netdev - Re: [PATCH 2/2] net: Implement SO_PASSCGROUP to enable passing cgroup path

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Date:	Wed, 16 Apr 2014 11:40:44 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	Simo Sorce <ssorce@...hat.com>, David Miller <davem@...emloft.net>,
	Tejun Heo <tj@...nel.org>, Daniel Walsh <dwalsh@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	lpoetter@...hat.com, cgroups@...r.kernel.org, kay@...hat.com,
	Network Development <netdev@...r.kernel.org>
Subject: Re: [PATCH 2/2] net: Implement SO_PASSCGROUP to enable passing cgroup path

On Wed, Apr 16, 2014 at 11:36 AM, Vivek Goyal <vgoyal@...hat.com> wrote:
> On Wed, Apr 16, 2014 at 10:29:08AM -0700, Andy Lutomirski wrote:
>
> [..]
>> >> Admittedly cgroups aren't currently as important as uid, but if this
>> >> changes, then SO_PASSCGROUP, as currently written, will have *exactly*
>> >> the same problem.
>> >
>> > Which is easy to foil by using SO_PEERCGROUP and find out who originally
>> > opened the socket, which is why that is also available!
>>
>> Then please remove SO_PASSCGROUP.
>
> SO_PASSCGROUP is important because SO_PEERCGROUP does not work with unix
> datagram sockets.

Right.  I forgot about that.

>
> Again going back to logging example, if some clients are logging to unix
> datagram sockets, SO_PASSCGROUP is the only option to figure out cgroup
> of client.

Hmm.  I think that, in your patch, the cgroup that is sent is the
cgroup of the caller of write/send/sendmsg.  What if you changed it to
use the same cgroup that SO_PEERCRED would use?  Would that still
work?

>>
>> I still haven't seen any explanation for what's wrong with requiring
>> senders to ask the kernel to transmit their cgroup.
>
> If nothing else, additional complexity and ovhead. Extra pair of messages
> need to be exchanged to request and then provide the information.
>
> How would it work in logging example? Every time logger receives a
> message, is it supposed to send another message to client to send
> SCM_CGROUP? That does not sound right.

No -- just have the logger send the cgroup with every message.  Yes,
it seems silly, but it's probably barely more expensive than with the
code in your patch.

--Andy
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