linux-kernel - Re: INFO: task hung in __get

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Date:   Wed, 15 May 2019 20:32:27 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     Jan Kara <jack@...e.cz>
Cc:     Jens Axboe <axboe@...nel.dk>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        syzbot <syzbot+10007d66ca02b08f0e60@...kaller.appspotmail.com>,
        dvyukov@...gle.com, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, syzkaller-bugs@...glegroups.com,
        linux-block@...r.kernel.org
Subject: Re: INFO: task hung in __get_super

On 2019/05/15 19:21, Jan Kara wrote:
> The question is how to fix this problem. The simplest fix I can see is that
> we'd just refuse to do LOOP_SET_FD if someone has the block device
> exclusively open as there are high chances such user will be unpleasantly
> surprised by the device changing under him. OTOH this has some potential
> for userspace visible regressions. But I guess it's worth a try. Something
> like attached patch?

(1) If I understand correctly, FMODE_EXCL is set at blkdev_open() only if O_EXCL
    is specified. How can we detect if O_EXCL was not used, for the reproducer
    ( https://syzkaller.appspot.com/text?tag=ReproC&x=135385a8a00000 ) is not
    using O_EXCL ?

(2) There seems to be no serialization. What guarantees that mount_bdev()
    does not start due to preempted after the check added by this patch?