From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-lf1-f41.google.com (mail-lf1-f41.google.com [209.85.167.41]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id D42286D13 for ; Wed, 18 Aug 2021 15:54:01 +0000 (UTC) Received: by mail-lf1-f41.google.com with SMTP id p38so5727271lfa.0 for ; Wed, 18 Aug 2021 08:54:01 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:subject:to:cc:references:from:message-id:date :user-agent:mime-version:in-reply-to:content-language :content-transfer-encoding; bh=BHMjZ/i4Omck1upCZSLIcKpTxWs6ScKdudtkHXwFkLE=; b=JlAMpmvQAcnvqxNk88BR3qSHSe9ZZa7k/OcVIY570riziDbSf3DLPuqNFMHWgV0I0n 5QMUFXPgwLIUgi3arHQZphJo8cgrCvg+jVgx967NQxQu08NppuIB460BQDGt8/mFkLrP MLTDlHl1qhAPFYhz16LezFv1Qgqyih6aXRqcW/gFiqnc11Dndqj0AlkE716+0EhDIo55 s3R1hJTvNL1/Njev/e3+3cYsskk1WcNwf6DZv6y/Ync8tt9vS+JlnwLrntuk50XmHMwb pCnzddyy2I63HxfTPOn97yfVOBCFyojMx+c8eQxPKDnxkKaQByZ30UrdGmeV5uOKjAXS YZIA== X-Gm-Message-State: AOAM532AndF3OC0hSNKW8nLUa2THlKcp9ZhjrE1pa5quAsVqFC/7K/U8 6MQ4MDvwLdRbjfWX8OPp/Es= X-Google-Smtp-Source: ABdhPJzX63+YdtxMD3d2t0KhRquyfJSWRbwehMnKvCRyitRNSB4q0CCa4GjIT1G2HjeO+ujGwlCaDg== X-Received: by 2002:a05:6512:1501:: with SMTP id bq1mr7028514lfb.286.1629302040079; Wed, 18 Aug 2021 08:54:00 -0700 (PDT) Received: from [10.68.32.40] (broadband-188-32-236-56.ip.moscow.rt.ru. [188.32.236.56]) by smtp.gmail.com with ESMTPSA id n18sm27302ljg.40.2021.08.18.08.53.59 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 18 Aug 2021 08:53:59 -0700 (PDT) Subject: Re: [PATCH] Revert "floppy: reintroduce O_NDELAY fix" To: Jiri Kosina Cc: linux-block@vger.kernel.org, linux-kernel@vger.kernel.org, regressions@lists.linux.dev, Mark Hounschell , Wim Osterholt , Kurt Garloff , stable@vger.kernel.org References: <20210808074246.33449-1-efremov@linux.com> From: Denis Efremov Message-ID: <5ca95b90-c6e0-99b3-b129-75dc05cfb1d4@linux.com> Date: Wed, 18 Aug 2021 18:53:58 +0300 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.11.0 Precedence: bulk X-Mailing-List: regressions@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Hi, On 8/16/21 10:17 AM, Jiri Kosina wrote: > On Sun, 8 Aug 2021, Denis Efremov wrote: > >> The patch breaks userspace implementations (e.g. fdutils) and introduces >> regressions in behaviour. Previously, it was possible to O_NDELAY open a >> floppy device with no media inserted or with write protected media without >> an error. Some userspace tools use this particular behavior for probing. >> >> It's not the first time when we revert this patch. Previous revert is in >> commit f2791e7eadf4 (Revert "floppy: refactor open() flags handling"). >> >> This reverts commit 8a0c014cd20516ade9654fc13b51345ec58e7be8. > > By reverting it you bring back the bugs that were fixed by it I agree with you, that O_NDELAY is broken for floppies (and always been). However, just by removing O_NDELAY we break many existing tools that use it for probing and ioctl-only opens. With the patch tools fail to open the device without a diskette and try to read a diskette if there is one (this is not as fast on a real hardware as in QEMU). I think that there should be a better fix that doesn't break existing tools. It appears that people still use software that depends on O_NDELAY in floppies. Same patch was already reverted in 2016 (presumably) by the same reason. > -- e.g. the > possibility to livelock mmap() on the returned fd to keep waiting on the > page unlock bit forever As far as I understand this is a problem only for syzkaller. And this is not a security issue nowadays since most distributions (I don't know exceptions) require at least "disk" group to access floppies. Do you know a link for the syzkaller reproducer? > or the functionality bug reported at [1], and > likely others. > > [1] https://bugzilla.suse.com/show_bug.cgi?id=1181018 The patch starts to return -ENXIO for O_NDELAY|O_RDONLY opens and devices without a diskette. I don't think this is an expected behavior during libblkid probing. Probably there is a better fix for [1], maybe even an additional workaround for floppies in libblkid. They already have workarounds for cdroms https://github.com/karelzak/util-linux/commit/dc30fd4383e57a0440cdb0e16ba5c4336a43b290 I started to add simple tests https://lkml.org/lkml/2021/8/18/845 However, I failed to reproduce mount bug [1], probably because I don't know how to configure cloudinit properly. I tried to reproduce a mount fail bug with open("/dev/fd0", O_NDELAY|O_RDONLY) and mount("/dev/fd0", ...) but it works. Looks like there should be something else in between... Regards, Denis