[Devel] Re: [PATCH 14/15] Destroy pid namespace on init's death

[Pavel Emelyanov]

Oleg Nesterov wrote:
> On 07/26, Pavel Emelyanov wrote:
>> @@ -895,6 +915,7 @@ fastcall NORET_TYPE void do_exit(long co
>> {
>> 	struct task_struct *tsk = current;
>> 	int group_dead;
>> +	struct pid_namespace *pid_ns = tsk->nsproxy->pid_ns;
>>
>> 	profile_task_exit(tsk);
>>
>> @@ -905,9 +926,10 @@ fastcall NORET_TYPE void do_exit(long co
>> 	if (unlikely(!tsk->pid))
>> 		panic("Attempted to kill the idle task!");
>> 	if (unlikely(tsk == task_child_reaper(tsk))) {
>> -		if (task_active_pid_ns(tsk) != &init_pid_ns)
>> -			task_active_pid_ns(tsk)->child_reaper =
>> -					init_pid_ns.child_reaper;
>> +		if (pid_ns != &init_pid_ns) {
>> +			zap_pid_ns_processes(pid_ns);
>> +			pid_ns->child_reaper = init_pid_ns.child_reaper;
>> +		}
>> 		else
>> 			panic("Attempted to kill init!");
> 
> No, no, this is wrong. Yes, the current code is buggy too, I'll send
> the fix.
> 
> I think this code should be moved below under the "if (group_dead)",
> and we should use tsk->group_leader.
> 
>> +void zap_pid_ns_processes(struct pid_namespace *pid_ns)
>> +{
>> +	int i;
>> +	int nr;
>> +	int nfree;
>> +	int options = WNOHANG|WEXITED|__WALL;
>> +
>> +repeat:
>> +	/*
>> +	 * We know pid == 1 is terminating. Find remaining pid_ts
>> +	 * in the namespace, signal them and then wait for them
>> +	 * exit.
>> +	 */
>> +	nr = next_pidmap(pid_ns, 1);
>> +	while (nr > 0) {
>> +		kill_proc_info(SIGKILL, SEND_SIG_PRIV, nr);
>> +		nr = next_pidmap(pid_ns, nr);
>> +	}
>> +
>> +	nr = next_pidmap(pid_ns, 1);
>> +	while (nr > 0) {
>> +		do_wait(nr, options, NULL, NULL, NULL);
> 
> When the first child of init exits, it sends SIGCHLD. After that,
> do_wait() will never sleep, so we are doing a busy-wait loop.
> Not good, especially when we have a niced child, can livelock.
> 
>> +		nr = next_pidmap(pid_ns, nr);
>> +	}
>> +
>> +	nfree = 0;
>> +	for (i = 0; i < PIDMAP_ENTRIES; i++)
>> +		nfree += atomic_read(&pid_ns->pidmap[i].nr_free);
>> +
>> +	/*
>> +	 * If pidmap has entries for processes other than 0 and 1, retry.
>> +	 */
>> +	if (nfree < (BITS_PER_PAGE * PIDMAP_ENTRIES - 2))
>> +		goto repeat;
> 
> This doesn't look right.
> 
> Suppose that some "struct pid" was pinned from the parent namespace.
> In that case zap_pid_ns_processes() will burn CPU until put_pid(), bad.

Nope. struct pid can be pinned, but the pidmap "fingerprint" cannot.
So as soon as the release_task() is called the pidmap becomes free and
we can proceed.

However I agree with the "burn CPU" issue - wait must sleep if needed.

> I think we can rely on forget_original_child() and do something like
> this:
> 
> 	zap_active_ns_processes(void)
> 	{
> 		// kill all tasks in our ns and below
> 		kill(-1, SIGKILL);

That would be too slow to walk through all the tasks in a node searching
for a couple of them we need. fing_ge_pid() looks better to me.

> 		do {
> 			clear_thread_flag(TIF_SIGPENDING);
> 		} while (wait(NULL) != -ECHLD);
> 	}
> 
> Oleg.
> 
> 

Thanks,
Pavel