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Re: [PATCH] vfs: Fix possible NULL pointer dereference in inode_permissi

To: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Subject: Re: [PATCH] vfs: Fix possible NULL pointer dereference in inode_permission()
From: Ben Myers <bpm@xxxxxxx>
Date: Fri, 10 Jan 2014 12:14:34 -0600
Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>, Theodore Ts'o <tytso@xxxxxxx>, Paul Moore <paul@xxxxxxxxxxxxxx>, Matthew Wilcox <matthew@xxxxxx>, Stephen Smalley <sds@xxxxxxxxxxxxx>, LKML <linux-kernel@xxxxxxxxxxxxxxx>, Steven Rostedt <rostedt@xxxxxxxxxxx>, xfs@xxxxxxxxxxx, Eric Paris <eparis@xxxxxxxxxx>, James Morris <james.l.morris@xxxxxxxxxx>, linux-fsdevel <linux-fsdevel@xxxxxxxxxxxxxxx>, stable <stable@xxxxxxxxxxxxxxx>, Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>, Paul McKenney <paulmck@xxxxxxxxxxxxxxxxxx>, Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
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In-reply-to: <20140110093148.GA26159@xxxxxxxxxxxxx>
References: <20140109214239.GD29910@xxxxxxxxxxxxxxxx> <20140109165012.391db81e@xxxxxxxxxxxxxxxxxx> <20140109223127.GM10323@xxxxxxxxxxxxxxxxxx> <CA+55aFzCTPYEQCPnLBi1CwmMTocVqCFiCuJ391HkVx1CMw61ug@xxxxxxxxxxxxxx> <20140109182523.5b50131f@xxxxxxxxxxxxxxxxxx> <20140109182756.17abaaa8@xxxxxxxxxxxxxxxxxx> <1389310626.15209.92.camel@localhost> <CA+55aFzd2nw=JU4s0u=PJbATK0bwhm0kot3zRH=anLLT6THRFQ@xxxxxxxxxxxxxx> <20140110000642.GN10323@xxxxxxxxxxxxxxxxxx> <20140110093148.GA26159@xxxxxxxxxxxxx>
User-agent: Mutt/1.5.20 (2009-06-14)
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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