Re: [PATCH v7 bpf-next 00/11] Socket migration for SO_REUSEPORT.

[Daniel Borkmann]

On 5/21/21 8:20 PM, Kuniyuki Iwashima wrote:
> The SO_REUSEPORT option allows sockets to listen on the same port and to
> accept connections evenly. However, there is a defect in the current
> implementation [1]. When a SYN packet is received, the connection is tied
> to a listening socket. Accordingly, when the listener is closed, in-flight
> requests during the three-way handshake and child sockets in the accept
> queue are dropped even if other listeners on the same port could accept
> such connections.
>
> This situation can happen when various server management tools restart
> server (such as nginx) processes. For instance, when we change nginx
> configurations and restart it, it spins up new workers that respect the new
> configuration and closes all listeners on the old workers, resulting in the
> in-flight ACK of 3WHS is responded by RST.
>
> To avoid such a situation, users have to know deeply how the kernel handles
> SYN packets and implement connection draining by eBPF [2]:
>
>    1. Stop routing SYN packets to the listener by eBPF.
>    2. Wait for all timers to expire to complete requests
>    3. Accept connections until EAGAIN, then close the listener.
>
>    or
>
>    1. Start counting SYN packets and accept syscalls using the eBPF map.
>    2. Stop routing SYN packets.
>    3. Accept connections up to the count, then close the listener.
>
> In either way, we cannot close a listener immediately. However, ideally,
> the application need not drain the not yet accepted sockets because 3WHS
> and tying a connection to a listener are just the kernel behaviour. The
> root cause is within the kernel, so the issue should be addressed in kernel
> space and should not be visible to user space. This patchset fixes it so
> that users need not take care of kernel implementation and connection
> draining. With this patchset, the kernel redistributes requests and
> connections from a listener to the others in the same reuseport group
> at/after close or shutdown syscalls.
>
> Although some software does connection draining, there are still merits in
> migration. For some security reasons, such as replacing TLS certificates,
> we may want to apply new settings as soon as possible and/or we may not be
> able to wait for connection draining. The sockets in the accept queue have
> not started application sessions yet. So, if we do not drain such sockets,
> they can be handled by the newer listeners and could have a longer
> lifetime. It is difficult to drain all connections in every case, but we
> can decrease such aborted connections by migration. In that sense,
> migration is always better than draining.
>
> Moreover, auto-migration simplifies user space logic and also works well in
> a case where we cannot modify and build a server program to implement the
> workaround.
>
> Note that the source and destination listeners MUST have the same settings
> at the socket API level; otherwise, applications may face inconsistency and
> cause errors. In such a case, we have to use the eBPF program to select a
> specific listener or to cancel migration.
>
> Special thanks to Martin KaFai Lau for bouncing ideas and exchanging code
> snippets along the way.
>
>
> Link:
>   [1] The SO_REUSEPORT socket option
>   https://lwn.net/Articles/542629/
>
>   [2] Re: [PATCH 1/1] net: Add SO_REUSEPORT_LISTEN_OFF socket option as drain mode
>   https://lore.kernel.org/netdev/1458828813.10868.65.camel@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/

This series needs review/ACKs from TCP maintainers. Eric/Neal/Yuchung please take
a look again.

Thanks,
Daniel