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 77F59C282C0 for ; Wed, 23 Jan 2019 12:40:34 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 4714121019 for ; Wed, 23 Jan 2019 12:40:34 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1548247234; bh=Xa/xSUgpdfNdKaKXklXJeYeWDgsOoNzWarFmPoqPj6Q=; h=Date:From:To:Cc:Subject:References:In-Reply-To:List-ID:From; b=OrCoofA4wqZ+/lWZ75r2wfbkbvOUvoQy2gwfAqD+jsqroX7Q26P9flCTqnEvsUp7I mL6CyOtREg7EvmMqXFJL9S+HsdG+Mz+StzUbnBxDL0ivmFDCxxBGaSoIFWRJ9wpG8k 8Mwxg+Y/zNgIZcBrOiYQQUbXtdO09egv+uUfjaNk= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727336AbfAWMkc (ORCPT ); Wed, 23 Jan 2019 07:40:32 -0500 Received: from mx2.suse.de ([195.135.220.15]:41464 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726175AbfAWMkc (ORCPT ); Wed, 23 Jan 2019 07:40:32 -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 E9E7BAD02; Wed, 23 Jan 2019 12:40:29 +0000 (UTC) Date: Wed, 23 Jan 2019 13:40:24 +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: <20190123124024.GY4087@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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190123122626.GA27968@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 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. > > > > I do agree that reporting errors is better than a simple catch all NULL > > > > but this should have been done when introduced rather than now when most > > > > callers simply check for NULL as a failure. > > > > > > I'm fixing up all the "NULL is a failure" callsites in the kernel, see > > > lkml for the first round of those patches. > > > > You are merely removing them, which doesn't really help for this patch. > > It doesn't hurt either, as if you really wanted to handle errors from debugfs > properly, you have to check for IS_ERR() as well, because the filesystem can be > compiled out (and then it returns an error pointer) I would assume that this would be achieved by a direct config dependency. E.g. BLK_DEBUG_FS. So the code doesn't even get compiled and wouldn't ever encounter the ERR_PTR. Yeah this whole debugfs API is broken and it would take a lot of time to unclutter it but it definitely is not that simple as what this patch does. -- Michal Hocko SUSE Labs