value is worthless.

In this case, I believe that changes after the check has been made 
won't hurt -- everything relevant to my work will be serialized by a 
separate lock.

> What are you trying to do?  If the process is cooperating, you don't really
> need that in the kernel, if it's not, the check is not usable...

I'm trying to work out a good way to reserve access rights to a device
-- even if that device doesn't exist yet.

Here's the story.  People have requested that the kernel add a
mechanism whereby a user program can get more-or-less exclusive access
rights to a USB device.  In fact, they'd like to reserve these rights
for any device plugged into a particular USB port.  So even if no
device is plugged into that port at the moment, the program should get
exclusive access as soon as a new device is detected there.

In order to prevent programs from dying without releasing their
exclusive rights, it seems natural to implement these rights as open
files.  Thus, opening file A(P) will give a program exclusive access
rights to any device plugged into USB port P.  Closing A(P) releases
the rights.  (The A(P) files would be implemented as single-open files, 
probably in sysfs.)

The problem is this.  Let D be the device plugged into port P.  When
some program opens D's device file, it's necessary to check whether
that same program has an open file reference for A(P), i.e., has opened
A(P), or has inherited a descriptor for A(P) from its parent, or has
been passed such a descriptor over a Unix socket, etc.  If not, and if 
A(P) is open (owned by someone else), then access to D is denied.

If you can think up a better way to implement these exclusive access 
rights, I'd be glad to hear it.

Alan Stern

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