From: Ian Kent <raven@themaw.net>
To: Miklos Szeredi <miklos@szeredi.hu>
Cc: David Howells <dhowells@redhat.com>,
Linus Torvalds <torvalds@linux-foundation.org>,
Al Viro <viro@zeniv.linux.org.uk>,
Casey Schaufler <casey@schaufler-ca.com>,
Stephen Smalley <sds@tycho.nsa.gov>,
Nicolas Dichtel <nicolas.dichtel@6wind.com>,
Christian Brauner <christian@brauner.io>,
andres@anarazel.de, Jeff Layton <jlayton@redhat.com>,
dray@redhat.com, Karel Zak <kzak@redhat.com>,
keyrings@vger.kernel.org, Linux API <linux-api@vger.kernel.org>,
linux-fsdevel@vger.kernel.org,
LSM <linux-security-module@vger.kernel.org>,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH 13/17] watch_queue: Implement mount topology and attribute change notifications [ver #5]
Date: Wed, 05 Aug 2020 09:53:46 +0800 [thread overview]
Message-ID: <c558fc4af785f62a2751be3b297d1ccbbfcfa969.camel@themaw.net> (raw)
In-Reply-To: <CAJfpeguFkDDhz7+70pSUv_j=xY5L08ESpaE+jER9vE5p+ZmfFw@mail.gmail.com>
On Tue, 2020-08-04 at 15:19 +0200, Miklos Szeredi wrote:
> On Tue, Aug 4, 2020 at 1:39 PM Ian Kent <raven@themaw.net> wrote:
> > On Mon, 2020-08-03 at 11:29 +0200, Miklos Szeredi wrote:
> > > On Thu, Jul 23, 2020 at 12:48 PM David Howells <
> > > dhowells@redhat.com>
> > > wrote:
> > >
> > > > > > __u32 topology_changes;
> > > > > > __u32 attr_changes;
> > > > > > __u32 aux_topology_changes;
> > > > >
> > > > > Being 32bit this introduces wraparound effects. Is that
> > > > > really
> > > > > worth it?
> > > >
> > > > You'd have to make 2 billion changes without whoever's
> > > > monitoring
> > > > getting a
> > > > chance to update their counters. But maybe it's not worth it
> > > > putting them
> > > > here. If you'd prefer, I can make the counters all 64-bit and
> > > > just
> > > > retrieve
> > > > them with fsinfo().
> > >
> > > Yes, I think that would be preferable.
> >
> > I think this is the source of the recommendation for removing the
> > change counters from the notification message, correct?
> >
> > While it looks like I may not need those counters for systemd
> > message
> > buffer overflow handling myself I think removing them from the
> > notification message isn't a sensible thing to do.
> >
> > If you need to detect missing messages, perhaps due to message
> > buffer
> > overflow, then you need change counters that are relevant to the
> > notification message itself. That's so the next time you get a
> > message
> > for that object you can be sure that change counter comparisons you
> > you make relate to object notifications you have processed.
>
> I don't quite get it. Change notification is just that: a
> notification. You need to know what object that notification
> relates
> to, to be able to retrieve the up to date attributes of said object.
>
> What happens if you get a change counter N in the notification
> message, then get a change counter N + 1 in the attribute retrieval?
> You know that another change happened, and you haven't yet processed
> the notification yet. So when the notification with N + 1 comes in,
> you can optimize away the attribute retrieve.
>
> Nice optimization, but it's optimizing a race condition, and I don't
> think that's warranted. I don't see any other use for the change
> counter in the notification message.
>
>
> > Yes, I know it isn't quite that simple, but tallying up what you
> > have
> > processed in the current batch of messages (or in multiple batches
> > of
> > messages if more than one read has been possible) to perform the
> > check
> > is a user space responsibility. And it simply can't be done if the
> > counters consistency is in question which it would be if you need
> > to
> > perform another system call to get it.
> >
> > It's way more useful to have these in the notification than
> > obtainable
> > via fsinfo() IMHO.
>
> What is it useful for?
Only to verify that you have seen all the notifications.
If you have to grab that info with a separate call then the count
isn't necessarily consistent because other notifications can occur
while you grab it.
My per-object rant isn't quite right, what's needed is a consistent
way to verify you have seen everything you were supposed to.
I think your point is that if you grab the info in another call and
it doesn't match you need to refresh and that's fine but I think it's
better to be able to verify you have got everything that was sent as
you go and avoid the need for the refresh more often.
>
> If the notification itself would contain the list of updated
> attributes and their new values, then yes, this would make sense. If
> the notification just tells us that the object was modified, but not
> the modifications themselves, then I don't see how the change counter
> in itself could add any information (other than optimizing the race
> condition above).
>
> Thanks,
> Miklos
>
> Thanks,
>
>
>
> > > > > > n->watch.info & NOTIFY_MOUNT_IS_RECURSIVE if true
> > > > > > indicates that
> > > > > > the notifcation was generated by an event (eg.
> > > > > > SETATTR)
> > > > > > that was
> > > > > > applied recursively. The notification is only
> > > > > > generated for the
> > > > > > object that initially triggered it.
> > > > >
> > > > > Unused in this patchset. Please don't add things to the API
> > > > > which are not
> > > > > used.
> > > >
> > > > Christian Brauner has patches for mount_setattr() that will
> > > > need to
> > > > use this.
> > >
> > > Fine, then that patch can add the flag.
> > >
> > > Thanks,
> > > Miklos
next prev parent reply other threads:[~2020-08-05 1:54 UTC|newest]
Thread overview: 45+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-03-18 15:03 [PATCH 00/17] pipe: Keyrings, mount and superblock notifications [ver #5] David Howells
2020-03-18 15:03 ` [PATCH 01/17] uapi: General notification queue definitions " David Howells
2020-03-18 15:03 ` [PATCH 02/17] security: Add hooks to rule on setting a watch " David Howells
2020-03-18 18:56 ` James Morris
2020-03-18 15:03 ` [PATCH 03/17] security: Add a hook for the point of notification insertion " David Howells
2020-03-18 18:57 ` James Morris
2020-03-18 15:03 ` [PATCH 04/17] pipe: Add O_NOTIFICATION_PIPE " David Howells
2020-03-18 15:03 ` [PATCH 05/17] pipe: Add general notification queue support " David Howells
2020-03-18 15:04 ` [PATCH 06/17] watch_queue: Add a key/keyring notification facility " David Howells
2020-03-18 19:04 ` James Morris
2020-03-18 15:04 ` [PATCH 07/17] Add sample notification program " David Howells
2020-03-18 15:04 ` [PATCH 08/17] pipe: Allow buffers to be marked read-whole-or-error for notifications " David Howells
2020-03-18 15:04 ` [PATCH 09/17] pipe: Add notification lossage handling " David Howells
2020-03-18 15:04 ` [PATCH 10/17] selinux: Implement the watch_key security hook " David Howells
2020-03-18 19:06 ` James Morris
2020-03-18 15:04 ` [PATCH 11/17] smack: Implement the watch_key and post_notification hooks " David Howells
2020-03-18 15:05 ` [PATCH 12/17] watch_queue: Add security hooks to rule on setting mount and sb watches " David Howells
2020-03-18 19:07 ` James Morris
2020-03-18 15:05 ` [PATCH 13/17] watch_queue: Implement mount topology and attribute change notifications " David Howells
2020-04-02 15:19 ` Miklos Szeredi
2020-06-14 3:07 ` Ian Kent
2020-06-15 8:44 ` Miklos Szeredi
2020-07-23 10:48 ` David Howells
2020-08-03 9:29 ` Miklos Szeredi
2020-08-04 11:38 ` Ian Kent
2020-08-04 13:19 ` Miklos Szeredi
2020-08-05 1:53 ` Ian Kent [this message]
2020-08-05 7:43 ` Miklos Szeredi
2020-08-05 11:36 ` Ian Kent
2020-08-05 11:56 ` Miklos Szeredi
2020-07-24 10:19 ` David Howells
2020-07-24 10:44 ` Ian Kent
2020-07-24 11:36 ` David Howells
2020-08-03 10:02 ` Miklos Szeredi
2020-08-03 10:08 ` David Howells
2020-08-03 10:18 ` David Howells
2020-08-03 11:17 ` Miklos Szeredi
2020-08-03 11:49 ` David Howells
2020-08-03 12:01 ` Ian Kent
2020-08-03 12:31 ` David Howells
2020-08-03 14:30 ` Ian Kent
2020-03-18 15:05 ` [PATCH 14/17] watch_queue: sample: Display mount tree " David Howells
2020-03-18 15:05 ` [PATCH 15/17] watch_queue: Introduce a non-repeating system-unique superblock ID " David Howells
2020-03-18 15:05 ` [PATCH 16/17] watch_queue: Add superblock notifications " David Howells
2020-03-18 15:05 ` [PATCH 17/17] watch_queue: sample: Display " David Howells
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