patchcontainer.sh

#!/bin/bash
# First, make sure that cgroups are mounted correctly.
CGROUP=/sys/fs/cgroup
: {LOG:=stdio}

[ -d $CGROUP ] || mkdir $CGROUP

mountpoint -q $CGROUP || 
    mount -n -t tmpfs -o uid=0,gid=0,mode=0755 cgroup $CGROUP || {
        echo "Could not make a tmpfs mount. Did you use -privileged?"
        exit 1
    }

if [ -d /sys/kernel/security ] && ! mountpoint -q /sys/kernel/security
then
    mount -t securityfs none /sys/kernel/security || {
        echo "Could not mount /sys/kernel/security."
        echo "AppArmor detection and -privileged mode might break."
    }
fi

# Mount the cgroup hierarchies exactly as they are in the parent system.
for SUBSYS in $(cut -d: -f2 /proc/1/cgroup)
do
        [ -d $CGROUP/$SUBSYS ] || mkdir $CGROUP/$SUBSYS
        mountpoint -q $CGROUP/$SUBSYS || 
                mount -n -t cgroup -o $SUBSYS cgroup $CGROUP/$SUBSYS

        # The two following sections address a bug which manifests itself
        # by a cryptic "lxc-start: no ns_cgroup option specified" when
        # trying to start containers withina container.
        # The bug seems to appear when the cgroup hierarchies are not
        # mounted on the exact same directories in the host, and in the
        # container.

        # Named, control-less cgroups are mounted with "-o name=foo"
        # (and appear as such under /proc/<pid>/cgroup) but are usually
        # mounted on a directory named "foo" (without the "name=" prefix).
        # Systemd and OpenRC (and possibly others) both create such a
        # cgroup. To avoid the aforementioned bug, we symlink "foo" to
        # "name=foo". This shouldn't have any adverse effect.
        echo $SUBSYS | grep -q ^name= && {
                NAME=$(echo $SUBSYS | sed s/^name=//)
                ln -s $SUBSYS $CGROUP/$NAME
        }

        # Likewise, on at least one system, it has been reported that
        # systemd would mount the CPU and CPU accounting controllers
        # (respectively "cpu" and "cpuacct") with "-o cpuacct,cpu"
        # but on a directory called "cpu,cpuacct" (note the inversion
        # in the order of the groups). This tries to work around it.
        [ $SUBSYS = cpuacct,cpu ] && ln -s $SUBSYS $CGROUP/cpu,cpuacct
done

# Note: as I write those lines, the LXC userland tools cannot setup
# a "sub-container" properly if the "devices" cgroup is not in its
# own hierarchy. Let's detect this and issue a warning.
grep -q :devices: /proc/1/cgroup ||
    echo "WARNING: the 'devices' cgroup should be in its own hierarchy."
grep -qw devices /proc/1/cgroup ||
    echo "WARNING: it looks like the 'devices' cgroup is not mounted."

# Now, close extraneous file descriptors.
pushd /proc/self/fd >/dev/null
for FD in *
do
    case "$FD" in
    # Keep stdin/stdout/stderr
    [012])
        ;;
    # Nuke everything else
    *)
        eval exec "$FD>&-"
        ;;
    esac
done
popd >/dev/null