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

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Message-ID: <Pine.LNX.4.44L0.0905211724050.3914-100000@iolanthe.rowland.org>
Date:	Thu, 21 May 2009 17:27:22 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Kay Sievers <kay.sievers@...y.org>,
	Pantelis Koukousoulas <pktoss@...il.com>
cc:	Al Viro <viro@...iv.linux.org.uk>,
	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, Kay Sievers wrote:

> Not sure, if we already discussed that with a conclusion: this
> "prevent access to a future device" interface needs to work at the pid
> level, or would a uid/gid check be sufficient?
> 
> Root can do all that stuff anyway, even with the locking in place.
> Uid/gid file permissions need to be applied to the "lock file", so
> specified non-root users can use that interface.
> 
> Maybe it would be good enough, to check that the one that opened the
> "lock file" has the same uid/gid as the on that tries to open the
> device when it has shown up?

I don't know; it depends on what people want.  Pantelis, would this be 
good enough for you?  That is, restrict the device either to programs 
having the same uid/gid or having the same pid as the process that 
opened the lock file?

(Note that the pid check alone isn't fully secure, since pid values get 
reused after a process ends.)

Alan Stern

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