[lxc-users] Cannot unlink/rename over host-level file that has been bind-mounted out of a container

[Serge Hallyn]

Since you are using / as the container rootfs, /empty is the same
inode in the container and on the host.  It is over-mounted.  So it
is busy.

Quoting Barry Jaspan (barry.jaspan at acquia.com):
> I have reduced the problem I described in my previous message to a much
> smaller test case. Here is an LXC config file that bind-mounts a single
> file, /other_file, from the host's root filesystem on top of another file,
> /empty, also from the host's root filesystem:
> 
> lxc.console = none
> lxc.rootfs = /
> lxc.mount.entry=/other_file empty none rw,bind 0 0
> 
> While the container is running, the host's /empty cannot be unlinked:
> 
> host# rm /empty /other_file
> host# touch /empty /other_file
> host# lxc-execute -n test -f test.config -- sleep 30 &
> [1] 2419
> host# strace -o rm.out rm /empty
> rm: cannot remove `/empty': Device or resource busy
> host# grep EBUSY rm.out
> unlinkat(AT_FDCWD, "/empty", 0)         = -1 EBUSY (Device or resource busy)
> host# fg
> lxc-execute -n test -f test.config -- sleep 30
> ^C
> host# rm /empty
> host#
> 
> Can anyone explain why the host's /empty cannot be unlinked even though it
> is bind-mounted out of the running container? Why is it "busy"?
> 
> Thanks,
> Barry
> 
> 
> 
> 
> On Mon, Dec 8, 2014 at 5:53 PM, Barry Jaspan <barry.jaspan at acquia.com>
> wrote:
> >
> > I'm using LXC on Ubuntu 12.04 (Precise) on EC2. I am creating a container
> > that uses the host's root filesystem. In the lxc config, I am bind-mounting
> > a different passwd file on top of /etc/passwd:
> >
> > lxc.mount.entry=/container/passwd etc/passwd none rw,bind 0 0
> >
> > As expected, /etc/passwd on the host and /etc/passwd in the container are
> > different inodes:
> >
> > host# ls -i /etc/passwd
> > 58046 /etc/passwd
> > host# ls -i /container/passwd
> > 287145 /container/passwd
> >
> > container# ls -i /etc/passwd
> > 287145 /etc/passwd
> >
> > What I did not expect is that now it is not possible to unlink /etc/passwd
> > at the host level (or inside the container, although I don't care about
> > that):
> >
> > host# rm /etc/passwd
> > rm: cannot remove `/etc/passwd': Device or resource busy
> > host#  chsh -s /bin/false root
> > chsh: failure while writing changes to /etc/passwd
> >
> > strace confirms that unlinkat() and rename() are returning EBUSY. (chsh
> > creates a replacement for /etc/passwd and then rename()'s it into place.)
> >
> > So, somehow, the fact that the host's /etc/passwd is *not* present in the
> > container marks its inode as busy. I do not think it is as simple as "an
> > inode that has been bind-mounted out of a namespace is busy" because a
> > simple bind-mount test case does not reproduce it:
> >
> > # echo foo > foo
> > # touch bar
> > # mount --bind ./foo ./bar
> > # cat bar
> > foo
> > # touch new
> > # mv new foo
> > #
> >
> > This seems related to http://lwn.net/Articles/570338/. However, in that
> > article, a file which is bind-mounted into another namespace causes EBUSY
> > from unlink/rename(). In my case, a file which is *not* bind-mounted into
> > another namespace is causing EBUSY.
> >
> > Can someone explain what is going on?
> >
> > Thanks,
> >
> > Barry
> >
> > --
> > Barry Jaspan
> > Senior Architect | Acquia <http://acquia.com>
> > barry.jaspan at acquia.com | (c) 617.905.2208 | (w) 781-313-8298
> >
> > Acquia Dev Cloud: You build killer websites. We do the rest.
> > <http://www.acquia.com/dev-cloud> <http://acquia.com/dev-cloud>
> > Acquia ranked #1 Software Vendor on the 2012 Inc 500
> > <http://www.acquia.com/about-us/newsroom/press-releases/inc-magazine-unveils-31st-annual-list-america-s-fastest-growing>
> >
> >
> 
> -- 
> Barry Jaspan
> Senior Architect | Acquia <http://acquia.com>
> barry.jaspan at acquia.com | (c) 617.905.2208 | (w) 781-313-8298
> 
> Acquia Dev Cloud: You build killer websites. We do the rest.
> <http://www.acquia.com/dev-cloud> <http://acquia.com/dev-cloud>
> Acquia ranked #1 Software Vendor on the 2012 Inc 500
> <http://www.acquia.com/about-us/newsroom/press-releases/inc-magazine-unveils-31st-annual-list-america-s-fastest-growing>

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