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From: Kirill Tkhai <ktkhai@virtuozzo.com>
To: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Oleg Nesterov <oleg@redhat.com>, <serge@hallyn.com>,
	<agruenba@redhat.com>, <linux-api@vger.kernel.org>,
	<linux-kernel@vger.kernel.org>, <paul@paul-moore.com>,
	<viro@zeniv.linux.org.uk>, <avagin@openvz.org>,
	<linux-fsdevel@vger.kernel.org>, <mtk.manpages@gmail.com>,
	<akpm@linux-foundation.org>, <luto@amacapital.net>,
	<gorcunov@openvz.org>, <mingo@kernel.org>,
	<keescook@chromium.org>
Subject: Re: [PATCH 2/2] pid_ns: Introduce ioctl to set vector of ns_last_pid's on ns hierarhy
Date: Wed, 3 May 2017 13:20:58 +0300	[thread overview]
Message-ID: <db243b0d-6fca-88d5-4a3f-2ef179fe1291@virtuozzo.com> (raw)
In-Reply-To: <8737cngdxi.fsf@xmission.com>

On 03.05.2017 00:13, Eric W. Biederman wrote:
> Kirill Tkhai <ktkhai@virtuozzo.com> writes:
> 
>> On 02.05.2017 19:33, Oleg Nesterov wrote:
>>> sorry for delay, vacation...
>>>
>>> On 04/28, Kirill Tkhai wrote:
>>>>
>>>> On 27.04.2017 19:22, Oleg Nesterov wrote:
>>>>>
>>>>> Ah, OK, I didn't notice the ns->child_reaper check in pidns_for_children_get().
>>>>>
>>>>> But note that it doesn't need tasklist_lock too.
>>>>
>>>> Hm, are there possible strange situations with memory ordering, when we see
>>>> ns->child_reaper of already died ns, which was placed in the same memory?
>>>> Do we have to use some memory barriers here?
>>>
>>> Could you spell please? I don't understand your concerns...
>>>
>>> I don't see how, say,
>>>
>>> 	static struct ns_common *pidns_for_children_get(struct task_struct *task)
>>> 	{
>>> 		struct ns_common *ns = NULL;
>>> 		struct pid_namespace *pid_ns;
>>>
>>> 		task_lock(task);
>>> 		if (task->nsproxy) {
>>> 			pid_ns = task->nsproxy->pid_ns_for_children;
>>> 			if (pid_ns->child_reaper) {
>                             ^^^^^^^^^^^^^^^^^^^^
>                             Oleg my apologies I missed this line earlier.
>                             This does look like a valid way to skip read_lock(&tasklist_lock);
>>> 				ns = &pid_ns->ns;
>>> 				get_pid_ns(ns);
>                                 ^^^^^^^^^^^^^ This needs to be:
>                                 get_pid_ns(pid_ns);
>                                 
>>> 			}
>>> 		}
>>> 		task_unlock(task);
>>>
>>> 		return ns;
>>> 	}
>>>
>>> can be wrong. It also looks more clean to me.
>>>
>>> ->child_reaper is not stable without tasklist, it can be dead/etc, but
>>> we do not care?
>>
>> I mean the following. We had a pid_ns1 with a child_reaper set. Then
>> it became dead, and a new pid_ns2 were allocated in the same memory.
> 
> task->nsproxy->pid_ns_for_children is always changed with
> task_lock(task) held.  See switch_task_namespaces (used by unshare and
> setns).  This also gives us the guarantee that the pid_ns reference
> won't be freed/reused in any for until task_lock(task) is dropped.

Now I've checked kmem_cache_zalloc() and it looks like it zeroes cache memory
content synchronous on allocation (it seems there is no pre-zeroed memory
for GFP_ZERO cases).

So, the zeroing happens before switch_task_namespaces() (and task_unlock())
and we're really safe after task_lock() in pidns_for_children_get().

Ok, I'll send new version of the patchset.

  reply	other threads:[~2017-05-03 10:21 UTC|newest]

Thread overview: 23+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-04-17 17:34 [PATCH 0/2] nsfs: Introduce ioctl to set vector of ns_last_pid's on pid ns hierarhy Kirill Tkhai
2017-04-17 17:36 ` [PATCH 1/2] nsfs: Add namespace-specific ioctl (NS_SPECIFIC_IOC) Kirill Tkhai
2017-04-17 17:36 ` [PATCH 2/2] pid_ns: Introduce ioctl to set vector of ns_last_pid's on ns hierarhy Kirill Tkhai
2017-04-19 20:27   ` Serge E. Hallyn
2017-04-24 19:03   ` Cyrill Gorcunov
2017-04-26 15:53   ` Oleg Nesterov
2017-04-26 16:11     ` Kirill Tkhai
2017-04-26 16:33       ` Kirill Tkhai
2017-04-26 16:32         ` Eric W. Biederman
2017-04-26 16:43           ` Kirill Tkhai
2017-04-26 17:01             ` Eric W. Biederman
2017-04-27 16:12       ` Oleg Nesterov
2017-04-27 16:17         ` Kirill Tkhai
2017-04-27 16:22           ` Oleg Nesterov
2017-04-28  9:17             ` Kirill Tkhai
2017-05-02 16:33               ` Oleg Nesterov
2017-05-02 17:22                 ` Eric W. Biederman
2017-05-02 17:33                 ` Kirill Tkhai
2017-05-02 21:13                   ` Eric W. Biederman
2017-05-03 10:20                     ` Kirill Tkhai [this message]
2017-04-27 16:39           ` Eric W. Biederman
2017-04-28  9:22             ` Kirill Tkhai
2017-04-27 16:16       ` Oleg Nesterov

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