Re: [RFC][PATCH] fanotify: allow setting FAN_CREATE in mount mark mask

[Amir Goldstein <amir73il@gmail.com>]

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.

> But with your proposal would both 1. and 2. still get a creation event?
>

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.

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.

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?
>

<shrug> :)

FMODE_NONOTIFY is there not because there was a requirement
not to send events, but because the path of the internal file is
"fake", so it has a weird looking path. After all there are many
other events that would be sent (not on open files).

At least I think that's the reason...

Thanks,
Amir.