Hello,

I accidentally make /mnt folder having no owner and group like this:
d????????? ? ? ? ? ? mnt

How to make it a normal folder as before with owner and group of root?

Thanks!
Crane

Likely, as the root user...

Tried, but it doesn't work. Here is the result:

crane@Ubuntu2204:~/yocto-pi$ sudo chown root:root /mnt

[sudo] password for crane:

chown: cannot access '/mnt': Protocol error

Something inaccessible probably got mounted there. Try: df /mnt; grep mnt /proc/mounts

Can you?: sudo rm /mnt; sudo umount -f /mnt

1 members found this post helpful.

you cannot add owner and group to anything, everything has owner and password always. You can only modify it. ??? means the information is not available, the device is not accessible.
Would be nice to know what is it, how is it mounted and if there was any error message in your logs about it.
How did you "produce" it at all?

Here are the results after running these commands:
crane@Ubuntu2204:~/yocto-pi$ df /mnt
df: /mnt: Protocol error
crane@Ubuntu2204:~/yocto-pi$ grep mnt /proc/mounts
nsfs /run/snapd/ns/snapd-desktop-integration.mnt nsfs rw 0 0
nsfs /run/snapd/ns/snap-store.mnt nsfs rw 0 0
vmshare /mnt vboxsf rw,nodev,relatime,iocharset=utf8,uid=0,gid=999,dmode=0770,fmode=0770,tag=VBoxAutomounter 0 0
crane@Ubuntu2204:~/yocto-pi$ sudo rm -r /mnt
rm: cannot remove '/mnt': Is a directory
crane@Ubuntu2204:~/yocto-pi$ sudo umount -f /mnt
crane@Ubuntu2204:~/yocto-pi$

Yes, it works to delete the foler /mnt using sudo umount -f /mnt.
Then I set up the shared folder with the mounting point /mnt again and it works to create the folder /mnt again.
The issue is addressed.

Yes, it seems not accessible any more.

Here are what's in the log:
crane@Ubuntu2204:~/yocto-pi$ sudo dmesg | grep mnt
[sudo] password for crane:
[ 5365.468560] 03:06:24.126084 automount vbsvcAutomounterMountIt: Successfully mounted 'vmshare' on '/mnt'
[ 5666.238214] 03:11:24.895731 automount vbsvcAutomounterMountIt: Successfully mounted 'vmshare' on '/mnt/'
[ 5895.652601] 03:15:14.310132 automount Error: vbsvcAutomounterMountIt: Failed to create mount path '/mnt/vmshare' for share 'vmshare': VERR_NET_PROTOCOL_ERROR
[ 5895.653087] 03:15:14.310630 automount Error: vbsvcAutomounterMountIt: Failed to create mount path '/mnt/vmshare' for share 'vmshare': VERR_NET_PROTOCOL_ERROR
[ 5895.653783] 03:15:14.311332 automount Error: vbsvcAutomounterMountIt: Failed to create mount path '/mnt/vmshare' for share 'vmshare': VERR_NET_PROTOCOL_ERROR
[ 5895.653933] 03:15:14.311487 automount Error: vbsvcAutomounterMountIt: Failed to create mount path '/mnt/vmshare' for share 'vmshare': VERR_NET_PROTOCOL_ERROR
crane@Ubuntu2204:~/yocto-pi$

This is produced when I play around with shared folder on VirtualBox. I don't know exactly what happened. Probably after I change the mount point back and forth for the shared folder in VirtualBox settings.

Thank all of you for your help!

nice. In that case I would suggest you to mark the thread as solved!
Don't forget, the mount point itself is a directory, but what you mounted is a device. In your case you removed the mounted device without umount-ing it, therefore the mount pointed to something invalid.
You need to keep the order: umount, rename/move mount point, mount again. Do not attempt to move a mounted device (or you will get an error like that).

1 members found this post helpful.

Ok got it. Thanks for the summary!

This time, it happens again, but not with a mount point.

This is how it happens. I found the folders in a folder are all executable. I don't like these folders with x attribute, so I tried to make the change.

crane@Ubuntu2204:~/farview$ ls -ltr kmodule/
total 20
drwxrwxr-x 2 crane crane 4096 Sep 5 17:06 start
drwxrwxr-x 2 crane crane 4096 Sep 6 13:41 simple
drwxrwxr-x 2 crane crane 4096 Sep 7 14:17 simple-rw
drwxrwxr-x 2 crane crane 4096 Sep 7 14:44 memory
drwxrwxr-x 2 crane crane 4096 Sep 10 00:58 devspi
crane@Ubuntu2204:~/farview$ sudo chmod -x -R kmodule/
[sudo] password for crane:
crane@Ubuntu2204:~/farview$ ls -ltr kmodule/
ls: cannot access 'kmodule/devspi': Permission denied
ls: cannot access 'kmodule/simple-rw': Permission denied
ls: cannot access 'kmodule/start': Permission denied
ls: cannot access 'kmodule/memory': Permission denied
ls: cannot access 'kmodule/simple': Permission denied
total 0
d????????? ? ? ? ? ? start
d????????? ? ? ? ? ? simple-rw
d????????? ? ? ? ? ? simple
d????????? ? ? ? ? ? memory
d????????? ? ? ? ? ? devspi
crane@Ubuntu2204:~/farview$

I am able to use "chmod +x -R" to change the attribute back to be executable and then they are accessible.

But I have no idea why chmod causes this result. Any idea?

The 'x' attribute on directories is not 'executable', it is 'traversable'. A directory cannot be executed, only traversed.

When you remove the 'x', or the traversability you can no longer see into its meta-data or see what is in it! Hence, d????????? ? ? ? ? ? start as it knows it is a directory and gets its name from the parent's meta-data, but can see nothing else.

Directories should always be traversable for those users who need to see inside them.

1 members found this post helpful.

The 'x' attribute on directories is not 'executable', it is 'traversable'. A directory cannot be executed, only traversed.

When you remove the 'x', or the traversability you can no longer see into its meta-data or see what is in it! Hence, d????????? ? ? ? ? ? start as it knows it is a directory and gets its name from the parent's meta-data, but can see nothing else.

Directories should always be traversable for those users who need to see inside them.

1 members found this post helpful.

Thanks astrogeek! I realize that. I change the files in the folder first. Then once I change the folders' attribute, it happens.

just one comment: "x" is usually not enough, you need to add "r" too. Without r the directory (or file) is unreadable, therefore you will not [be able to] see the content of it.
additionally we have more flags (like X, s, t), but I think that is another story. see for example here: https://linuxize.com/post/chmod-command-in-linux/

Thanks pan64 for sharing. Good to know.