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

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Date:   Thu, 18 Apr 2019 16:10:20 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Christian Brauner <christian@...uner.io>
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 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.

But again, I am fine with CLONE_THREAD | CLONE_PIDFD.

Oleg.