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@xxxxxxxxxx> 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@xxxxxxxxxx> 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
