From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752178AbaHVVNV (ORCPT ); Fri, 22 Aug 2014 17:13:21 -0400 Received: from mail-la0-f50.google.com ([209.85.215.50]:55353 "EHLO mail-la0-f50.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751796AbaHVVNR (ORCPT ); Fri, 22 Aug 2014 17:13:17 -0400 Date: Sat, 23 Aug 2014 01:13:14 +0400 From: Cyrill Gorcunov To: Andrew Morton Cc: linux-kernel@vger.kernel.org, keescook@chromium.org, tj@kernel.org, avagin@openvz.org, ebiederm@xmission.com, hpa@zytor.com, serge.hallyn@canonical.com, xemul@parallels.com, segoon@openwall.com, kamezawa.hiroyu@jp.fujitsu.com, mtk.manpages@gmail.com, jln@google.com Subject: Re: [patch 4/4] prctl: PR_SET_MM -- Introduce PR_SET_MM_MAP operation, v3 Message-ID: <20140822211314.GD25918@moon> References: <20140804172255.109539743@openvz.org> <20140804172610.965949916@openvz.org> <20140821155115.8433bb37bf631b8ae8340f84@linux-foundation.org> <20140822063242.GI14072@moon> <20140821234912.f007e2fb.akpm@linux-foundation.org> <20140822203809.GC25918@moon> <20140822134628.5df9cda87d744e35d794b5c2@linux-foundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20140822134628.5df9cda87d744e35d794b5c2@linux-foundation.org> 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 Fri, Aug 22, 2014 at 01:46:28PM -0700, Andrew Morton wrote: > On Sat, 23 Aug 2014 00:38:09 +0400 Cyrill Gorcunov wrote: > > > > > > > Or will we? What happens if we later decide that some additional field > > > needs to be added? Do we version the interface? Add a new prctl() > > > mode? Let's cook up a plan for that and at least add to changelog? > > > > I don't expect to change it anytime soon but we still have an option -- > > if we decide to extend or shrink it we always can use sizeof/offsetof > > helpers to check which exactly version userspace asks us to use. > > How does that work? We just have a blob of bytes coming in from > userspace. Not just blob. We have it as a structure where all fields have a constant size. Say we have struct prctl_mm_map { __u64 start_code; __u64 start_code; __u64 some-new-field; }; in the kernel, so its size will be 24 bytes but userspace uses old definition without @some-new-field member (16 bytes). So when we get a reguest with 16 bytes from userspace we can find the userspace have passed old definition. It's not as explicit as if we would have some @version field in struct prctl_mm_mmap, but looks fine for me. Still I can add @version into the structure if you prefer. > > As far as I understand the mm_struct is not the structure which > > changes that frequently, right? > > We might find existing things which criu wants to access. And criu > lives forever, yes? The mm_struct is likely to change over that time > period ;) Hopefully criu will live long enough so I would have a chance to update prctl_mm_map accordingly :) Still the good thing is that once mm_struct get changed the kernel fails to build in sys.c and the change will be noticed immediately so we update sys.c as well.