linux-kernel - Re: [PATCH] ipc,sem block sem_lock on sma->lock during sma initialization

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Date:	Sun, 23 Nov 2014 13:36:51 -0800
From:	Davidlohr Bueso <dave@...olabs.net>
To:	Rik van Riel <riel@...hat.com>
Cc:	Manfred Spraul <manfred@...orfullife.com>,
	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Rafael Aquini <aquini@...hat.com>, 1vier1@....de
Subject: Re: [PATCH] ipc,sem block sem_lock on sma->lock during sma
 initialization

On Sun, 2014-11-23 at 16:03 -0500, Rik van Riel wrote:
> On 11/23/2014 01:23 PM, Manfred Spraul wrote:
> > Hi Rik,
> > 
> > On 11/21/2014 08:52 PM, Rik van Riel wrote:
> >> When manipulating just one semaphore with semop, sem_lock only
> >> takes that single semaphore's lock. This creates a problem during
> >> initialization of the semaphore array, when the data structures
> >> used by sem_lock have not been set up yet. The sma->lock is
> >> already held by newary, and we just have to make sure everything
> >> else waits on that lock during initialization.
> >> 
> >> Luckily it is easy to make sem_lock wait on the sma->lock, by
> >> pretending there is a complex operation in progress while the sma
> >> is being initialized.
> > That's not sufficient, as sma->sem_nsems is accessed before
> > calling sem_lock(), both within find_alloc_undo() and within
> > semtimedop().
> > 
> > The root problem is that sma->sem_nsems and sma->sem_base are
> > accessed without any locks, this conflicts with the approach that
> > sma starts to exist as not yet initialized but locked and is
> > unlocked after the initialization is completed.
> > 
> > Attached is an idea. It did pass a few short tests. What do you
> > think?
> 
> This was my other idea for fixing the issue; unfortunately
> I didn't think of it until after I sent the first patch :)

Yep, this is what I was mentioning as well.

> You are right that without that change, we can return the
> wrong error codes to userspace.
> 
> I will give the patch a try, though I have so far been unable
> to reproduce the bug that the customer reported, so I am unlikely
> to give much in the way of useful testing results...
> 
> Andrew, feel free to give Manfred's patch my
> 
> Acked-by: Rik van Riel <riel@...hat.com>

Acked-by: Davidlohr Bueso <dave@...olabs.net>

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