linux-kernel - Re: [RFC] situation with fput() locking (was Re: [PULL REQUEST] : ima-appraisal patches)

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Message-ID: <20120420031509.GE6871@ZenIV.linux.org.uk>
Date:	Fri, 20 Apr 2012 04:15:09 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	linux-fsdevel@...r.kernel.org, James Morris <jmorris@...ei.org>,
	linux-security-module@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	David Safford <safford@...ux.vnet.ibm.com>,
	Dmitry Kasatkin <dmitry.kasatkin@...el.com>,
	Mimi Zohar <zohar@...ux.vnet.ibm.com>,
	David Miller <davem@...emloft.net>
Subject: Re: [RFC] situation with fput() locking (was Re: [PULL REQUEST] :
 ima-appraisal patches)

On Fri, Apr 20, 2012 at 03:54:38AM +0100, Al Viro wrote:

> and filp_close() would, if that turns out to be possible, call fput_nodefer()
> instead of fput().  If we *do* have places where we need deferral in
> filp_close() (and I'm fairly sure that any such place is a deadlock right
> now), well, we'll need a variant of filp_close() sans the call of fput...()
> and those places would call that, followed by full (deferring) fput().

BTW, some of those filp_close() are simply wrong - e.g. a big pile in
drivers/media/video/cx25821/*.c should be fput().

And yes, we have at least some under mutex - binder_lock held by
binder_ioctl(), which ends up doing binder_transaction() with its
failure cleanup hitting close_filp().  On an arbitrary struct file.
It *probably* doesn't deadlock on the spot, since binder_release()
itself contains a deferral mechanism (perhaps they'd spotted the
deadlock, perhaps there are other reasons to have it).
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