From mboxrd@z Thu Jan  1 00:00:00 1970
From: Al Viro <viro@ZenIV.linux.org.uk>
Subject: Re: How do I make a clean mount namespace?
Date: Thu, 24 Apr 2014 03:39:25 +0100
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To: Andy Lutomirski <luto@amacapital.net>
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On Tue, Apr 22, 2014 at 03:12:11PM -0700, Andy Lutomirski wrote:
> I want to set up a little container.  So I unshare the mount namespace
> and mount something somewhere (say /mnt) that I want to be my new
> root.  Now what?
> 
> pivot_root("/mnt", "/mnt/garbage") seems to frequently return -EBUSY.

RTFM.  Literally - man 2 pivot_root and look for the only place where
it mentions EBUSY.

If you get that error, check what you've got in /proc/mounts (in the
namespace your process is in, obviously) just before the syscall.
With these arguments you really want /mnt to be a mountpoint.  If your
new root really lives on the same fs as the old one, just do
mount --bind /mnt /mnt before any other mounts.