On Tue, Mar 30, 2021 at 3:53 PM Christian Brauner
[off-list ref] wrote:

On Tue, Mar 30, 2021 at 03:33:23PM +0300, Amir Goldstein wrote:

quoted

On Tue, Mar 30, 2021 at 3:12 PM Christian Brauner
[off-list ref] wrote:

quoted

On Sun, Mar 28, 2021 at 06:56:24PM +0300, Amir Goldstein wrote:

quoted

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.

quoted

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.