linux-kernel - Re: [PATCH] vfs: Fix possible NULL pointer dereference in inode

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Date:	Fri, 10 Jan 2014 12:14:34 -0600
From:	Ben Myers <bpm@....com>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	Al Viro <viro@...IV.linux.org.uk>, Theodore Ts'o <tytso@....edu>,
	Paul Moore <paul@...l-moore.com>,
	Matthew Wilcox <matthew@....cx>,
	Stephen Smalley <sds@...ho.nsa.gov>,
	LKML <linux-kernel@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>, xfs@....sgi.com,
	Eric Paris <eparis@...hat.com>,
	James Morris <james.l.morris@...cle.com>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	stable <stable@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] vfs: Fix possible NULL pointer dereference in
 inode_permission()

Christoph,

On Fri, Jan 10, 2014 at 01:31:48AM -0800, Christoph Hellwig wrote:
> On Fri, Jan 10, 2014 at 12:06:42AM +0000, Al Viro wrote:
> > Check what XFS is doing ;-/  That's where those call_rcu() have come from.
> > Sure, we can separate the simple "just do call_rcu(...->free_inode)" case
> > and hit it whenever full ->free_inode is there and ->destroy_inode isn't.
> > Not too pretty, but removal of tons of boilerplate might be worth doing
> > that anyway.  But ->destroy_inode() is still needed for cases where fs
> > has its own idea of inode lifetime rules.  Again, check what XFS is doing
> > in that area...
> 
> Btw, I'd really love to get rid of the XFS ->destroy_inode abuse, it's
> been a long time thorn in the flesh.

I believe this behavior is related to freeing of an inode cluster.

> What's really needed there to make XFS behave more similar to everyone
> else is a way for the filesystem to say: "I can't actually free this
> inode right now, but I'll come back to you later".

This test might read something like:  "If my link count has gone to zero, and I
am the last inode in my cluster to be freed, and there are other inodes from my
cluster incore, I cannot be freed."

Should be doable.  Maybe there are other reasons.

-Ben
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