Re: [GIT PULL] kdbus for 4.1-rc1

Rik van Riel
2015-04-15T21:08:08Z

On 04/15/2015 04:22 PM, Andy Lutomirski wrote:
> On Wed, Apr 15, 2015 at 9:44 AM, Havoc Pennington <hp@pobox.com> wrote:
>> On Wed, Apr 15, 2015 at 12:00 PM, Rik van Riel <riel@redhat.com> wrote:
>>> On 04/15/2015 07:06 AM, One Thousand Gnomes wrote:
>>>
>>>>> that anyone here does either. In the many years I've spent working on
>>>>> this, dbus has seemed to be odd, and strange, to the way that the kernel
>>>>> has normally worked, because it is. And that's not a bad thing, it's
>>>>> just different, and for us to support real needs and requirements of our
>>>>> users, is the requirement of the Linux kernel.
>>>>
>>>> There are I think a set of intertwined problems here
>>>>
>>>> - An efficient delivery system for multicast messages delivered locally
>>>> (be that MPI, dbus whatever - it's not "dbus or nothing")
>>>>
>>>> - A kernel side dynamic namespace to describe what goes where
>>>>
>>>> - A kernel side security model to describe who may receive what, and
>>>> which additional information/tags/cred info
>>>>
>>>> - Something that provides state to stuff that needs it (and probably
>>>> belongs in userspace - dbus name service etc)
>>>>
>>>> - Something that maps dbus and other models onto the kernel security
>>>> model (and we have tools like EBPF which are very powerful)
>>>>
>>>> - Something that maps the kernel layer onto models like MPI-3
>>
>> When trying to split apart problems, for dbus it's important to keep
>> ordering guarantees.
>>
>> That is, with dbus if I send a broadcast message, then send a unicast
>> request to another client, then drop the connection causing the bus to
>> broadcast that I've dropped; then the other client will see those
>> things in that order - the broadcast, then the request, and then that
>> I've dropped the connection.
>
> This leads me to a potentially interesting question: where's the
> buffering? If there's a bus with lots of untrusted clients and one of
> them broadcasts data faster than all receivers can process it, where
> does it go?
>
> At least with a userspace solution, it's clear what the OOM killer
> should kill when this happens. Unless it's PID 1. Sigh.

It may be useful to do the buffering (and general interception
of any message that cannot be delivered) in a userspace program.

Not only to get the buffers out of the kernel and into swappable
memory, but also so people could re-use the same infrastructure
for things like cluster communication (or communication between
different containers) - the userspace daemons could take care of
routing messages to and from the outside.

They could also be useful to keep some of the policy stuff
outside of the kernel, if only to ensure that the kernel side
policy is not set in stone, and people can do things differently
in the future if they want to.