lxc-attach コマンドメモ

lxc 0.9.0 以降は userns 対応もあって lxc-attach コマンドが大幅に書き直されている．こんな仕組みみたい．

	/* For the cgroup attaching logic to work in conjunction with pid and user namespaces,
	 * we need to have the following hierarchy:
	 *
	 *     lxc-attach [process executed externally]
	 *         | socketpair(cgroup_ipc_sockets)
	 *         | fork()           -> child
	 *         |                       | setns()
	 *         |                       | fork()    -> grandchild
	 *         |                       |                   | initialize
	 *         |                       |                   | signal parent
	 *         |                       |<------------------|----+
	 *         |                       | signal parent     |
	 *         |<----------------------|-----+             |
	 *         | add to cgroups        |                   |
	 *         | signal child -------->|                   |
	 *         |                       | signal child ---->|
	 *         | waitpid()             | waitpid()         | exec()
	 *         |                       |<------------------| exit()
	 *         |<----------------------| exit()
	 *         | exit()
	 *
	 * The rationale is the following: The first parent is needed because after
	 * setns() (mount + user namespace) we can't access the cgroup filesystem
	 * to add the pid to the corresponding cgroup. Therefore, we need to do that
	 * in a process executed on the host, so that's why we need to fork and wait
	 * for it to have done some initialization (cgroups may restrict certain
	 * operations so we have to do that in the end) and use IPC for signaling.
	 *
	 * Then in the child process we do the setns(). However, a process is never
	 * really attached to a pid namespace (never changes its pid, doesn't appear
	 * in the pid namespace /proc), only child processes of that process are
	 * truely inside the new pid namespace. That's why we need to fork() again
	 * after setns() before performing final initializations, then signal our
	 * parent, which signals the primary process, which does cgroup adding,
	 * which then signals to the grandchild that it can exec().
	 */