MSG_ZEROCOPY doesn't work on half-open TCP sockets

[Marek Majkowski]

Hi,

Current implementation of MSG_ZEROCOPY for TCP requires the socket to
be ESTABLISHED:
https://elixir.bootlin.com/linux/v5.0-rc1/source/net/ipv4/tcp.c#L1188

if (sk->sk_state != TCP_ESTABLISHED) {
    err = -EINVAL;
    goto out_err;
}

In TCP it's totally fine to have half-open sockets, for example:

shutdown(5, SHUT_RD)

Moves the socket from ESTABLISHED to CLOSE_WAIT. In such TCP state
it's possible to continue sending data. This is not supported by
MSG_ZEROCOPY, which will fail with EINVAL in such case. I think it's a
bug.

Marek