Re: [patch 1/2] fork_connector: add a fork connector

[Jay Lan <jlan@engr.sgi.com>]

Evgeniy Polyakov wrote:
> On Tue, 22 Mar 2005 10:26:19 -0800
> Ram <linuxram@us.ibm.com> wrote:
>
>
> > On Mon, 2005-03-21 at 23:07, Guillaume Thouvenin wrote:
> >
> > > On Mon, 2005-03-21 at 12:52 -0800, Ram wrote:
> > >
> > > >     If a bunch of applications are listening for fork events, 
> > > >     your patch allows any application to turn off the 
> > > >     fork event notification?  Is this the right behavior?
> > >
> > > Yes it is. The main management is done by application so, if several
> > > applications are listening for fork events you need to choose which one
> > > will turn off the fork connector. 
> > >
> > > I want to keep this turn on/off mechanism simple but if it's needed I
> > > can manage the variable "cn_fork_enable" as a counter. Thus the callback
> > > could be something like:
> > >
> > > static void cn_fork_callback(void *data)
> > > {
> > >  int start; 
> > >  struct cn_msg *msg = (struct cn_msg *)data;
> > >
> > >  if (cn_already_initialized && (msg->len == sizeof(cn_fork_enable))) {
> > >    memcpy(&start, msg->data, sizeof(cn_fork_enable));
> > >    if (start)
> > >      cn_fork_enable++;
> > >    else
> > >      cn_fork_enable > 0 ? cn_fork_enable-- : 0;
> > >  }
> > > }
> >
> > I think a better way is:
> >
> >   Providing a different connector channel called the administrator 
> >   channel which can be used only by a super-user, and gives you
> >   the ability to switch on or off any connector channel including the
> >   fork-connector channel.
>
>
> Only super-user can bind netlink socket to multicast group.
>
>
> >   For lack of better term I am using the word 'channel' to mean
> >   something that carries events of particular type through the
> >   connector-infrastructure.
>
>
> I still do not see why it is needed.
> Super-user can run ip command and turn network interface off
> not waiting while apache or named exits or unbind.

You can turn off a network interface and turn it back on without
closing a network application and it will continue to work.

> In theory I can create some kind of userspace registration mechanism,
> when userspace application reports it's pid to the connector, 
> and then it sends data to the specified pids, but does not 
> allow controlling from userspace.
> But I really do not think it is a good idea to permit
> non-priviledged userspace processes to know about deep
> kernel internals through connector's messages.

I see this issue less a case of bad guys vs. good guys. I see it
as various components doing system related work, but there is
no mechanism of knowing who is on who is off by today's patch. A
service listening to the fork connector can request to turn off
cn_fork_enable on exit and inadquately affect other services/daemons
listening to the same connector. It is not acceptable in my opinion.

The idea of implementing fork connector enabling/disabling was so
that the kernel does not waste time writing to the sockets if no
application listening. If implementing such a feature costs
more than it saves, maybe the fork connector should simply always
send?

Thanks,
- jay


> > RP
> >
> >
> >
> > > What do you think about this implementation? 
> > >
> > > Guillaume
>
>
>
> 	Evgeniy Polyakov
>
> Only failure makes us experts. -- Theo de Raadt

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