linux-kernel - Re: [PATCH 02/46] fs: d

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Message-ID: <20101208065955.GA14846@amd>
Date:	Wed, 8 Dec 2010 17:59:55 +1100
From:	Nick Piggin <npiggin@...nel.dk>
To:	Dave Chinner <david@...morbit.com>
Cc:	Nick Piggin <npiggin@...nel.dk>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 02/46] fs: d_validate fixes

On Wed, Dec 08, 2010 at 12:53:44PM +1100, Dave Chinner wrote:
> On Sat, Nov 27, 2010 at 08:44:32PM +1100, Nick Piggin wrote:
> > d_validate has been broken for a long time.
> > 
> > kmem_ptr_validate does not guarantee that a pointer can be dereferenced
> > if it can go away at any time. Even rcu_read_lock doesn't help, because
> > the pointer might be queued in RCU callbacks but not executed yet.
> > 
> > So the parent cannot be checked, nor the name hashed. The dentry pointer
> > can not be touched until it can be verified under lock. Hashing simply
> > cannot be used.
> > 
> > Instead, verify the parent/child relationship by traversing parent's
> > d_child list. It's slow, but only ncpfs and the destaged smbfs care
> > about it, at this point.
> 
> I'd drop the previous revert patch and just convert the RCU hash
> traversal straight to the d_child traversal code you introduce here.
> This is a much better explanation of why the d_validate mechanism
> needs to be changed, and the revert is really an unnecessary extra
> step...

Has to be backported, though. Patch that is to be reverted obviously
adds more brokenness and is a good example that you cannot dget() under
rcu read protection even if the rest of the surrounding function is
bugfree. I wouldn't have thought it's a big deal.



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