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Re: [Libvir] Remote patch, 2007/02/19

Daniel P. Berrange wrote:
On Mon, Feb 19, 2007 at 03:30:25PM +0000, Richard W.M. Jones wrote:
Just for everyone's information.


I'm really not at all a fan of the magic cookie being passed around all the
time on the wire


I understand that SunRPC is at its core a stateless protocol, because it is
intended to be able to run apps over both unreliable datagram & reliable
stream sockets. For libvirt's purposes though I don't see us ever caring
about running over anything other than TCP / UNIX domain sockets which are
stateful and reliable.

Right, and the other thing to observe is that in fact SunRPC over TCP / Unix domain sockets doesn't automatically reconnect anyway, as I once thought it did. We shouldn't try to do manual reconnection because that has all sorts of other interesting ways to fail and in any case would make the code really too complicated. [Gory details and test code in case anyone is interested: http://et.redhat.com/~rjones/sunrpc_reconnection/ ]

With that in mind I'd venture to suggest we ditch the whole idea of cookies

Every method on the server end is already given a
     'struct svc_req *req'

This struct contains a field

     ' SVCXPRT *rq_xprt;'

Which represents the data transport of the client. And the SVCXPRT struct
has as its first member the '  int xp_sock' which is the socket associated
with the client.
So we can trivially & securely map from a client's TCP connetion to the
virConnectPtr without needing any magic cookies.

What concerns me here is that xp_sock is just a file descriptor and fds can be reused. It is also an fd that could be any of:
 * a TCPv6 socket
 * a TCPv4 socket
 * a Unix domain socket
* on the client side, a socketpair (which on Linux is a funny type of Unix domain socket) So finding something unique about it may be tricky. What happens if two clients connect in succession over the local Unix domain socket?

I need to think about this some more, so watch this space ...

Also worth noting is that cookies may represent other server-side objects, in particular domains and networks. We can have multiple domains per connection. The relationship between networks and connections is complicated (and I don't pretend to understand it at the moment either). I will be thinking about this too ...


Emerging Technologies, Red Hat  http://et.redhat.com/~rjones/
64 Baker Street, London, W1U 7DF     Mobile: +44 7866 314 421
 "[Negative numbers] darken the very whole doctrines of the equations
 and make dark of the things which are in their nature excessively
 obvious and simple" (Francis Maseres FRS, mathematician, 1759)

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