Re: [Qemu-devel] [PATCH] RFC: net/socket: learn to talk with a unix dgram socket

[Jason Wang <jasowang@redhat.com>]

On 2018/11/10 上午3:56, Marc-André Lureau wrote:
> -net socket has a fd argument, and may be passed pre-opened sockets.
>
> TCP sockets use framing.
> UDP sockets have datagram boundaries.
>
> When given a unix dgram socket, it will be able to read from it, but
> will attempt to send on the dgram_dst, which is unset. The other end
> will not receive the data.
>
> Let's teach -net socket to recognize a UNIX DGRAM socket, and use the
> regular send() command (without dgram_dst).
>
> This makes running slirp out-of-process possible that
> way (python pseudo-code):
>
> a, b = socket.socketpair(socket.AF_UNIX, socket.SOCK_DGRAM)
>
> subprocess.Popen('qemu -net socket,fd=%d -net user' % a.fileno(), shell=True)
> subprocess.Popen('qemu ... -net nic -net socket,fd=%d' % b.fileno(), shell=True)
>
> (to make slirp a seperate project altogether, we would have to have
> some compatibility code and/or deprecate various options & HMP
> commands for dynamic port forwarding etc - but this looks like a
> reachable goal)
>
> Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>


I believe instead of supporting unnamed sockets, we should also support 
named one through cli?


> ---
>   net/socket.c | 25 +++++++++++++++++++++----
>   1 file changed, 21 insertions(+), 4 deletions(-)
>
> diff --git a/net/socket.c b/net/socket.c
> index 7095eb749f..8a9c30892d 100644
> --- a/net/socket.c
> +++ b/net/socket.c
> @@ -119,9 +119,13 @@ static ssize_t net_socket_receive_dgram(NetClientState *nc, const uint8_t *buf,
>       ssize_t ret;
>   
>       do {
> -        ret = qemu_sendto(s->fd, buf, size, 0,
> -                          (struct sockaddr *)&s->dgram_dst,
> -                          sizeof(s->dgram_dst));
> +        if (s->dgram_dst.sin_family != AF_UNIX) {
> +            ret = qemu_sendto(s->fd, buf, size, 0,
> +                              (struct sockaddr *)&s->dgram_dst,
> +                              sizeof(s->dgram_dst));
> +        } else {
> +            ret = send(s->fd, buf, size, 0);
> +        }


Any reason that send is a must here? send(2) said:
        call

            send(sockfd, buf, len, flags);

        is equivalent to

            sendto(sockfd, buf, len, flags, NULL, 0);


Thanks


>       } while (ret == -1 && errno == EINTR);
>   
>       if (ret == -1 && errno == EAGAIN) {
> @@ -322,6 +326,15 @@ static NetSocketState *net_socket_fd_init_dgram(NetClientState *peer,
>       int newfd;
>       NetClientState *nc;
>       NetSocketState *s;
> +    SocketAddress *sa;
> +    SocketAddressType sa_type;
> +
> +    sa = socket_local_address(fd, errp);
> +    if (!sa) {
> +        return NULL;
> +    }
> +    sa_type = sa->type;
> +    qapi_free_SocketAddress(sa);
>   
>       /* fd passed: multicast: "learn" dgram_dst address from bound address and save it
>        * Because this may be "shared" socket from a "master" process, datagrams would be recv()
> @@ -365,8 +378,12 @@ static NetSocketState *net_socket_fd_init_dgram(NetClientState *peer,
>                    "socket: fd=%d (cloned mcast=%s:%d)",
>                    fd, inet_ntoa(saddr.sin_addr), ntohs(saddr.sin_port));
>       } else {
> +        if (sa_type == SOCKET_ADDRESS_TYPE_UNIX) {
> +            s->dgram_dst.sin_family = AF_UNIX;
> +        }
> +
>           snprintf(nc->info_str, sizeof(nc->info_str),
> -                 "socket: fd=%d", fd);
> +                 "socket: fd=%d %s", fd, SocketAddressType_str(sa_type));
>       }
>   
>       return s;