From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-3.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_PASS, USER_AGENT_MUTT autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 123A6C282C0 for ; Wed, 23 Jan 2019 13:40:28 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id D283320870 for ; Wed, 23 Jan 2019 13:40:27 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1548250827; bh=wNmxIfrUB4BRy9ctI6UXPdU/HJMFau3Lnl6h6P/v0Qk=; h=Date:From:To:Cc:Subject:References:In-Reply-To:List-ID:From; b=P521YdRkTFNad672fac43R3xxKKwZ74EcYa/aibdLdMQUZGqbWbk0x/RqcXX+kBZT mpqaWRx1YIfBkaMINhDCB2nb8BrXq2oXVuol4slbVl+pO1E3xN6uS/2nvkJKcKSoJo JIykReswvKMHttGNh9CnoXN353qJa1KoymTbMxf8= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726234AbfAWNk0 (ORCPT ); Wed, 23 Jan 2019 08:40:26 -0500 Received: from mail.kernel.org ([198.145.29.99]:55358 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726029AbfAWNkZ (ORCPT ); Wed, 23 Jan 2019 08:40:25 -0500 Received: from localhost (5356596B.cm-6-7b.dynamic.ziggo.nl [83.86.89.107]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 948AB20870; Wed, 23 Jan 2019 13:40:23 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1548250824; bh=wNmxIfrUB4BRy9ctI6UXPdU/HJMFau3Lnl6h6P/v0Qk=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=RjOxJze67PXHuG6KNbV0GJbOetwtYZmjBU7oRbwbKeGeBIJth+aGD0c8sPcNyzpT0 7b00qc2Q2UqPj576pNUuYLlr1+ffXvfX2DMOdAP04XH2+laNBBe1UZJ++nzjX5vZP6 F4qvWh2WgZJ1ua1ifKgd5s9jGySs2slXfhvIv/3s= Date: Wed, 23 Jan 2019 14:40:21 +0100 From: Greg Kroah-Hartman To: Michal Hocko Cc: linux-kernel@vger.kernel.org, Masami Hiramatsu , Ulf Hansson , Gary R Hook , Heiko Carstens Subject: Re: [PATCH 2/2] debugfs: return error values, not NULL Message-ID: <20190123134021.GB24700@kroah.com> References: <20190123102702.GA17123@kroah.com> <20190123102814.GB17123@kroah.com> <20190123110628.GV4087@dhcp22.suse.cz> <20190123115535.GA31237@kroah.com> <20190123121350.GX4087@dhcp22.suse.cz> <20190123122626.GA27968@kroah.com> <20190123124024.GY4087@dhcp22.suse.cz> <20190123130057.GB21318@kroah.com> <20190123130917.GZ4087@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190123130917.GZ4087@dhcp22.suse.cz> User-Agent: Mutt/1.11.2 (2019-01-07) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jan 23, 2019 at 02:09:17PM +0100, Michal Hocko wrote: > On Wed 23-01-19 14:00:57, Greg KH wrote: > > On Wed, Jan 23, 2019 at 01:40:24PM +0100, Michal Hocko wrote: > > > On Wed 23-01-19 13:26:26, Greg KH wrote: > > > > On Wed, Jan 23, 2019 at 01:13:50PM +0100, Michal Hocko wrote: > > > > > On Wed 23-01-19 12:55:35, Greg KH wrote: > > > > > > On Wed, Jan 23, 2019 at 12:06:28PM +0100, Michal Hocko wrote: > > > > > > > On Wed 23-01-19 11:28:14, Greg KH wrote: > > > > > > > > When an error happens, debugfs should return an error pointer value, not > > > > > > > > NULL. This will prevent the totally theoretical error where a debugfs > > > > > > > > call fails due to lack of memory, returning NULL, and that dentry value > > > > > > > > is then passed to another debugfs call, which would end up succeeding, > > > > > > > > creating a file at the root of the debugfs tree, but would then be > > > > > > > > impossible to remove (because you can not remove the directory NULL). > > > > > > > > > > > > > > > > So, to make everyone happy, always return errors, this makes the users > > > > > > > > of debugfs much simpler (they do not have to ever check the return > > > > > > > > value), and everyone can rest easy. > > > > > > > > > > > > > > How come this is safe at all? Say you are creating a directory by > > > > > > > debugfs_create_dir and then feed the return value to debugfs_create_files > > > > > > > as a parent. In case of error you are giving it an invalid pointer and > > > > > > > likely blow up unless I miss something. > > > > > > > > > > > > debugfs_create_files checks for invalid parents and will just refuse to > > > > > > create the file. It's always done that. > > > > > > > > > > I must be missing something because debugfs_create_files does > > > > > d_inode(parent)->i_private = data; > > > > > as the very first thing and that means that it dereferences an invalid > > > > > pointer right there. > > > > > > > > debugfs_create_file() -> __debugfs_create_file() -> start_creating() > > > > and that function checks if parent is an error, which it aborts on, or > > > > if it is NULL, it sets parent to a valid value: > > > > > > > > /* If the parent is not specified, we create it in the root. > > > > * We need the root dentry to do this, which is in the super > > > > * block. A pointer to that is in the struct vfsmount that we > > > > * have around. > > > > */ > > > > if (!parent) > > > > parent = debugfs_mount->mnt_root; > > > > > > > > I don't see any line that looks like: > > > > > d_inode(parent)->i_private = data; > > > > in Linus's tree right now, what kernel version are you referring to? > > > > > > Ohh, my bad. I have looked at debugfs_create_files which is a mq helper > > > around debugfs_create_file. But that is a good example why this patch is > > > dangerous anyway. blk_mq_debugfs_register simply checks for NULL and > > > debugfs_create_files doesn't expect ERR_PTR here. So you would have to > > > check each and every user to make sure you can do that. > > > > Ah, I already have that patch in my "to add a proper changelog" queue, > > it's below and fixes that problem. > > OK, fair enough. I am just wondering how many more gems like that are > lurking there. Do not get me wrong but a broken error handling is rarely > fixed by removing it. I think you all are the "lucky" one here :) I did audit the whole kernel tree already, with the exception of the gpu drivers, as they are even more insane than block drivers... > And Cc: stable is completely inappropriate IMNSHO. This is just adding a > risk without a large benefit. What risk? Again, the _ONLY_ way that NULL is returned here is if you are out of memory when you try to create that debugfs file/directory. Given that we usually don't even fail small kmallocs, I doubt this can even ever happen. > Moreover all these changes should be posted in a single patch thread so > that everybody can see the final outcome. No one wants to see a 200+ patch thread and be cc:ed on them all. thanks, greg k-h