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In-Reply-To: <20200306204523.GD23230@ZenIV.linux.org.uk>
From:   Miklos Szeredi <miklos@szeredi.hu>
Date:   Fri, 6 Mar 2020 21:51:12 +0100
Message-ID: <CAJfpegsRjHHy5JwB8S6-f0+SXRPmfaJJnaKj8nJvwVQzNCqU=A@mail.gmail.com>
Subject: Re: [PATCH 00/17] VFS: Filesystem information and notifications [ver #17]
To:     Al Viro <viro@zeniv.linux.org.uk>
Cc:     Ian Kent <raven@themaw.net>, David Howells <dhowells@redhat.com>,
        Christian Brauner <christian.brauner@ubuntu.com>,
        James Bottomley <James.Bottomley@hansenpartnership.com>,
        Steven Whitehouse <swhiteho@redhat.com>,
        Miklos Szeredi <mszeredi@redhat.com>,
        Christian Brauner <christian@brauner.io>,
        Jann Horn <jannh@google.com>,
        "Darrick J. Wong" <darrick.wong@oracle.com>,
        Linux API <linux-api@vger.kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        lkml <linux-kernel@vger.kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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On Fri, Mar 6, 2020 at 9:45 PM Al Viro <viro@zeniv.linux.org.uk> wrote:
>
> On Fri, Mar 06, 2020 at 08:38:44PM +0000, Al Viro wrote:
> > On Fri, Mar 06, 2020 at 08:37:05PM +0000, Al Viro wrote:
> >
> > > You are misreading mntput_no_expire(), BTW - your get_mount() can
> > > bloody well race with umount(2), hitting the moment when we are done
> > > figuring out whether it's busy but hadn't cleaned ->mnt_ns (let alone
> > > set MNT_DOOMED) yet.  If somebody calls umount(2) on a filesystem that
> > > is not mounted anywhere else, they are not supposed to see the sucker
> > > return 0 until the filesystem is shut down.  You break that.

Ah, good point.

> >
> > While we are at it, d_alloc_parallel() requires i_rwsem on parent held
> > at least shared.

Okay.

> Egads...  Let me see if I got it right - you are providing procfs symlinks
> to objects on the internal mount of that thing.  And those objects happen
> to be directories, so one can get to their parent that way.  Or am I misreading
> that thing?

Yes.

Thanks,
Miklos