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

[David Miller <davem-fT/PcQaiUtIeIZ0/mPfg9Q@public.gmane.org>]

From: Vivek Goyal <vgoyal-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
Date: Tue, 15 Apr 2014 20:20:10 -0400

> On Tue, Apr 15, 2014 at 02:53:13PM -0700, Andy Lutomirski wrote:
>> On Tue, Apr 15, 2014 at 2:15 PM, Vivek Goyal <vgoyal-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org> wrote:
>> > This patch implements socket option SO_PASSCGROUP along the lines of
>> > SO_PASSCRED.
>> >
>> > If SO_PASSCGROUP is set, then recvmsg() will get a control message
>> > SCM_CGROUP which will contain the cgroup path of sender. This cgroup
>> > belongs to first mounted hierarchy in the sytem.
>> >
>> > SCM_CGROUP control message can only be received and sender can not send
>> > a SCM_CGROUP message. Kernel automatically generates one if receiver
>> > chooses to receive one.
>> >
>> > This works both for unix stream and datagram sockets.
>> >
>> > cgroup information is passed only if either the sender or receiver has
>> > SO_PASSCGROUP option set. This means for existing workloads they should
>> > not see any significant performance impact of this change.
>> 
>> This is odd.  Shouldn't an SCM_CGROUP cmsg be generated when the
>> receiver has SO_PASSCGROUP set and the sender passes SCM_CGROUP to
>> sendmsg?
> 
> How can receiver trust the cgroup info generated by sender. It needs to
> be generated by kernel so that receiver can trust it.
> 
> And if receiver needs to know cgroup of sender, receiver can just set
> SO_PASSCGROUP on socket and receiver should get one SCM_CGROUP message
> with each message received.

I completely agree.