I got it slightly wrong, and it's even worse than this. As far as I understand it, the current semantics of MSG_ZEROCOPY on TCP make it close to unusable. The problem is that the remote party can move your MSG_ZEROCOPY socket from ESTABLISHED to CLOSE_WAIT without your involvement. This will mean that even though the program can still send() data to the socket, MSG_ZEROCOPY operations will fail with EINVAL.
In other words: because the socket needs to be ESTABLISHED for MSG_ZEROCOPY to work, and because remote party can send FIN and move the socket to CLOSE_WAIT, a sending party must implement a fallback from EINVAL return code on the transmission code. An adversarial client who does shutdown(SHUT_WR), will trigger EINVAL in the sender.. Marek On Wed, Jan 9, 2019 at 1:01 PM Marek Majkowski <ma...@cloudflare.com> wrote: > > 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
