From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755512Ab2LMNPc (ORCPT ); Thu, 13 Dec 2012 08:15:32 -0500 Received: from mx1.redhat.com ([209.132.183.28]:22963 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753444Ab2LMNPa (ORCPT ); Thu, 13 Dec 2012 08:15:30 -0500 Date: Thu, 13 Dec 2012 14:15:40 +0100 From: Oleg Nesterov To: Srikar Dronamraju Cc: Ingo Molnar , Peter Zijlstra , Ananth N Mavinakayanahalli , Anton Arapov , linux-kernel@vger.kernel.org Subject: Re: [PATCH 3/7] uprobes: Kill the pointless inode/uc checks in register/unregister Message-ID: <20121213131540.GA11862@redhat.com> References: <20121123202741.GA18858@redhat.com> <20121123202806.GA18887@redhat.com> <20121210061901.GF22164@linux.vnet.ibm.com> <20121210191232.GA32462@redhat.com> <20121213103548.GC29086@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20121213103548.GC29086@linux.vnet.ibm.com> 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 12/13, Srikar Dronamraju wrote: > > * Oleg Nesterov [2012-12-10 20:12:32]: > > > On 12/10, Srikar Dronamraju wrote: > > > > > > * Oleg Nesterov [2012-11-23 21:28:06]: > > > > > > > register/unregister verifies that inode/uc != NULL. For what? > > > > This really looks like "hide the potential problem", the caller > > > > should pass the valid data. > > > > > > > > > > Agree that users should pass valid data. > > > I do understand that we expect the users to be knowledge-able. > > > Also users are routed thro in-kernel api that does this check. > > > > > > However from an api perspective, if a user passes invalid data, do we > > > want the system to crash. > > > > > > Esp if kernel can identify that users has indeed passed wrong info. I do agree > > > that users can still pass invalid data that kernel maynot be able to > > > identify in most cases. > > > > inode != NULL can't verify that it actually points to the valid inode, > > NULL is only one example of invalid data. > > > > I agree, sometimes it makes sense to protect against the stupid mistakes, > > but if we want to check against NULL we should do > > > > if (WARN_ON(!inode)) > > return; > > > > agree, that warn_on is better than a simple check and this one if (WARN_ON(inode < PAGE_OFFSET)) is even better ;) > > Especially in uprobe_unregister(). The current code is really "hide > > the possible problem" and nothing more. It is better to crash imho > > than silently return. > > > > > > register() also checks uc->next == NULL, probably to prevent the > > > > double-register but the caller can do other stupid/wrong things. > > > > > > Users can surely do more stupid things. But this is again something that > > > kernel can identify. By allowing a double-register of a consumer, thats > > > already registered, we might end up allowing circular loop of consumers. > > > > I understand. But in this case we should document that uc->next must > > be cleared before uprobe_register(). Or add init_consumer(). > > > > And we should change uprobe_unregister() to clear uc->next as well. > > I think that the code like this > > > > uprobe_register(uc); > > uprobe_unregister(uc); > > > > uprobe_register(uc); > > > > should work. Currently it doesn't because of this check. > > > > yes, these should work and makes a case to nullify ->next on unregister. > > However, what if someone tries > > uprobe_register(uc1); > uprobe_register(uc2); > uprobe_register(uc1); > > i.e somebody tries to re-register uc1, while its active and has a valid > next. After the re-registration of uc1, the uprobe->consumers will no more reference uc2. Yes. And even without uprobe_register(uc2) the result won't be good. This is like list_add(node). > Should we leave this case as a fool shooting himself? IMHO yes, or we should create init_consumer() or at least document that the private ->next member should be nullified. But let me repeat, > > So I still think these checks are pointless and (at least in unregister) > > even harmful. Yes, but I am not going to argue if you want to keep these checks. Oleg.