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

Mon Dec 15 19:52:20 UTC 2014

Quoting Barry Jaspan (barry.jaspan at acquia.com):
> On Mon, Dec 15, 2014 at 2:19 PM, Serge Hallyn <serge.hallyn at ubuntu.com>
> wrote:
> >
> > Since you are using / as the container rootfs, /empty is the same
> > inode in the container and on the host.
> 
> 
> No, it isn't. Within the container, /empty is the inode of the host's
> /other_file, because /other_file is bind-mounted over /empty:
> 
> lxc.mount.entry=/other_file empty none rw,bind 0 0

/other_file is bind-mounted *over* /empty.  So /empty is busy.

> host# ls -li /empty /other_file
> 57876 -rw-r--r-- 1 root root 0 Dec 15 19:26 /empty
> 58108 -rw-r--r-- 1 root root 0 Dec 15 19:26 /other_file
> host# lxc-execute -n test -f test.config -- ls -li /empty /other_file
> 58108 -rw-r--r-- 1 root root 0 Dec 15 19:26 /empty
> 58108 -rw-r--r-- 1 root root 0 Dec 15 19:26 /other_file
> 
> I would expect the host's /other_file to be busy, but not the host's /empty.
> 
> Barry
> 
> 
> 
> 
> 
> 
> 
> > 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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> >
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> 
> 
> 
> -- 
> 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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