Can I change devices permissions with chmod?

Can I change devices permissions with chmod?

• Before read: I don't want to use any udev rule. I need to change (programmatically) the permissions of some device. But first in order to underestand what I have to do (in code) I want to make the job using just chmod command. So far, I've tried this: root# ls -l /dev/sdb root# brw-rw-rw- 1 root disk 8, 16 Apr 7 05:27 /dev/sdb root# chmod 0600 /dev/sdb root# ls -l /dev/sdb root# brw------- 1 root disk 8, 16 Apr 7 05:27 /dev/sdb as you can see, /dev/sdb has read and write permissions only for the owner (wich is root). But I'm still able to create new files and read files from my connected flash drive. What I'm missing. How can I use chmod to prevent users to write to some device? Can be done what I'm asking for? Thank in advance.

• Answer:

  If I understand you right then there is some file system on /dev/sdb that you have mounted. What matters here are the permissions in the file system that resides on /dev/sdb, the permissions of /dev/sdb are completely irrelevant for your question. Except that with permissions 0666 anyone can bypass the access control mechanisms for that file system and access the content on the device arbitrarily, but this is a different issue. If you want to restrict access to the files within the file system, then you have to assign appropriate ownership and permissions to the files (beginning with the root of that file system). For file systems like FAT mount(8) lets you set the ownership and access permissions for all files within the file system. If you want to expose the entire tree to certain users only and hide it from all others, then mount it somewhere where only those users have access to. But note that any user can see that something is being mounted (mount(8) or df(1) will show them). chroot(1) is not going to help you at all.