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

From: Lorenz Bauer <lmb@cloudflare.com>
To: John Fastabend <john.fastabend@gmail.com>
Cc: Alexei Starovoitov <alexei.starovoitov@gmail.com>,
	Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	kernel-team <kernel-team@cloudflare.com>,
	Networking <netdev@vger.kernel.org>, bpf <bpf@vger.kernel.org>
Subject: Re: [PATCH 0/5] Return fds from privileged sockhash/sockmap lookup
Date: Fri, 13 Mar 2020 11:03:26 +0000	[thread overview]
Message-ID: <CACAyw989zaAe2UDHcOPODbSo=WDPpQzD6kX1h5z_4xBfKP+oQg@mail.gmail.com> (raw)
In-Reply-To: <5e6a8e48240a9_6a322aac933b85c029@john-XPS-13-9370.notmuch>

On Thu, 12 Mar 2020 at 19:32, John Fastabend <john.fastabend@gmail.com> wrote:
>
> The restriction that the maps can not grow/shrink is perhaps limiting a
> bit. I can see how resizing might be useful. In my original load balancer
> case a single application owned all the socks so there was no need to
> ever pull them back out of the map. We "knew" where they were. I think
> resize ops could be added without to much redesign. Or a CREATE flag could
> be used to add it as a new entry if needed. At some point I guess someone
> will request it as a feature for Cilium for example. OTOH I'm not sure
> off-hand how to use a dynamically sized table for load balancing. I
> should know the size because I want to say something about the hash
> distribution and if the size is changing do I still know this? I really
> haven't considered it much.

I agree, magically changing the size of a sockmap isn't useful. We don't
want to do load-balancing, but still need stable indices into the map:

- derive some sort of ID from the skb
- look up the ID in the sockmap
- return the socket as the result of the program

If the ID changes we need to coordinate this with the eBPF, or at least
update some other map in a race-free way.

[...]

>
> Rather than expose the fd's to user space would a map copy api be
> useful? I could imagine some useful cases where copy might be used
>
>  map_copy(map *A, map *B, map_key *key)
>
> would need to sort out what to do with key/value size changes. But
> I can imagine for upgrades this might be useful.

I guess that would be a way to approach it. I'd probably find a primitive
to copy a whole map atomically more useful, but haven't really thought
about it much.

>
> Another option I've been considering the need for a garbage collection
> thread trigger at regular intervals. This BPF program could do the
> copy from map to map in kernel space never exposing fds out of kernel

So, have a dummy prog that has both maps, and copies from old to new.
Invoke that from user space via BPF_PROG_TEST_RUN?

I guess that would work, but falls back to being "protected" by
CAP_SYS_ADMIN. It's just more cumbersome than doing it in user space!

Lorenz

-- 
Lorenz Bauer  |  Systems Engineer
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