From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753852AbaILJZG (ORCPT ); Fri, 12 Sep 2014 05:25:06 -0400 Received: from www.linutronix.de ([62.245.132.108]:46356 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753625AbaILJZD (ORCPT ); Fri, 12 Sep 2014 05:25:03 -0400 Date: Fri, 12 Sep 2014 11:24:49 +0200 (CEST) From: Thomas Gleixner To: Dave Hansen cc: Qiaowei Ren , "H. Peter Anvin" , Ingo Molnar , x86@kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v8 08/10] x86, mpx: add prctl commands PR_MPX_REGISTER, PR_MPX_UNREGISTER In-Reply-To: Message-ID: References: <1410425210-24789-1-git-send-email-qiaowei.ren@intel.com> <1410425210-24789-9-git-send-email-qiaowei.ren@intel.com> <541239F1.2000508@intel.com> User-Agent: Alpine 2.10 (DEB 1266 2009-07-14) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII X-Linutronix-Spam-Score: -1.0 X-Linutronix-Spam-Level: - X-Linutronix-Spam-Status: No , -1.0 points, 5.0 required, ALL_TRUSTED=-1,SHORTCIRCUIT=-0.0001 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, 12 Sep 2014, Thomas Gleixner wrote: > On Thu, 11 Sep 2014, Dave Hansen wrote: > > Well, we use it to figure out whether we _potentially_ need to tear down > > an VM_MPX-flagged area. There's no guarantee that there will be one. > > So what you are saying is, that if user space sets the pointer to NULL > via the unregister prctl, kernel can safely ignore vmas which have the > VM_MPX flag set. I really can't follow that logic. > > mmap_mpx(); > prctl(enable mpx); > do lots of crap which uses mpx; > prctl(disable mpx); > > So after that point the previous use of MPX is irrelevant, just > because we set a pointer to NULL? Does it just look like crap because > I do not get the big picture how all of this is supposed to work? do_bounds() will happily map new BTs no matter whether the prctl was invoked or not. So what's the value of the prctl at all? The mapping is flagged VM_MPX. Why is this not sufficient? Thanks, tglx