linux-kernel - Re: [PATCH v2 1/2] fork: extend clone3() to support CLONE_SET

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Message-ID: <20190802132419.GD20111@redhat.com>
Date:   Fri, 2 Aug 2019 15:24:20 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Adrian Reber <areber@...hat.com>
Cc:     Christian Brauner <christian@...uner.io>,
        Eric Biederman <ebiederm@...ssion.com>,
        Pavel Emelianov <xemul@...tuozzo.com>,
        Jann Horn <jannh@...gle.com>,
        Dmitry Safonov <0x7f454c46@...il.com>,
        linux-kernel@...r.kernel.org, Andrei Vagin <avagin@...il.com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Radostin Stoyanov <rstoyanov1@...il.com>
Subject: Re: [PATCH v2 1/2] fork: extend clone3() to support CLONE_SET_TID

On 08/02, Oleg Nesterov wrote:
>
> On 08/02, Adrian Reber wrote:
> >
> > On Wed, Jul 31, 2019 at 07:41:36PM +0200, Oleg Nesterov wrote:
> > > But the main question is how it can really help if ns->level > 0, unlikely
> > > CRIU will ever need to clone the process with the same pid_nr == set_tid
> > > in the ns->parent chain.
> >
> > Not sure I understand what you mean. For CRIU only the PID in the PID
> > namespace is relevant.
>
> If it runs "inside" this namespace. But in this case alloc_pid() should
> use nr == set_tid only once in the main loop, when i == ns->level.

and I just noticed that your patch does exactly this, it clears set_tid
after the 1st usage when i == ns->level.

> > > So may be kernel_clone_args->set_tid should be pid_t __user *set_tid_array?
> > > Or I missed something ?
> >
> > Not sure why and how an array would be needed. Could you give me some
> > more details why you think this is needed.
>
> IIURC, criu can restore the process tree along with nested pid namespaces.
>
> how can this patch help in this case?
>
> But again, perhaps I missed something. I forgot everything about criu.

Yes... I guess in this case it can "safely" modify ns_last_pid in the child
namespaces it needs to create.

So Adrian, sorry for confusion, I think your patch is fine.

Oleg.