can't umount /mnt/sdb1: Operation not permitted

[hiro]

Then you would still be able to use something like mount -o users, no?
At least that's my interpretation after many years of mounting stuff.

[tinypoodle]

Is your issue with the permissions of 'mount' or with the mechanism permissions of busybox applets are handled?

[hiro]

you're right tinypoodle, I mixed these two issues up.

[roberts]

I have the same problem with umount.  I have found the cause of the issue:  spurious group write permission on /etc/busybox.conf

~$ ls -l /etc/busybox.conf
-rw--w----    1 root     staff          140 Mar  4  2010 /etc/busybox.conf
~$ umount /mnt/sda3
umount: can't umount /mnt/sda3: Operation not permitted
~$ sudo chmod g-w /etc/busybox.conf
~$ ls -l /etc/busybox.conf
-rw-------    1 root     staff          140 Mar  4  2010 /etc/busybox.conf
~$ umount /mnt/sda3
~$

I propose to correct this in next version

+1 on the Good Catch.
Applied to 4.7.5