From: Christian Brauner <christian.brauner@ubuntu.com>
To: Amir Goldstein <amir73il@gmail.com>
Cc: Jan Kara <jack@suse.cz>,
linux-fsdevel <linux-fsdevel@vger.kernel.org>,
Linux API <linux-api@vger.kernel.org>,
Miklos Szeredi <miklos@szeredi.hu>
Subject: Re: [RFC][PATCH] fanotify: allow setting FAN_CREATE in mount mark mask
Date: Tue, 30 Mar 2021 16:17:03 +0200 [thread overview]
Message-ID: <20210330141703.lkttbuflr5z5ia7f@wittgenstein> (raw)
In-Reply-To: <CAOQ4uxjPhrY55kJLUr-=2+S4HOqF0qKAAX27h2T1H1uOnxM9pQ@mail.gmail.com>
On Tue, Mar 30, 2021 at 04:54:02PM +0300, Amir Goldstein wrote:
> On Tue, Mar 30, 2021 at 3:53 PM Christian Brauner
> <christian.brauner@ubuntu.com> wrote:
> >
> > On Tue, Mar 30, 2021 at 03:33:23PM +0300, Amir Goldstein wrote:
> > > On Tue, Mar 30, 2021 at 3:12 PM Christian Brauner
> > > <christian.brauner@ubuntu.com> wrote:
> > > >
> > > > On Sun, Mar 28, 2021 at 06:56:24PM +0300, Amir Goldstein wrote:
> > > > > Add a high level hook fsnotify_path_create() which is called from
> > > > > syscall context where mount context is available, so that FAN_CREATE
> > > > > event can be added to a mount mark mask.
> > > > >
> > > > > This high level hook is called in addition to fsnotify_create(),
> > > > > fsnotify_mkdir() and fsnotify_link() hooks in vfs helpers where the mount
> > > > > context is not available.
> > > > >
> > > > > In the context where fsnotify_path_create() will be called, a dentry flag
> > > > > flag is set on the new dentry the suppress the FS_CREATE event in the vfs
> > > > > level hooks.
> > > >
> > > > Ok, just to make sure this scheme would also work for overlay-style
> > > > filesystems like ecryptfs where you possible generate two notify events:
> > > > - in the ecryptfs layer
> > > > - in the lower fs layer
> > > > at least when you set a regular inode watch.
> > > >
> > > > If you set a mount watch you ideally would generate two events in both
> > > > layers too, right? But afaict that wouldn't work.
> > > >
> > > > Say, someone creates a new link in ecryptfs the DENTRY_PATH_CREATE
> > > > flag will be set on the new ecryptfs dentry and so no notify event will
> > > > be generated for the ecryptfs layer again. Then ecryptfs calls
> > > > vfs_link() to create a new dentry in the lower layer. The new dentry in
> > > > the lower layer won't have DCACHE_PATH_CREATE set. Ok, that makes sense.
> > > >
> > > > But since vfs_link() doesn't have access to the mnt context itself you
> > > > can't generate a notify event for the mount associated with the lower
> > > > fs. This would cause people who a FAN_MARK_MOUNT watch on that lower fs
> > > > mount to not get notified about creation events going through the
> > > > ecryptfs layer. Is that right? Seems like this could be a problem.
> > > >
> > >
> > > Not sure I follow what the problem might be.
> > >
> > > FAN_MARK_MOUNT subscribes to get only events that were
> > > generated via that vfsmount - that has been that way forever.
> > >
> > > A listener may subscribe to (say) FAN_CREATE on a certain
> > > mount AND also also on a specific parent directory.
> > >
> > > If the listener is watching the entire ecryptfs mount and the
> > > specific lower directory where said vfs_link() happens, both
> > > events will be reported. One from fsnotify_create_path() and
> > > the lower from fsnotify_create().
> > >
> > > If one listener is watching the ecryptfs mount and another
> > > listener is watching the specific ecryptfs directory, both
> > > listeners will get a single event each. They will both get
> > > the event that is emitted from fsnotify_path_create().
> > >
> > > Besides I am not sure about ecryptfs, but overlayfs uses
> > > private mount clone for accessing lower layer, so by definition
> >
> > I know. That's why I was using ecryptfs as an example which doesn't do
> > that (And I think it should be switched tbh.). It simply uses
> > kern_path() and then stashes that path.
> >
> > My example probably would be something like:
> >
> > mount -t ext4 /dev/sdb /A
> >
> > 1. FAN_MARK_MOUNT(/A)
> >
> > mount --bind /A /B
> >
> > 2. FAN_MARK_MOUNT(/B)
> >
> > mount -t ecryptfs /B /C
> >
> > 3. FAN_MARK_MOUNT(/C)
> >
> > let's say I now do
> >
> > touch /C/bla
> >
> > I may be way off here but intuitively it seems both 1. and 2. should get
> > a creation event but not 3., right?
> >
>
> Why not 3?
> You explicitly set a mark on /C requesting to be notified when
> objects are created via /C.
Sorry, that was a typo. I meant to write, both 2. and 3. should get a
creation event but not 1.
>
> > But with your proposal would both 1. and 2. still get a creation event?
> >
Same obvious typo. The correct question would be: with your proposal do
2. and 3. both get an event?
Because it feels like they both should since /C is mounted on top of /B
and ecryptfs acts as a shim. Both FAN_MARK_MOUNT(/B) and
FAN_MARK_MOUNT(/C) should get a creation event after all both will have
mnt->mnt_fsnotify_marks set.
>
> They would not get an event, because fsnotify() looks for CREATE event
> subscribers on inode->i_fsnotify_marks and inode->i_sb_s_fsnotify_marks
> and does not find any.
Well yes, but my example has FAN_MARK_MOUNT(/B) set. So fanotify
_should_ look at
(!mnt || !mnt->mnt_fsnotify_marks) &&
and see that there are subscribers and should notify the subscribers in
/B even if the file is created through /C.
My point is with your solution this can't be handled and I want to make
sure that this is ok. Because right now you'd not be notified about a
new file having been created in /B even though mnt->mnt_fsnotify_marks
is set and the creation went through /B via /C.
_Unless_ we switch to an argument like overlayfs and say "This is a
private mount which is opaque and so we don't need to generate events.".
Overlayfs handles this cleanly due to clone_private_mount() which will
shed all mnt->mnt_fsnotify_marks and ecryptfs should too if that is the
argument we follow, no?
>
> The vfs_create() -> fsnotify_create() hook passes data_type inode to
> fsnotify() so there is no fsnotify_data_path() to extract mnt event
> subscribers from.
Right, that was my point. You don't have the mnt context for the
underlying fs at a time when e.g. call vfs_link() which ultimately calls
fsnotify_create/link() which I'm saying might be a problem.
>
> The same fate would be to files created by overlayfs, nfsd and cachefiles.
>
> Only the create event on /C/bla from the syscall context would
> call fsnoity_path_create() and result with path data in fsnotify(), so
> the mnt event subscriber would be found.
>
> > > users cannot watch the underlying overlayfs operations using
> > > a mount mark. Also, overlayfs suppresses fsnotify events on
> > > underlying files intentionally with FMODE_NONOTIFY.
> >
> > Probably ecryptfs should too?
It really feels like ecryptfs should do clone_private_mnt() and probably
cachefiles too. I mentioned this to David just a few weeks ago actually.
Christian
next prev parent reply other threads:[~2021-03-30 14:18 UTC|newest]
Thread overview: 61+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-03-28 15:56 [RFC][PATCH] fanotify: allow setting FAN_CREATE in mount mark mask Amir Goldstein
2021-03-30 7:31 ` Christian Brauner
2021-03-30 9:31 ` Amir Goldstein
2021-03-30 16:24 ` Amir Goldstein
2021-03-31 10:08 ` Christian Brauner
2021-03-31 10:57 ` Amir Goldstein
2021-04-08 11:44 ` open_by_handle_at() in userns Amir Goldstein
2021-04-08 12:55 ` Christian Brauner
2021-04-08 14:15 ` J. Bruce Fields
2021-04-08 15:54 ` Amir Goldstein
2021-04-08 16:08 ` J. Bruce Fields
2021-04-08 16:48 ` Frank Filz
2021-04-08 15:34 ` Amir Goldstein
2021-04-08 15:41 ` Christian Brauner
2021-03-30 12:12 ` [RFC][PATCH] fanotify: allow setting FAN_CREATE in mount mark mask Christian Brauner
2021-03-30 12:33 ` Amir Goldstein
2021-03-30 12:53 ` Christian Brauner
2021-03-30 12:55 ` Christian Brauner
2021-03-30 13:54 ` Amir Goldstein
2021-03-30 14:17 ` Christian Brauner [this message]
2021-03-30 14:56 ` Amir Goldstein
2021-03-31 9:46 ` Christian Brauner
2021-03-31 11:29 ` Amir Goldstein
2021-03-31 12:17 ` Christian Brauner
2021-03-31 12:59 ` Amir Goldstein
2021-03-31 12:54 ` Jan Kara
2021-03-31 14:06 ` Amir Goldstein
2021-03-31 20:59 ` fsnotify path hooks Amir Goldstein
2021-04-01 10:29 ` Jan Kara
2021-04-01 14:18 ` Amir Goldstein
2021-04-02 8:20 ` Amir Goldstein
2021-04-04 10:27 ` LSM and setxattr helpers Amir Goldstein
2021-04-05 12:23 ` Christian Brauner
2021-04-05 14:47 ` Mimi Zohar
2021-04-06 15:43 ` Amir Goldstein
2021-04-05 16:18 ` Casey Schaufler
2021-04-06 8:35 ` fsnotify path hooks Jan Kara
2021-04-06 18:49 ` Amir Goldstein
2021-04-08 12:52 ` Jan Kara
2021-04-08 15:11 ` Amir Goldstein
2021-04-09 10:08 ` Jan Kara
2021-04-09 10:45 ` Christian Brauner
2021-04-20 6:01 ` Amir Goldstein
2021-04-20 11:41 ` Christian Brauner
2021-04-20 11:58 ` Amir Goldstein
2021-04-20 13:38 ` Christian Brauner
2021-04-09 13:22 ` Amir Goldstein
2021-04-09 14:30 ` Al Viro
2021-04-09 14:39 ` Christian Brauner
2021-04-09 14:46 ` Al Viro
2021-04-09 15:20 ` Christian Brauner
2021-04-09 16:06 ` Amir Goldstein
2021-04-09 16:09 ` Amir Goldstein
2021-04-18 18:51 ` Amir Goldstein
2021-04-19 8:08 ` Amir Goldstein
2021-04-19 16:41 ` Amir Goldstein
2021-04-19 17:02 ` Al Viro
2021-04-19 22:04 ` Amir Goldstein
2021-04-20 7:53 ` Amir Goldstein
2021-03-31 13:06 ` [RFC][PATCH] fanotify: allow setting FAN_CREATE in mount mark mask J. Bruce Fields
2021-03-30 12:20 ` Amir Goldstein
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