RE: [RFC bpf-next 0/5] Extend SOCKMAP to store listening sockets

Jakub Sitnicki wrote:

This patch set is a follow up on a suggestion from LPC '19 discussions to
make SOCKMAP (or a new map type derived from it) a generic type for storing
established as well as listening sockets.

We found ourselves in need of a map type that keeps references to listening
sockets when working on making the socket lookup programmable, aka BPF
inet_lookup [1].  Initially we repurposed REUSEPORT_SOCKARRAY but found it
problematic to extend due to being tightly coupled with reuseport
logic (see slides [2]). So we've turned our attention to SOCKMAP instead.

As it turns out the changes needed to make SOCKMAP suitable for storing
listening sockets are self-contained and have use outside of programming
the socket lookup. Hence this patch set.

With these patches SOCKMAP can be used in SK_REUSEPORT BPF programs as a
drop-in replacement for REUSEPORT_SOCKARRAY for TCP. This can hopefully
lead to code consolidation between the two map types in the future.

Having said that, the main intention here is to lay groundwork for using
SOCKMAP in the next iteration of programmable socket lookup patches.

I'm looking for feedback if there's anything fundamentally wrong with
extending SOCKMAP map type like this that I might have missed.

I think this looks good. The main reason I blocked it off before is mostly
because I had no use-case for it and the complication with what to do with
child sockets. Clearing the psock state seems OK to me if user wants to
add it back to a map they can simply grab it again from a sockops event.

By the way I would eventually like to see the lookup hook return the
correct type (PTR_TO_SOCKET_OR_NULL) so that the verifier "knows" the type
and the socket can be used the same as if it was pulled from a sk_lookup
helper.

Thanks,
Jakub

[1] https://lore.kernel.org/bpf/20190828072250.29828-1-jakub@cloudflare.com/ (local)
[2] https://linuxplumbersconf.org/event/4/contributions/487/attachments/238/417/Programmable_socket_lookup_LPC_19.pdf