linux-kernel - Re: [RFC]: Possible race condition in kernel futex code

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Message-ID: <20151009113552.GC3816@twins.programming.kicks-ass.net>
Date:	Fri, 9 Oct 2015 13:35:52 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Hans Zuidam <h.zuidam@...puter.org>,
	Jaccon Bastiaansen <jaccon.bastiaansen@...il.com>,
	x86@...nel.org, mingo@...hat.com, "H. Peter Anvin" <hpa@...or.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	stable@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [RFC]: Possible race condition in kernel futex code

On Fri, Oct 09, 2015 at 11:25:09AM +0100, Thomas Gleixner wrote:
> Hans,
> 
> On Fri, 9 Oct 2015, Hans Zuidam wrote:
> > On 9 okt. 2015, at 11:06, Thomas Gleixner <tglx@...utronix.de> wrote:
> > > You cannot use an explicit 32bit read. We need an access which
> > > handles the fault gracefully.
> >
> > The reason for the explicit read suggestion is to avoid the
> > _builtin_constant_p() in __copy_from_user_nocheck().  The GCC manual
> > says that there may be situations where it returns 0 even though the
> > argument is a constant.
> 
> That's insane at best.

Right, but I bet that is for cases where constant propagation completely
fails, and this is a trivial case, I have no problem relying on it.
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