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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id E774BC433EF for ; Thu, 20 Jan 2022 12:13:24 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232127AbiATMNY (ORCPT ); Thu, 20 Jan 2022 07:13:24 -0500 Received: from smtp-out1.suse.de ([195.135.220.28]:43468 "EHLO smtp-out1.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230093AbiATMNY (ORCPT ); Thu, 20 Jan 2022 07:13:24 -0500 Received: from relay2.suse.de (relay2.suse.de [149.44.160.134]) by smtp-out1.suse.de (Postfix) with ESMTP id EBD0721709; Thu, 20 Jan 2022 12:13:22 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.cz; s=susede2_rsa; t=1642680802; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=XbTaUwDkXc2YpBVQ5zhYci9z5sdLls8fCr2xhba8dfU=; b=EbevpEwrW5eOA9Y9SJvV2hxHzKq0KDJow+vT+A+j7fwl6LG1uAXmWK7Mxdq//YAGdHBPas Ned7yhlx6PGRZFueBLkrpp4FGcl62sQrX6XvabNpmlB0WzcvIHH9DGyIWh1v8lhxgk7NPU 6kVoTslpjM+fGrnVhBQrl/RfYiNIPOs= DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=suse.cz; s=susede2_ed25519; t=1642680802; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=XbTaUwDkXc2YpBVQ5zhYci9z5sdLls8fCr2xhba8dfU=; b=L6jcJHwOA0W2upHKbukNTQ+fv79iSUqSqSyY2UPrU7GPyeKI9CKVbBQoluYvfjOfEyin1Q vaZI7TFgOJwAjxCg== Received: from quack3.suse.cz (unknown [10.100.200.198]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by relay2.suse.de (Postfix) with ESMTPS id DD128A3B87; Thu, 20 Jan 2022 12:13:22 +0000 (UTC) Received: by quack3.suse.cz (Postfix, from userid 1000) id 93EF3A05D9; Thu, 20 Jan 2022 13:13:19 +0100 (CET) Date: Thu, 20 Jan 2022 13:13:19 +0100 From: Jan Kara To: Karel Zak Cc: Jan Kara , util-linux@vger.kernel.org, linux-block@vger.kernel.org, Tetsuo Handa Subject: Re: Racy loop device reuse logic Message-ID: <20220120121319.kernpjn45jqgween@quack3.lan> References: <20220113154735.hdzi4cqsz5jt6asp@quack3.lan> <20220119085247.duhblxzp6joukarw@quack3.lan> <20220119113900.tm5j65wzxglggsig@ws.net.home> <20220119213449.jydoqbyzjudqmikk@quack3.lan> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20220119213449.jydoqbyzjudqmikk@quack3.lan> Precedence: bulk List-ID: X-Mailing-List: util-linux@vger.kernel.org On Wed 19-01-22 22:34:49, Jan Kara wrote: > On Wed 19-01-22 12:39:00, Karel Zak wrote: > > On Wed, Jan 19, 2022 at 09:52:47AM +0100, Jan Kara wrote: > > > Ping? Any opinion? > > > > Sorry for the delay. > > > > > On Thu 13-01-22 16:47:35, Jan Kara wrote: > > > > Hello, > > > > > > > > Tetsuo has been doing some changes to the loop device shutdown in the > > > > kernel and that broke LTP that is doing essentially the following loop: > > > > > > > > while :; do mount -o loop,ro isofs.iso isofs/; umount isofs/; done > > > > > > > > And this loop is broken because of a subtle interaction with systemd-udev > > > > that also opens the loop device. The race seems to be in mount(8) handling > > > > itself and the altered kernel timing makes it happen. It look like: > > > > > > > > bash systemd-udev > > > > mount -o loop,ro isofs.iso isofs/ > > > > /dev/loop0 is created and bound to isofs.iso, autoclear is set for > > > > loop0 > > > > opens /dev/loop0 > > > > umount isofs/ > > > > loop0 still lives because systemd-udev still has device open > > > > mount -o loop,ro isofs.iso isofs/ > > > > gets to mnt_context_setup_loopdev() > > > > loopcxt_find_overlap() > > > > sees loop0 is still valid and with proper parameters > > > > reuse = true; > > > > close /dev/loop0 > > > > last fd closed => loop0 is > > > > cleaned up > > > > loopcxt_get_fd() > > > > opens loop0 but it is no longer the device we wanted! > > > > calls mount(2) which fails because we cannot read from the loop device > > > > > > > > It seems to me that mnt_context_setup_loopdev() should actually recheck > > > > that loop device parameters still match what we need after opening > > > > /dev/loop0 (if LOOP_GET_STATUS ioctl succeeds on the fd, you are guaranteed > > > > the loop device is in that state and will not be torn down under your > > > > hands). What do you think? > > > > Seems like elegant solution. Please, send a patch. > > OK, will do. Patch sent which fixes the problem for me. Related to this, I have found out that loop device reuse detection code sometimes fails with EPERM (if I run the mount loop long enough). I have tracked this down to e.g. loopcxt_get_sizelimit() calling ul_path_read_u64() which fails (loop device just got torn down) and ul_path_read_u64() always returns -1 on error which gets somewhat confusingly translated to EPERM error. I think this should be handled more gracefully so I'll send patches to fix this as well. Honza -- Jan Kara SUSE Labs, CR