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 B712BC433EF for ; Thu, 13 Jan 2022 15:47:38 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229787AbiAMPri (ORCPT ); Thu, 13 Jan 2022 10:47:38 -0500 Received: from smtp-out2.suse.de ([195.135.220.29]:47654 "EHLO smtp-out2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229496AbiAMPrh (ORCPT ); Thu, 13 Jan 2022 10:47:37 -0500 Received: from relay2.suse.de (relay2.suse.de [149.44.160.134]) by smtp-out2.suse.de (Postfix) with ESMTP id 4B5DC1F3CD; Thu, 13 Jan 2022 15:47:36 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.cz; s=susede2_rsa; t=1642088856; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type; bh=tuADTLT4KlmQGRFiFRwWkhr8g0fsZXyfY0BNVq8KCqc=; b=lLj/BHXvz5/VQjOGPfL9pnTFryJLBrCxPCSQbsd1IQokWzfq+jTHqCLL7ONsvauV2ACFds 55z6EgrrogpHQGFQDbnHpmmQJ1vA+uspynsi6YGBTK6YkfonAdQ3nbJVv2XaxaaCb81I+H hho53hi+THsrQVukXfRw/BBu/M6ZPYg= DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=suse.cz; s=susede2_ed25519; t=1642088856; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type; bh=tuADTLT4KlmQGRFiFRwWkhr8g0fsZXyfY0BNVq8KCqc=; b=w5/bzTB8dOBeLfbqv/YkU81XDWMwUWM6ZBSeTwJoVgboPLfEzCioC11jp9fNFEQEB8tkZT kjnqD3ivWDr+2aCw== Received: from quack3.suse.cz (unknown [10.163.28.18]) (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 3D47BA3B83; Thu, 13 Jan 2022 15:47:36 +0000 (UTC) Received: by quack3.suse.cz (Postfix, from userid 1000) id BB91AA05E2; Thu, 13 Jan 2022 16:47:35 +0100 (CET) Date: Thu, 13 Jan 2022 16:47:35 +0100 From: Jan Kara To: Karel Zak Cc: util-linux@vger.kernel.org, linux-block@vger.kernel.org, Tetsuo Handa Subject: Racy loop device reuse logic Message-ID: <20220113154735.hdzi4cqsz5jt6asp@quack3.lan> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Precedence: bulk List-ID: X-Mailing-List: util-linux@vger.kernel.org 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? Honza -- Jan Kara SUSE Labs, CR