Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758718Ab3EWL4c (ORCPT ); Thu, 23 May 2013 07:56:32 -0400 Received: from mx1.redhat.com ([209.132.183.28]:28573 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1758100Ab3EWL4a convert rfc822-to-8bit (ORCPT ); Thu, 23 May 2013 07:56:30 -0400 Date: Thu, 23 May 2013 07:56:20 -0400 From: Jeff Layton To: Stanislav Kinsbursky Cc: "Eric W. Biederman" , , , , , , , , , , , Subject: Re: [RFC PATCH] fs: call_usermodehelper_root helper introduced Message-ID: <20130523075620.21abf79a@tlielax.poochiereds.net> In-Reply-To: <519DFFA9.3010606@parallels.com> References: <20130522072840.27720.85023.stgit@localhost.localdomain> <878v36ex6n.fsf@xmission.com> <519DCE5D.6070204@parallels.com> <87k3mq9fsu.fsf@xmission.com> <519DF109.9010309@parallels.com> <20130523073108.13afafa6@tlielax.poochiereds.net> <519DFFA9.3010606@parallels.com> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8BIT Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4501 Lines: 100 On Thu, 23 May 2013 15:38:17 +0400 Stanislav Kinsbursky wrote: > 23.05.2013 15:31, Jeff Layton пишет: > > On Thu, 23 May 2013 14:35:53 +0400 > > Stanislav Kinsbursky wrote: > > > >> 23.05.2013 14:00, Eric W. Biederman пишет: > >>> Stanislav Kinsbursky writes: > >>> > >>>> 22.05.2013 21:33, Eric W. Biederman пишет: > >>>>> Stanislav Kinsbursky writes: > >>>>> > >>>>>> Usermode helper executes all binaries in global "init" root context. This > >>>>>> doesn't allow to call a binary from other root context (for example in a > >>>>>> container). > >>>>>> Currently, both containerized NFS client and NFS server requires an ability to > >>>>>> execute a binary in a container's root context. Root swap can be done in > >>>>>> "init" callback, passed by UMH caller. > >>>>>> But since we have 2 callers already (and more of them are expected to appear > >>>>>> in future) and because set_fs_root() in not exported, it looks reasonable to > >>>>>> add one more generic UMH helper to generic fs code. > >>>>>> Root path reference must be hold by the caller, since it will be put on UMH > >>>>>> thread exit. > >>>>> > >>>>> Awesome. With this patch as an uprivilieged user I get to pick which > >>>>> binary the kernel will execute. At least if nfs and nfsd ever runs in a > >>>>> user namespace (something that looks like only matter of time). > >>>>> > >>>> > >>>> Not really. Only by using a kernel module to call the UMH. > >>>> And an unprivileged can't load a module as far a I know. > >>>> I.e. NFSd, for example, will use unprivileged user's root to perform this call. > >>> > >>> To help me understand the context which instances of call user mode > >>> helper are you expecting to use this facility? > >>> > >> > >> Ok. Here is how the NFSd uses UMH: > >> UMH is used on NFSd service to start user-space client tracker daemon > >> ("/sbin/nfsdcltarck"), which is used to store some per-client locks data on > >> persistent storage. > >> > >>>>> I think this is a seriously bad idea. > >>>>> > >>>>> Why can't we do this in userspace with setns as we do with the core dump > >>>>> helper? > >>>>> > >>>> > >>>> Could you, please, clarify, how setns can help here? > >>> > >>> setns can change the mount namespace, and chroot can change to root > >>> directory in the specified mount namespace. Essentially you can enter > >>> into a containers complete context (pid, mnt, root, etc) comming from > >>> the outside. > >>> > >> > >> So, you are actually suggesting to move the binary start from the kernel to user-space. > >> IOW, you are suggesting to do not using UMH at all. > >> Am I right? > >> I don't know the reasons, why it was done by using UMH and not in userspace. > >> Could you clarify this, Jeff? > >> > > > > nfsdcltrack is a "one-shot" program for managing and querying the nfsd > > client tracking database. When knfsd needs to query or modify the > > db, it uses the UMH infrastructure to call this program that does > > what's requested and then exits. > > > > So, I'm not sure I really understand your question. It wasn't done in > > userspace since the whole purpose of this program is to handle upcalls > > from the kernel. > > > > The question is what was the reason to start this binary from kernel by UMH? Manipulating and querying the client tracking database is an infrequent event, so having a continuously running daemon is wasteful and means that the admin has to ensure that it's running. A UMH upcall is much simpler and generally "just works" if the program is present. > I.e. why it can't be started by some user-space process before or after NFSd start? > I don't familiar with this client tracking facility and that's the only reason why I'm asking. > This program is not a daemon that runs continuously. It's only called when the kernel needs to manipulate the database. Are you asking whether we could turn this into a continuously running daemon? If so then the answer is "yes", but that's not really a good idea either. In fact, we had that with the nfsdcld program, but no one liked it (including me) for the reasons I detailed above. -- Jeff Layton -- 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/