Re: [PATCH] prctl: propagate has_child_subreaper flag to every descendant

[Pavel Tikhomirov <ptikhomirov@virtuozzo.com>]

Add to cc Lennart Poettering <lennart@poettering.net>

On 01/23/2017 02:55 PM, Oleg Nesterov wrote:
> On 01/22, Pavel Tikhomirov wrote:
>>
>>>
>>> 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.
>
> This is clear, but this can only happen if 2 forks 3 and after that
> sets is_child_subreaper, right?
>
> And if someone actually does this then your patch can break this
> application, no?
>
> IOW. Currently CRIU can't restore the process tree with the same
> has_child_subreaper bits if some process forks before
> prctl(PR_SET_CHILD_SUBREAPER). It restores the tree as if prctl()
> was called before the 1st fork.
>
> So you change the semantics of PR_SET_CHILD_SUBREAPER and now CRIU
> is fine simply because you remove this feature: the sub-reaper can
> no longer pre-fork the children which should reparent to the previous
> reaper.
>
> I won't really argure, but I am not sure this is good idea...

If one task uses these feature now it must be very carefull: if some our 
ancestor have enabled is_child_subreaper somewhere up the tree, forked 
our tree and after that disabled is_child_subreaper, so we already have 
has-flag and all children will inherit has-flag irrelevant to what is 
our order of fork/prctl-ing to become subreaper.

And only one way to check if the task has no has_child_subreaper flag is 
to create some childs kill them and see to where they will reparent, but 
I doubt that someone is doing these now.

> At least I think this should be clearly documented.

Yes, I surely need to add some documentation here. Thanks for mentioning 
that! - Will do.

>
>>> 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.)
>
> I'll send a patch, probably a generic helper makes sense.
>
> Btw task_is_descendant() looks wrong at first glance.
>
> Oleg.
>

-- 
Best regards, Tikhomirov Pavel
Software Developer, Virtuozzo.