Operation not permitted
It started when dpkg could not upgrade the util-linux package because the file /usr/bin/delpart could not be symlinked. So I tried to delete the file.
sudo rm /usr/bin/delpart
rm: cannot remove `/usr/bin/delpart': Operation not permitted
All operations on the file failed. I tried mv, fsck, reboot into rescue, etc.
So I googled “linux ext4 ‘Operation not permitted”. This did not help much, but I noticed a link about ext2 extended attributes. I have never used extended attributes, so I did a quick read of the man page for lsattr and chattr.
cd /usr/bin
sudo lsattr delpart
---D-a-----tT-- delpart
That was a strange set of attributes. So I compared to another random file.
sudo lsattr zip
-------------e- zip
Once the problem is found, the solution is straightforward
sudo chattr +e -DatT delpart
sudo lsattr delpart
-------------e- delpart
The question now is how the file got to this state. I can only speculate that a fsck run “repaired” this corrupted file to this strange but consistent state. I wonder if there are other surprises waiting for me on this disk.