From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755822AbaHYPFv (ORCPT ); Mon, 25 Aug 2014 11:05:51 -0400 Received: from mx1.redhat.com ([209.132.183.28]:48144 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751812AbaHYPFt (ORCPT ); Mon, 25 Aug 2014 11:05:49 -0400 Date: Mon, 25 Aug 2014 17:03:01 +0200 From: Oleg Nesterov To: Hugh Dickins Cc: Cyrill Gorcunov , Andrew Morton , Manfred Spraul , Davidlohr Bueso , Kees Cook , Tejun Heo , Andrew Vagin , "Eric W. Biederman" , "H. Peter Anvin" , Serge Hallyn , Pavel Emelyanov , Vasiliy Kulikov , KAMEZAWA Hiroyuki , Michael Kerrisk , Julien Tinnes , linux-kernel@vger.kernel.org Subject: Re: [PATCH 1/1] ipc/shm: fix the historical/wrong mm->start_stack check Message-ID: <20140825150301.GB3021@redhat.com> References: <20140823144246.GA6281@redhat.com> <20140823144327.GA6299@redhat.com> <20140823152240.GH25918@moon> <20140823155832.GA9406@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 08/24, Hugh Dickins wrote: > > On Sat, 23 Aug 2014, Oleg Nesterov wrote: > > On 08/23, Cyrill Gorcunov wrote: > > > > > > On Sat, Aug 23, 2014 at 04:43:27PM +0200, Oleg Nesterov wrote: > > > > The ->start_stack check in do_shmat() looks ugly and simply wrong. > > > > > > > > 1. ->start_stack is only valid right after exec(), the application > > > > can switch to another stack and even unmap this area. > > > > > > > > 2. The reason for this check is not clear at all. The application > > > > should know what it does. And why 4 pages? And why in fact it > > > > requires 5 pages? > > > > > > > > 3. This wrongly assumes that the stack can only grown down. > > > > > > > > Personally I think we should simply kill this check, but I did not > > > > dare to do this. So the patch only fixes the 1st problem (mostly to > > > > avoid the usage of mm->start_stack) and ignores VM_GROWSUP. > > > > > > > > Signed-off-by: Oleg Nesterov > > > Reviewed-by: Cyrill Gorcunov > > Yes, much better to use find_vma than have this strange stray use > of unreliable start_stack. > > Acked-by: Hugh Dickins Thanks! > though like Manfred I didn't quite see how overflow was impossible > on unfamiliar architectures. And you can't see, because the comment is simply wrong, I'll send v2. > > > I don't understand this check either, the comment above it says nothing > > > but only commits what code is doing not explaining why. > > > > Yes, and this check predates the git history. I even looked into > > git://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git but this > > change was added by the huge "v2.5.0.7 -> v2.5.0.8" update in 2002, > > and obviously without any explanation (apart from "fix up proper shmat > > semantics", but this connects SHM_REMAP itself). > > I'd say it comes earlier, from Christoph Rohland's 2.4.17-pre7's > "Add missing checks on shmat()", though I didn't find more than that. > > We can all understand wanting to leave a gap below the growsdown stack, > but of course could argue about growsup and 1 or 4 or 5 or whatever: And it is not clear to me why the kernel should care at all, > okay that we're all more interested in just removing that start_stack. so perhaps v2 should simply remove it? Or do you think it would be safer to not do this? Oleg.