Re: [PATCH 0/5] Return fds from privileged sockhash/sockmap lookup

On Sat, 14 Mar 2020 at 02:58, Alexei Starovoitov
[off-list ref] wrote:

I'm not following. There is skb->sk. Why do you need to lookup sk ? Because
your hook is before demux and skb->sk is not set? Then move your hook to after?

I think we're arguing in circles because in this thread I haven't seen the
explanation of the problem you're trying to solve. We argued about your
proposed solution and got stuck. Can we restart from the beginning with all
details?

Yes, that's a good idea. I mentioned this in passing in my cover
letter, but should
have provided more context.

Jakub is working on a patch series to add a BPF hook to socket dispatch [1] aka
the inet_lookup function. The core idea is to control skb->sk via a BPF program.
Hence, we can't use skb->sk.

Introducing this hook poses another problem: we need to get the struct sk from
somewhere. The canonical way in BPF is to use the lookup_sk helpers. Of course
that doesn't work, since our hook would invoke itself. So we need a
data structure
that can hold sockets, to be used by programs attached on the new hook.

Jakub's RFC patch set used REUSEPORT_SOCKARRAY for this. During LPC '19
we got feedback that sockmap is probably the better choice. As a
result, Jakub started
working on extending sockmap TCP support and after a while I joined to add UDP.

Now, we are looking at what our control plane could look like. Based
on the inet-tool
work that Marek Majkowski has done [2], we currently have the following set up:

* An LPM map that goes from IP prefix and port to an index in a sockmap
* A sockmap that holds sockets
* A BPF program that performs the business logic

inet-tool is used to update the two maps to add and remove mappings on the fly.
Essentially, services donate their sockets either via fork+exec or SCM_RIGHTS on
a Unix socket.

Once we have inserted a socket in the sockmap, it's not possible to
retrieve it again.
This makes it impossible to change the position of a socket in the
map, to resize the
map, etc. with our current design.

One way to work around this is to add a persistent component to our
control plane:
a process can hold on to the sockets and re-build the map when necessary. The
downsides are that upgrading the service is non-trivial (since we need
to pass the
socket fds) and that a failure of this service is catastrophic. Once
it happens, we
probably have to reboot the machine to get it into a workable state again.

We'd like to avoid a persistent service if we can. By allowing to look
up fds from the
sockmap, we could make this part of our control plane more robust.

1: https://www.youtube.com/watch?v=qRDoUpqvYjY
2: https://github.com/majek/inet-tool

I hope this explanation helps, sorry for not being more thorough in the original
cover letter!

Lorenz

-- 
Lorenz Bauer  |  Systems Engineer
6th Floor, County Hall/The Riverside Building, SE1 7PB, UK

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