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[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 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org 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