Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751424AbaKFVR5 (ORCPT ); Thu, 6 Nov 2014 16:17:57 -0500 Received: from mail.linuxfoundation.org ([140.211.169.12]:55097 "EHLO mail.linuxfoundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751167AbaKFVRy (ORCPT ); Thu, 6 Nov 2014 16:17:54 -0500 Date: Thu, 6 Nov 2014 13:17:54 -0800 From: Greg Kroah-Hartman To: =?iso-8859-1?Q?M=E5ns_Rullg=E5rd?= Cc: Christian Riesch , Jiri Slaby , linux-kernel@vger.kernel.org, Peter Hurley , stable@vger.kernel.org Subject: Re: [PATCH] n_tty: Add memory barrier to fix race condition in receive path Message-ID: <20141106211754.GA32000@kroah.com> References: <20141106203832.GB30170@kroah.com> <20141106205644.GA31435@kroah.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Nov 06, 2014 at 09:01:36PM +0000, M?ns Rullg?rd wrote: > Greg Kroah-Hartman writes: > > > On Thu, Nov 06, 2014 at 08:49:01PM +0000, M?ns Rullg?rd wrote: > >> Greg Kroah-Hartman writes: > >> > >> > On Thu, Nov 06, 2014 at 12:39:59PM +0100, Christian Riesch wrote: > >> >> The current implementation of put_tty_queue() causes a race condition > >> >> when re-arranged by the compiler. > >> >> > >> >> On my build with gcc 4.8.3, cross-compiling for ARM, the line > >> >> > >> >> *read_buf_addr(ldata, ldata->read_head++) = c; > >> >> > >> >> was re-arranged by the compiler to something like > >> >> > >> >> x = ldata->read_head > >> >> ldata->read_head++ > >> >> *read_buf_addr(ldata, x) = c; > >> >> > >> >> which causes a race condition. Invalid data is read if data is read > >> >> before it is actually written to the read buffer. > >> > > >> > Really? A compiler can rearange things like that and expect things to > >> > actually work? How is that valid? > >> > >> This is actually required by the C spec. There is a sequence point > >> before a function call, after the arguments have been evaluated. Thus > >> all side-effects, such as the post-increment, must be complete before > >> the function is called, just like in the example. > >> > >> There is no "re-arranging" here. The code is simply wrong. > > > > Ah, ok, time to dig out the C spec... > > > > Anyway, because of this, no need for the wmb() calls, just rearrange the > > logic and all should be good, right? Christian, can you test that > > instead? > > Weakly ordered SMP systems probably need some kind of barrier. I didn't > look at it carefully. It shouldn't need a barier, as it is a sequence point with the function call. Well, it's an inline function, but that "shouldn't" matter here, right? thanks, greg k-h -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/