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=-3.7 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,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 6370AC282C0 for ; Wed, 23 Jan 2019 13:09:23 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 321DE20855 for ; Wed, 23 Jan 2019 13:09:23 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1548248963; bh=iDwBI/t4xFRVlLFQq4IzY0VVRnVQGIG6gT9vVHAKbrc=; h=Date:From:To:Cc:Subject:References:In-Reply-To:List-ID:From; b=CDoev/7dT1WJPaHlX9ocoTtesFJePfeMXYkScUEDzBpl4ywIzN4sFm3l+aAzLAbXZ JBXevofR5dOZwQMl9wQQPAYrmiZXCKnb4kkp102d2wZoqysfn0I05CiAE1c5W+GE+4 te2zwVw5zrViGdUsMM4FtxQ5Po0mIWRBjhi/kFrQ= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726906AbfAWNJV (ORCPT ); Wed, 23 Jan 2019 08:09:21 -0500 Received: from mx2.suse.de ([195.135.220.15]:45538 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726057AbfAWNJV (ORCPT ); Wed, 23 Jan 2019 08:09:21 -0500 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id 07C16AE80; Wed, 23 Jan 2019 13:09:19 +0000 (UTC) Date: Wed, 23 Jan 2019 14:09:17 +0100 From: Michal Hocko To: Greg Kroah-Hartman 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: <20190123130917.GZ4087@dhcp22.suse.cz> 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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190123130057.GB21318@kroah.com> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. And Cc: stable is completely inappropriate IMNSHO. This is just adding a risk without a large benefit. Moreover all these changes should be posted in a single patch thread so that everybody can see the final outcome. -- Michal Hocko SUSE Labs