util-linux-2.27.1 - umount -a -O _netdev problem, _netdev ignored

[Zoltan <zoltan@z2zcorp.com>]

Hello,

There is a problem in the code of util-linux-2.27.1, in the umount binary.

The command

umount -a -O _netdev

does _not_ umount anything, despite an /etc/fstab with the likes of

...
alpha:/var/mail       /var/mail      nfs _netdev,soft,tcp
...

The "mount -a -O _netdev" command works as expected.  However, the
/run/mount/utab files is empty, and I was unable to get the "umount -a -O
_netdev" to work as advertised.  Running it with "--verbose" flag merely
confirmed that everything was being ": ignored".

I have a physical /etc/mtab (not symlink), and the "_netdev" option is missing
from /etc/mtab file, but that's probably expected???

If I manually add it (_netdev) to /etc/mtab, then the umount -a -O _netdev
does work.  But /run/mount/utab is empty.  Trying to step through the code
gets pretty brutal, and the *fs population is still hidden inside some
macro...and I can never find where the /run/mount/utab is even being parsed in
the umount code-path.

This is with a root-over-NFS setup, in case that matters.  Probably shouldn't.
 Kernel is 4.3.0.

Is this  a regression?  Seems there were issues before...wondering if it
re-surfaced.

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