Re: [RFC net-next v1 1/2] udp: Introduce UDP_STOP_RCV option for UDP

[Kuniyuki Iwashima <kuniyu@amazon.com>]

From: Jiayuan Chen <jiayuan.chen@linux.dev>
Date: Thu,  1 May 2025 11:51:08 +0800
> For some services we are using "established-over-unconnected" model.
> 
> '''
> // create unconnected socket and 'listen()'
> srv_fd = socket(AF_INET, SOCK_DGRAM)
> setsockopt(srv_fd, SO_REUSEPORT)
> bind(srv_fd, SERVER_ADDR, SERVER_PORT)
> 
> // 'accept()'
> data, client_addr = recvmsg(srv_fd)
> 
> // create a connected socket for this request
> cli_fd = socket(AF_INET, SOCK_DGRAM)
> setsockopt(cli_fd, SO_REUSEPORT)
> bind(cli_fd, SERVER_ADDR, SERVER_PORT)
> connect(cli, client_addr)
> ...
> // do handshake with cli_fd
> '''
> 
> This programming pattern simulates accept() using UDP, creating a new
> socket for each client request. The server can then use separate sockets
> to handle client requests, avoiding the need to use a single UDP socket
> for I/O transmission.
> 
> But there is a race condition between the bind() and connect() of the
> connected socket:
> We might receive unexpected packets belonging to the unconnected socket
> before connect() is executed, which is not what we need.
> (Of course, before connect(), the unconnected socket will also receive
> packets from the connected socket, which is easily resolved because
> upper-layer protocols typically require explicit boundaries, and we
> receive a complete packet before creating a connected socket.)
> 
> Before this patch, the connected socket had to filter requests at recvmsg
> time, acting as a dispatcher to some extent. With this patch, we can
> consider the bind and connect operations to be atomic.

SO_ATTACH_REUSEPORT_EBPF is what you want.

The socket won't receive any packets until the socket is added to
the BPF map.

No need to reinvent a subset of BPF functionalities.