linux-kernel - Re: [patch 1/5] futex: Make unlock

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Date:	Mon, 16 Jun 2014 15:49:45 -0700
From:	Darren Hart <darren@...art.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Davidlohr Bueso <davidlohr@...com>,
	Kees Cook <kees@...flux.net>, wad@...omium.org
Subject: Re: [patch 1/5] futex: Make unlock_pi more robust

On Tue, 2014-06-17 at 00:28 +0200, Thomas Gleixner wrote:
> On Tue, 17 Jun 2014, Thomas Gleixner wrote:
> > On Mon, 16 Jun 2014, Darren Hart wrote:
> > > On Wed, 2014-06-11 at 20:45 +0000, Thomas Gleixner wrote:
> > > In wake_futex_pi we verify ownership by matching pi_state->owner ==
> > > current, but here the only test is the TID value, which is set by
> > > userspace - which we don't trust...
> > > 
> > > I'm trying to determine if it matters in this case... if there are no
> > > waiters, is the pi_state still around? If so, it does indeed matter, and
> > > we should be verifying.
> > 
> > Erm. The whole point of this patch is to do:
> > 
> >      - Find existing state first and handle it.
> > 
> >      - If no state exists and TID == current, take it
> > 
> >      - Otherwise create state
> 
> Duh, that was the lock path. But here the point is:
> 
>       - Find existing state first and handle it.
> 
>       - If no state exists and TID == current, release it
> 

Right, I understood your meaning, and I withdraw the concern.

> The retry is obvious, right?

Yes.

-- 
Darren Hart
Intel Open Source Technology Center


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