[lxc-users] Cannot unlink/rename over host-level file that has been bind-mounted out of a container
Barry Jaspan
barry.jaspan at acquia.com
Mon Dec 15 20:30:34 UTC 2014
On Mon, Dec 15, 2014 at 2:52 PM, Serge Hallyn <serge.hallyn at ubuntu.com>
wrote:
>
> /other_file is bind-mounted *over* /empty. So /empty is busy.
>
Okay, I get it. The inode which is the host's /empty is being used as a
mount-point. It makes sense that being a mount-point makes the inode busy.
My goal is to use the host's /etc inside the container, but to replace
/etc/passwd+group+shadow within the container with unique per-container
versions, while still allowing commands like useradd/chsh/etc, which unlink
/etc/passwd, to work. My current work-around is to create a hardlink farm
of /etc to another path and replace passwd+group+shadow with empty files:
cp -al /etc /etc_farm
rm /etc_farm/{passwd,group,shadow}
touch /etc_farm/{passwd,group,shadow}
I can then mount /etc_farm as the container's /etc, and mount a unique
per-container passwd+group+shadow over the container's /etc versions, and
the host's /etc/passwd+group+shadow are not EBUSY. This works fine. The
downside is that any changes to the host's /etc are not reflected in
/etc_farm unless I regenerate /etc_farm.
Does anyone have another suggested approach?
Barry
>
> > 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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>
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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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