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=-6.8 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI, SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS 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 3F974C54FCC for ; Tue, 21 Apr 2020 06:55:40 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id F097E20738 for ; Tue, 21 Apr 2020 06:55:39 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=kernel.org header.i=@kernel.org header.b="rOvT1pf7" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org F097E20738 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=linuxfoundation.org Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id 7D3F68E0005; Tue, 21 Apr 2020 02:55:39 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 785708E0003; Tue, 21 Apr 2020 02:55:39 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 69BEC8E0005; Tue, 21 Apr 2020 02:55:39 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0159.hostedemail.com [216.40.44.159]) by kanga.kvack.org (Postfix) with ESMTP id 510E68E0003 for ; Tue, 21 Apr 2020 02:55:39 -0400 (EDT) Received: from smtpin13.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay02.hostedemail.com (Postfix) with ESMTP id 0DDCE4DC8 for ; Tue, 21 Apr 2020 06:55:39 +0000 (UTC) X-FDA: 76730951598.13.bait93_142d1c7556931 X-HE-Tag: bait93_142d1c7556931 X-Filterd-Recvd-Size: 6950 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by imf43.hostedemail.com (Postfix) with ESMTP for ; Tue, 21 Apr 2020 06:55:38 +0000 (UTC) Received: from localhost (83-86-89-107.cable.dynamic.v4.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 BFF8D2072D; Tue, 21 Apr 2020 06:55:36 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1587452137; bh=zJ60yvTqB3tFZq6e5i0uFpw0aBM0sbkbs4gVEDscwcc=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=rOvT1pf71YXBkdb+1cDESYAuVzGz9Pfk05UHBuTjyKfXBGV8Jwl3nLgA8xfppe4Gy 3rQRUzHq43ryklylYJLxHNa0hUAmBU4i2wDKLh3QDBBLU8xmzuY6g/vZ/rjhX3FvfR l9NazMlSeEbaWkalX0L3KNeQ72f9/by4M0/j7BI8= Date: Tue, 21 Apr 2020 08:55:35 +0200 From: Greg KH To: Luis Chamberlain Cc: Bart Van Assche , axboe@kernel.dk, viro@zeniv.linux.org.uk, rostedt@goodmis.org, mingo@redhat.com, jack@suse.cz, ming.lei@redhat.com, nstange@suse.de, akpm@linux-foundation.org, mhocko@suse.com, yukuai3@huawei.com, linux-block@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2 08/10] blktrace: add checks for created debugfs files on setup Message-ID: <20200421065535.GC347130@kroah.com> References: <20200419194529.4872-1-mcgrof@kernel.org> <20200419194529.4872-9-mcgrof@kernel.org> <38240225-e48e-3035-0baa-4929948b23a3@acm.org> <20200419230537.GG11244@42.do-not-panic.com> <20200420114038.GE3906674@kroah.com> <20200420184445.GK11244@42.do-not-panic.com> <20200420201101.GB302402@kroah.com> <20200420202046.GN11244@42.do-not-panic.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200420202046.GN11244@42.do-not-panic.com> X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Mon, Apr 20, 2020 at 08:20:46PM +0000, Luis Chamberlain wrote: > On Mon, Apr 20, 2020 at 10:11:01PM +0200, Greg KH wrote: > > On Mon, Apr 20, 2020 at 06:44:45PM +0000, Luis Chamberlain wrote: > > > On Mon, Apr 20, 2020 at 01:40:38PM +0200, Greg KH wrote: > > > > On Sun, Apr 19, 2020 at 04:17:46PM -0700, Bart Van Assche wrote: > > > > > On 4/19/20 4:05 PM, Luis Chamberlain wrote: > > > > > > On Sun, Apr 19, 2020 at 03:57:58PM -0700, Bart Van Assche wrote: > > > > > > > On 4/19/20 12:45 PM, Luis Chamberlain wrote: > > > > > > > > Even though debugfs can be disabled, enabling BLK_DEV_IO_TRACE will > > > > > > > > select DEBUG_FS, and blktrace exposes an API which userspace uses > > > > > > > > relying on certain files created in debugfs. If files are not created > > > > > > > > blktrace will not work correctly, so we do want to ensure that a > > > > > > > > blktrace setup creates these files properly, and otherwise inform > > > > > > > > userspace. > > > > > > > > > > > > > > > > Signed-off-by: Luis Chamberlain > > > > > > > > --- > > > > > > > > kernel/trace/blktrace.c | 8 +++++--- > > > > > > > > 1 file changed, 5 insertions(+), 3 deletions(-) > > > > > > > > > > > > > > > > diff --git a/kernel/trace/blktrace.c b/kernel/trace/blktrace.c > > > > > > > > index 9cc0153849c3..fc32a8665ce8 100644 > > > > > > > > --- a/kernel/trace/blktrace.c > > > > > > > > +++ b/kernel/trace/blktrace.c > > > > > > > > @@ -552,17 +552,19 @@ static int blk_trace_create_debugfs_files(struct blk_user_trace_setup *buts, > > > > > > > > struct dentry *dir, > > > > > > > > struct blk_trace *bt) > > > > > > > > { > > > > > > > > - int ret = -EIO; > > > > > > > > - > > > > > > > > bt->dropped_file = debugfs_create_file("dropped", 0444, dir, bt, > > > > > > > > &blk_dropped_fops); > > > > > > > > + if (!bt->dropped_file) > > > > > > > > + return -ENOMEM; > > > > > > > > bt->msg_file = debugfs_create_file("msg", 0222, dir, bt, &blk_msg_fops); > > > > > > > > + if (!bt->msg_file) > > > > > > > > + return -ENOMEM; > > > > > > > > bt->rchan = relay_open("trace", dir, buts->buf_size, > > > > > > > > buts->buf_nr, &blk_relay_callbacks, bt); > > > > > > > > if (!bt->rchan) > > > > > > > > - return ret; > > > > > > > > + return -EIO; > > > > > > > > return 0; > > > > > > > > } > > > > > > > > > > > > > > I should have had a look at this patch before I replied to the previous > > > > > > > patch. > > > > > > > > > > > > > > Do you agree that the following code can be triggered by > > > > > > > debugfs_create_file() and also that debugfs_create_file() never returns > > > > > > > NULL? > > > > > > > > > > > > If debugfs is enabled, and not that we know it is in this blktrace code, > > > > > > as we select it, it can return ERR_PTR(-ERROR) if an error occurs. > > > > > > > > > > This is what I found in include/linux/debugfs.h in case debugfs is disabled: > > > > > > > > > > static inline struct dentry *debugfs_create_file(const char *name, > > > > > umode_t mode, struct dentry *parent, void *data, > > > > > const struct file_operations *fops) > > > > > { > > > > > return ERR_PTR(-ENODEV); > > > > > } > > > > > > > > > > I have not found any code path that can cause debugfs_create_file() to > > > > > return NULL. Did I perhaps overlook something? If not, it's not clear to me > > > > > why the above patch adds checks that check whether debugfs_create_file() > > > > > returns NULL? > > > > > > > > Short answer, yes, it can return NULL. Correct answer is, you don't > > > > care, don't check the value and don't do anything about it. It's > > > > debugging code, userspace doesn't care, so just keep moving on. > > > > > > Thing is this code *exposes* knobs to userspace for an API that *does* > > > exepect those files to exist. That is, blktrace *relies* on these > > > debugfs files to exist. So the kconfig which enables blktrace > > > CONFIG_BLK_DEV_IO_TRACE selects DEBUG_FS. > > > > That's nice, but again, no kernel code should do anything different > > depending on what debugfs happens to be doing at that point in time. > > So even if the debugfs files were *not* created, and this code executes only > if DEBUG_FS, you don't think we should inform userspace if the blktrace > setup ioctl, which sets up these debugfs, didn't happen? > > The "recovery" here would just be to destroy the blktrace setup, and > inform userspace that the blktrace setup ioctl failed. Hm, ok, but comment the heck out of this saying _why_ you are testing the return value, and how that differs from 99% of the other users of this function in the kernel tree please. Otherwise I will end up removing the checks again with my semi-regular sweep of the tree... thanks, greg k-h