io_uring/net: fail zc send when unsupported by socket

If a protocol doesn't support zerocopy it will silently fall back to
copying. This type of behaviour has always been a source of troubles
so it's better to fail such requests instead.

Cc: <[email protected]> # 6.0
Signed-off-by: Pavel Begunkov <[email protected]>
Link: https://lore.kernel.org/r/2db3c7f16bb6efab4b04569cd16e6242b40c5cb3.1666346426.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <[email protected]>

Author: isilence

io_uring/net.c

@@ -1056,6 +1056,8 @@ int io_send_zc(struct io_kiocb *req, unsigned int issue_flags)
 	sock = sock_from_file(req->file);
 	if (unlikely(!sock))
 		return -ENOTSOCK;
+	if (!test_bit(SOCK_SUPPORT_ZC, &sock->flags))
+		return -EOPNOTSUPP;
 
 	msg.msg_name = NULL;
 	msg.msg_control = NULL;