* https://www.stgraber.org/2014/01/17/lxc-1-0-unprivileged-containers/

上記の非特権コンテナの要件に

  * Kernel: 3.13 + a couple of staging patches (which Ubuntu has in its kernel)
  * User namespaces enabled in the kernel
  * A very recent version of shadow that supports subuid/subgid
  * Per-user cgroups on all controllers (which I turned on a couple of weeks ago)
  * LXC 1.0 beta2 or higher (released two days ago)
  * A version of PAM with a loginuid patch that’s yet to be in any released version

とある．

  * まず lxc/src/lxc/cgroup.c で<code c>
static bool find_cgroup_hierarchies(struct cgroup_meta_data *meta_data,
        bool all_kernel_subsystems, bool all_named_subsystems,
        const char **subsystem_whitelist)
{
        FILE *proc_self_cgroup;
        char *line = NULL;
        size_t sz = 0;
        int r;
        bool bret = false;
        size_t hierarchy_capacity = 0;

        proc_self_cgroup = fopen_cloexec("/proc/self/cgroup", "r");
        /* if for some reason (because of setns() and pid namespace for example)
,
         * /proc/self is not valid, we try /proc/1/cgroup... */
        if (!proc_self_cgroup)
                proc_self_cgroup = fopen_cloexec("/proc/1/cgroup", "r");
        if (!proc_self_cgroup)
                return false;

        while (getline(&line, &sz, proc_self_cgroup) != -1) {
                /* file format: hierarchy:subsystems:group,
                 * we only extract hierarchy and subsystems
                 * here */
</code>こんなコードがあり，/proc/self/cgroup から「現在の cgroup」を取得するようになってる
  * Ubuntu 14.04 だと一般ユーザでログインすると<code>karma@lxctest02:~$ id
uid=1000(karma) gid=1000(karma) groups=1000(karma),27(sudo)
karma@lxctest02:~$ cat /proc/self/cgroup 
11:name=systemd:/user/1000.user/5.session
10:hugetlb:/user/1000.user/5.session
9:perf_event:/user/1000.user/5.session
8:blkio:/user/1000.user/5.session
7:freezer:/user/1000.user/5.session
6:devices:/user/1000.user/5.session
5:memory:/user/1000.user/5.session
4:cpuacct:/user/1000.user/5.session
3:cpu:/user/1000.user/5.session
2:cpuset:/user/1000.user/5.session</code>
  * ここは書き込み権がある <code>karma@lxctest02:~$ LANG=C ls -ld /sys/fs/cgroup/cpu/user/1000.user/5.session
drwxr-xr-x 2 karma karma 0 Jan 20 10:37 /sys/fs/cgroup/cpu/user/1000.user/5.session</code>
  * ちなみに普通に echo $$ > tasks みたいに cgroup に登録する．以下のコード <code>#!/bin/bash
for c in /sys/fs/cgroup/*
do
  sudo mkdir $c/karma
  sudo chown -R karma: $c/karma
  if [ `basename $c` = "cpuset" ]; then
    echo 0 > $c/karma/cpuset.cpus
    echo 0 > $c/karma/cpuset.mems
  fi
  echo $PID > $c/karma/tasks
done</code> を実行すると，当然 <code>karma@lxctest02:~$ cat /proc/self/cgroup 
11:name=systemd:/karma
10:hugetlb:/karma
9:perf_event:/karma
8:blkio:/karma
7:freezer:/karma
6:devices:/karma
5:memory:/karma
4:cpuacct:/karma
3:cpu:/karma
2:cpuset:/karma</code>
  * /sys/fs/cgroup/user/1000.user/5.session みたいなのは誰が作ってるのか? どうも systemd のデーモンのどれかっぽい．(pam_loginuid 使って?) <code c>int cg_path_get_session(const char *path, char **session) {
        const char *e, *n;
        char *s;

        assert(path);
        assert(session);

        e = path_startswith(path, "/user/");
        if (!e)
                return -ENOENT;

        /* Skip the user name */
        e = skip_label(e);
        if (!e)
                return -ENOENT;

        n = strchrnul(e, '/');
        if (n - e < 8)
                return -ENOENT;
        if (memcmp(n - 8, ".session", 8) != 0)
                return -ENOENT;

        s = strndup(e, n - e - 8);
        if (!s)
                return -ENOMEM;

        *session = s;
        return 0;
}</code> systemd/src/shared/cgroup-util.c