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Description of problem:
unsure(1) shows "--user" and "-U" option, while when trying the option, unsure doesn't work.

Version-Release number of selected component (if applicable):
util-linux-2.23.2-26.el7_2.2.x86_64

How reproducible:
Just run the unsure command as follows

Steps to Reproduce:
1.sudo yum install util-linux
2.sudo unshare -m -u -i -n -p -f --user --mount-proc bash


Actual results:
[pkger@dhcp debug]$ sudo unshare -m -u -i -n -p -f --user --mount-proc bash
unshare: unshare failed: Invalid argument
[pkger@dhcp debug]$ 

Expected results:
A new bash process in the new namespace

Additional info:

The user namespaces are "Technology Preview" and disabled by default, see kernel implementation https://bugzilla.redhat.com/show_bug.cgi?id=1138782

If I good remember you need to enable it on kernel command line
(something like grubby --args=user_namespace.enable=1 + reboot).