On Tue, Mar 30, 2021 at 04:54:02PM +0300, Amir Goldstein wrote:

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

quoted

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.

Sorry, that was a typo. I meant to write, both 2. and 3. should get a
creation event but not 1.

quoted

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.

quoted

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?

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