Re: QEMU Sockets Networking Backend Multicast Networking Fix

[Jason Wang]

On 2020/2/15 下午6:39, Markus Armbruster wrote:
> Jason, please have a look.
>
> Faisal Al-Humaimidi <address@hidden> writes:
>
> > Hello QEMU developers,
> >
> > I have noticed a bug in the `mcast` option of the `socket` networking
> > backend, where I simply cannot join a multicast group (tested in Windows 10
> > with QEMU 4.2.0 release). I have found a fix to the problem. The problem
> > was mainly due to the fact that QEMU was binding to the multicast address,
> > and not the local address or the default INADDR_ANY (0.0.0.0) if no local
> > address is used.
> >
> > Here's the patch text (as well as attached with this email), that outlines
> > my fix:
> >
> > ```
> > diff -uarN qemu-4.2.0.original/net/socket.c qemu-4.2.0.modified/net/socket.c
> > --- qemu-4.2.0.original/net/socket.c 2019-12-12 10:20:48.000000000 -0800
> > +++ qemu-4.2.0.modified/net/socket.c 2020-02-14 10:30:16.395973453 -0800
> > @@ -253,6 +253,15 @@
> >           goto fail;
> >       }
> >
> > +    /* Preserve the multicast address, and bind to a non-multicast group
> > (e.g. a
> > +     * local address).
> > +     */
> > +    struct in_addr group_addr = mcastaddr->sin_addr;
> > +    if (localaddr) {
> > +        mcastaddr->sin_addr = *localaddr;
> > +    } else {
> > +        mcastaddr->sin_addr.s_addr = htonl(INADDR_ANY);
> > +    }
> >       ret = bind(fd, (struct sockaddr *)mcastaddr, sizeof(*mcastaddr));


This looks wrong, AFAIK the local address should be added through 
IP_ADD_MEMBERSHIP which is already handled in this function I believe.

Thanks


> >       if (ret < 0) {
> >           error_setg_errno(errp, errno, "can't bind ip=%s to socket",
> > @@ -260,7 +269,10 @@
> >           goto fail;
> >       }
> >
> > -    /* Add host to multicast group */
> > +    /* Restore the multicast address. */
> > +    mcastaddr->sin_addr = group_addr;
> > +
> > +    /* Add host to multicast group. */
> >       imr.imr_multiaddr = mcastaddr->sin_addr;
> >       if (localaddr) {
> >           imr.imr_interface = *localaddr;
> > @@ -277,7 +289,7 @@
> >           goto fail;
> >       }
> >
> > -    /* Force mcast msgs to loopback (eg. several QEMUs in same host */
> > +    /* Force mcast msgs to loopback (eg. several QEMUs in same host). */
> >       loop = 1;
> >       ret = qemu_setsockopt(fd, IPPROTO_IP, IP_MULTICAST_LOOP,
> >                             &loop, sizeof(loop));
> > @@ -287,7 +299,7 @@
> >           goto fail;
> >       }
> >
> > -    /* If a bind address is given, only send packets from that address */
> > +    /* If a bind address is given, only send packets from that address. */
> >       if (localaddr != NULL) {
> >           ret = qemu_setsockopt(fd, IPPROTO_IP, IP_MULTICAST_IF,
> >                                 localaddr, sizeof(*localaddr));
> > ```
> >
> > Regards,
> > Faisal Al-Humaimidi