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Date:   Mon, 5 Apr 2021 18:23:49 +0000
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Christian Brauner <christian.brauner@ubuntu.com>
Cc:     Jens Axboe <axboe@kernel.dk>,
        syzbot <syzbot+c88a7030da47945a3cc3@syzkaller.appspotmail.com>,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        syzkaller-bugs@googlegroups.com, io-uring@vger.kernel.org
Subject: Re: [syzbot] WARNING in mntput_no_expire (2)
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On Mon, Apr 05, 2021 at 07:08:01PM +0200, Christian Brauner wrote:

> Ah dentry count of -127 looks... odd.

dead + 1...

void lockref_mark_dead(struct lockref *lockref)
{
        assert_spin_locked(&lockref->lock);
	lockref->count = -128;
}

IOW, a leaked (uncounted) reference to dentry, that got dget() called on
it after dentry had been freed.

	IOW, current->fs->pwd.dentry happens to point to an already freed
struct dentry here.  Joy...

	Could you slap

spin_lock(&current->fs->lock);
WARN_ON(d_count(current->fs->pwd.dentry) < 0);
spin_unlock(&current->fs->lock);

before and after calls of io_issue_sqe() and see if it triggers?  We definitely
are seeing buggered dentry refcounting here.