From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751765Ab2LJTMX (ORCPT ); Mon, 10 Dec 2012 14:12:23 -0500 Received: from mx1.redhat.com ([209.132.183.28]:64944 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751446Ab2LJTMW (ORCPT ); Mon, 10 Dec 2012 14:12:22 -0500 Date: Mon, 10 Dec 2012 20:12:32 +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: <20121210191232.GA32462@redhat.com> References: <20121123202741.GA18858@redhat.com> <20121123202806.GA18887@redhat.com> <20121210061901.GF22164@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20121210061901.GF22164@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 12/10, 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; 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. So I still think these checks are pointless and (at least in unregister) even harmful. But I won't insist too much, I can drop this patch if you do not change your mind. Oleg.