linux-kernel - Re: [PATCH v2 2/5] clone: add CLONE

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Date:   Thu, 18 Apr 2019 16:15:07 +0200
From:   Christian Brauner <christian@...uner.io>
To:     Oleg Nesterov <oleg@...hat.com>
CC:     torvalds@...ux-foundation.org, viro@...iv.linux.org.uk,
        jannh@...gle.com, dhowells@...hat.com, linux-api@...r.kernel.org,
        linux-kernel@...r.kernel.org, serge@...lyn.com, luto@...nel.org,
        arnd@...db.de, ebiederm@...ssion.com, keescook@...omium.org,
        tglx@...utronix.de, mtk.manpages@...il.com,
        akpm@...ux-foundation.org, cyphar@...har.com,
        joel@...lfernandes.org, dancol@...gle.com
Subject: Re: [PATCH v2 2/5] clone: add CLONE_PIDFD

On April 18, 2019 4:10:20 PM GMT+02:00, Oleg Nesterov <oleg@...hat.com> wrote:
>On 04/18, Christian Brauner wrote:
>>
>> On Thu, Apr 18, 2019 at 03:12:07PM +0200, Oleg Nesterov wrote:
>> > Should we allow CLONE_THREAD | CLONE_PIDFD ?
>>
>> I think so, yes. I have thought about this.
>
>OK, I won't insist. But let me explain why did I ask.
>
>> Yes, due to CLONE_FILES |
>> CLONE_VM you'd necessarily hand the pidfd to the child but threads
>are
>> no security boundary in the first place.
>
>No, no, I am not not worried about security. CLONE_PARENT | CLONE_PIDFD
>looks more problematic to me, but I see nothing dangerous
>security-wise..
>
>I agree that CLONE_THREAD | CLONE_PIDFD may be usefule, but I am not
>sure
>we should allow this from the very begining, until we have a "real"
>use-case.
>
>IIUC, we are going to make it pollable soon. OK, but
>proc_tgid_base_poll()
>(which should be turned into pidfd_poll) simply can't work if
>pid_task() is
>not a group leader. poll(pidfd) will hang forever if pidfd was created
>by
>CLONE_THREAD | CLONE_PIDFD.
>
>Sure, we can (should?) improve pidfd_poll() but this will need more
>nasty
>changes in the core kernel code. Do we really need/want this? Right now
>it
>is not clear to me. Instead, we can simply disallow
>CLONE_THREAD|CLONE_PIDFD
>until we decide that yes, we want to poll sub-threads.

If you think that makes the polling work simpler for Joel then for sure.
And yes, I have argued for "disable until someone needs it" often before so I can't really argue the other way around here. :)
I'll send an updated version soon.

Christian

>
>But again, I am fine with CLONE_THREAD | CLONE_PIDFD.
>
>Oleg.