Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751328AbaKFUtI (ORCPT ); Thu, 6 Nov 2014 15:49:08 -0500 Received: from unicorn.mansr.com ([81.2.72.234]:50671 "EHLO unicorn.mansr.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750706AbaKFUtF convert rfc822-to-8bit (ORCPT ); Thu, 6 Nov 2014 15:49:05 -0500 From: =?iso-8859-1?Q?M=E5ns_Rullg=E5rd?= To: Greg Kroah-Hartman 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 References: <20141106203832.GB30170@kroah.com> Date: Thu, 06 Nov 2014 20:49:01 +0000 In-Reply-To: <20141106203832.GB30170@kroah.com> (Greg Kroah-Hartman's message of "Thu, 6 Nov 2014 12:38:32 -0800") Message-ID: User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/24.4 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Transfer-Encoding: 8BIT Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. -- M?ns Rullg?rd mans@mansr.com -- 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/