From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S938763AbdAEUty (ORCPT ); Thu, 5 Jan 2017 15:49:54 -0500 Received: from mga03.intel.com ([134.134.136.65]:35243 "EHLO mga03.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751162AbdAEUtp (ORCPT ); Thu, 5 Jan 2017 15:49:45 -0500 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.33,322,1477983600"; d="scan'208";a="919376707" Subject: Re: [RFC, PATCHv2 29/29] mm, x86: introduce RLIMIT_VADDR To: Andy Lutomirski References: <20161227015413.187403-1-kirill.shutemov@linux.intel.com> <20161227015413.187403-30-kirill.shutemov@linux.intel.com> <5a3dcc25-b264-37c7-c090-09981b23940d@intel.com> <20170105192910.q26ozg4ci4i3j2ai@black.fi.intel.com> <161ece66-fbf4-cb89-3da6-91b4851af69f@intel.com> Cc: "Kirill A. Shutemov" , Linus Torvalds , Andrew Morton , X86 ML , Thomas Gleixner , Ingo Molnar , Arnd Bergmann , "H. Peter Anvin" , Andi Kleen , linux-arch , "linux-mm@kvack.org" , "linux-kernel@vger.kernel.org" , Linux API From: Dave Hansen Message-ID: <978d5f1a-ec4d-f747-93fd-27ecfe10cb88@intel.com> Date: Thu, 5 Jan 2017 12:49:44 -0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.5.1 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 01/05/2017 12:14 PM, Andy Lutomirski wrote: >> I'm not sure I'm comfortable with this. Do other rlimit changes cause >> silent data corruption? I'm pretty sure doing this to MPX would. >> > What actually goes wrong in this case? That is, what combination of > MPX setup of subsequent allocations will cause a problem, and is the > problem worse than just a segfault? IMO it would be really nice to > keep the messy case confined to MPX. The MPX bounds tables are indexed by virtual address. They need to grow if the virtual address space grows. There's an MSR that controls whether we use the 48-bit or 57-bit layout. It basically decides whether we need a 2GB (48-bit) or 1TB (57-bit) bounds directory. The question is what we do with legacy MPX applications. We obviously can't let them just allocate a 2GB table and then go let the hardware pretend it's 1TB in size. We also can't hand the hardware using a 2GB table an address >48-bits. Ideally, I'd like to make sure that legacy MPX can't be enabled if this RLIMIT is set over 48-bits (really 47). I'd also like to make sure that legacy MPX is active, that the RLIMIT can't be raised because all hell will break loose when the new addresses show up. Remember, we already have (legacy MPX) binaries in the wild that have no knowledge of this stuff. So, we can implicitly have the kernel bump this rlimit around, but we can't expect userspace to do it, ever.