Re: [Qemu-devel] [PATCH v2 1/5] sockets: ensure we can bind to both ipv4 & ipv6 separately

["Daniel P. Berrange" <berrange@redhat.com>]

On Mon, May 22, 2017 at 10:26:09AM -0500, Eric Blake wrote:
> On 05/19/2017 01:03 PM, Daniel P. Berrange wrote:
> > When binding to an IPv6 socket we currently force the
> > IPV6_V6ONLY flag to off. This means that the IPv6 socket
> > will accept both IPv4 & IPv6 sockets when QEMU is launched
> > with something like
> > 
> >   -vnc :::1
> > 
> > While this is good for that case, it is bad for other
> > cases. For example if an empty hostname is given,
> > getaddrinfo resolves it to 2 addresses 0.0.0.0 and ::,
> > in that order. We will thus bind to 0.0.0.0 first, and
> > then fail to bind to :: on the same port. The same
> > problem can happen if any other hostname lookup causes
> > the IPv4 address to be reported before the IPv6 address.
> > 
> > When we get an IPv6 bind failure, we should re-try the
> > same port, but with IPV6_V6ONLY turned on again, to
> > avoid clash with any IPv4 listener.
> > 
> > This ensures that
> > 
> >   -vnc :1
> > 
> > will bind successfully to both 0.0.0.0 and ::, and also
> > avoid
> > 
> >   -vnc :1,to=2
> > 
> > from mistakenly using a 2nd port for the :: listener.
> > 
> > Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
> > ---
> >  util/qemu-sockets.c | 31 +++++++++++++++++++++++--------
> >  1 file changed, 23 insertions(+), 8 deletions(-)
> > 
> 
> >          for (p = port_min; p <= port_max; p++) {
> > +#ifdef IPV6_V6ONLY
> > +            /* listen on both ipv4 and ipv6 */
> > +            int v6only = 0;
> > +#endif
> >              inet_setport(e, p);
> > +#ifdef IPV6_V6ONLY
> > +        rebind:
> > +            if (e->ai_family == PF_INET6) {
> 
> The rebind: label could go here with no change in semantics and one less
> conditional when compiled without optimization. But hopefully the
> compiler is smart enough to see that under -O2, so I don't see a reason
> for you to change the code.

Also bear in mind the running time of this method is going to be
dominated by the getaddrinfo() call most of the time, so I don't
think there's any measurable perf difference to moving the label.

> > +                qemu_setsockopt(slisten, IPPROTO_IPV6, IPV6_V6ONLY, &v6only,
> > +                                sizeof(v6only));
> > +            }
> > +#endif
> >              if (bind(slisten, e->ai_addr, e->ai_addrlen) == 0) {
> >                  goto listen;
> >              }
> > +
> > +#ifdef IPV6_V6ONLY
> > +            /* If we got EADDRINUSE from an IPv6 bind & V6ONLY is unset,
> > +             * it could be that the IPv4 port is already claimed, so retry
> > +             * with V6ONLY set
> > +             */
> > +            if (e->ai_family == PF_INET6 && errno == EADDRINUSE && !v6only) {
> > +                v6only = 1;
> > +                goto rebind;
> > +            }
> > +#endif
> > +
> 
> Reviewed-by: Eric Blake <eblake@redhat.com>
> 
> -- 
> Eric Blake, Principal Software Engineer
> Red Hat, Inc.           +1-919-301-3266
> Virtualization:  qemu.org | libvirt.org
> 




Regards,
Daniel
-- 
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