From: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
To: Oleg Nesterov <oleg@redhat.com>
Cc: Ingo Molnar <mingo@redhat.com>,
Peter Zijlstra <peterz@infradead.org>,
Andrew Morton <akpm@linux-foundation.org>,
Cyrill Gorcunov <gorcunov@openvz.org>,
John Stultz <john.stultz@linaro.org>,
Thomas Gleixner <tglx@linutronix.de>,
Nicolas Pitre <nicolas.pitre@linaro.org>,
Michal Hocko <mhocko@suse.com>,
Stanislav Kinsburskiy <skinsbursky@virtuozzo.com>,
Mateusz Guzik <mguzik@redhat.com>, <linux-kernel@vger.kernel.org>,
Pavel Emelyanov <xemul@virtuozzo.com>,
Konstantin Khorenko <khorenko@virtuozzo.com>
Subject: Re: [PATCH] prctl: propagate has_child_subreaper flag to every descendant
Date: Sun, 22 Jan 2017 13:11:46 +0300 [thread overview]
Message-ID: <4908be49-d3c3-366d-0fd1-05249ef4ecef@virtuozzo.com> (raw)
In-Reply-To: <20170120181359.GA17205@redhat.com>
Sorry I had some problem with mail-agent, resend to be on the safe side.
On 01/20/2017 09:14 PM, Oleg Nesterov wrote:
> On 01/19, Pavel Tikhomirov wrote:
>>
>> Having these two
>> differently behaving groups can lead to confusion. Also it is
>> a problem for CRIU, as when we restore process tree we need to
>> somehow determine which descendants belong to which group and
>> much harder - to put them exactly to these group.
>
> Hmm. could you explain how this change helps CRIU? I mean, why
> restorer can't do prctl(CHILD_SUBREAPER) before the first fork?
Imagine we have these tree in pidns:
1: has_child_subreaper == 0 && is_child_subreaper == 0
|-2: has_child_subreaper == 0 && is_child_subreaper == 1
| |-3: has_child_subreaper == 0 && is_child_subreaper == 0
| | |-5: has_child_subreaper == 0 && is_child_subreaper == 0
| |-4: has_child_subreaper == 1 && is_child_subreaper == 0
| | |-6: has_child_subreaper == 1 && is_child_subreaper == 0
before c/r: If 4 dies 6 will reparent to 2, if 3 dies 5 will reparent to 1.
after c/r: (where restorer had is_child_subreaper == 1, everybody in the
tree will have has_child_subreaper == 1) Everybody will reparent to 2.
>
> Anyway, afaics the patch is sub-optimal and not correct...
>
>> --- a/include/linux/sched.h
>> +++ b/include/linux/sched.h
>> @@ -1715,6 +1715,8 @@ struct task_struct {
>> struct signal_struct *signal;
>> struct sighand_struct *sighand;
>>
>> + struct list_head csr_descendant;
>> +
>
> You don't need this new member and descendants_lock. task_struct has
> the ->real_parent pointer so you can work the tree without recursion.
Sorry I don't get how I can walk down the tree of all descendants with
help of ->real_parent pointer, can you please point on some example or
explain a bit more? (I see task_is_descendant() in
security/yama/yama_lsm.c but we will need to check it for every process,
not only descendants, the latter can be a lot faster.)
>
>> +static void prctl_set_child_subreaper(struct task_struct *reaper, bool arg2)
>> +{
>> + LIST_HEAD(descendants);
>> +
>> + reaper->signal->is_child_subreaper = arg2;
>> + if (!arg2)
>> + return;
>> +
>> + spin_lock(&descendants_lock);
>> + read_lock(&tasklist_lock);
>> +
>> + list_add(&reaper->csr_descendant, &descendants);
>> +
>> + while (!list_empty(&descendants)) {
>> + struct task_struct *tsk;
>> + struct task_struct *p;
>> +
>> + tsk = list_first_entry(&descendants, struct task_struct,
>> + csr_descendant);
>> +
>> + list_for_each_entry(p, &tsk->children, sibling) {
>
> This is not enough. Every thread has its own ->children list, you need
> to walk the sub-threads as well.
Will do.
>
>> + * If we've found child_reaper - skip descendants in
>> + * it's subtree as they will never get out pidns
>> + */
>> + if (is_child_reaper(task_pid(p)))
>> + continue;
>
> Again, a child reaper can be multi-threaded, this check can be false
> negative.
>
> Probably is_child_reaper() should be renamed somehow and a new helper
> makes sense... something like
Will do.
>
> bool task_is_child_reaper(struct task_struct *p)
> {
> return same_thread_group(p, task_active_pid_ns(p)->child_reaper);
> }
>
> Oleg.
>
--
Best regards, Tikhomirov Pavel
Software Developer, Virtuozzo.
next prev parent reply other threads:[~2017-01-22 10:12 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-01-19 16:43 [PATCH] prctl: propagate has_child_subreaper flag to every descendant Pavel Tikhomirov
2017-01-20 18:14 ` Oleg Nesterov
2017-01-22 10:00 ` Pavel Tikhomirov
2017-01-22 10:11 ` Pavel Tikhomirov [this message]
2017-01-23 11:55 ` Oleg Nesterov
2017-01-23 12:52 ` task_is_descendant() cleanup Oleg Nesterov
2017-01-25 21:59 ` Kees Cook
2017-01-30 13:49 ` Oleg Nesterov
2017-01-23 14:30 ` [PATCH] prctl: propagate has_child_subreaper flag to every descendant Pavel Tikhomirov
2017-01-23 16:06 ` Oleg Nesterov
2017-01-23 11:57 ` [PATCH] introduce the walk_process_tree() helper Oleg Nesterov
2017-01-23 12:07 ` Oleg Nesterov
2017-01-24 15:01 ` Pavel Tikhomirov
2017-01-23 16:44 ` setns() && PR_SET_CHILD_SUBREAPER Oleg Nesterov
2017-01-23 18:21 ` Eric W. Biederman
2017-01-24 14:07 ` Oleg Nesterov
2017-01-24 15:24 ` Eric W. Biederman
2017-01-30 18:16 ` Oleg Nesterov
2017-01-30 18:17 ` [PATCH] exit: fix the setns() && PR_SET_CHILD_SUBREAPER interaction Oleg Nesterov
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