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

[Jason Wang]

On 2018/11/14 下午9:01, Marc-André Lureau wrote:
> Hi
>
> On Wed, Nov 14, 2018 at 7:46 AM Jason Wang <address@hidden> wrote:
> > 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 <address@hidden>
> >
> > I believe instead of supporting unnamed sockets, we should also support
> > named one through cli?
> This could be a later patch, I have no need for it yet. Perhaps it
> should be a chardev then?


I mean something like: -socket id=ud0,path=/tmp/XXX


> > > ---
> > >    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);
> Yes they should be equivalent, but then we need to add ?: operators
> for the dest arguments. I preferred to have an if() instead.
>
> thanks


One possible issue here is I'm not sure there's a equivalent send() in 
e.g non POSIX system.

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;