[Libguestfs] [PATCH nbdkit] Add support for AF_VSOCK.

[Richard W.M. Jones]

On Fri, Oct 18, 2019 at 11:16:16AM -0500, Eric Blake wrote:
> On 10/18/19 10:38 AM, Richard W.M. Jones wrote:
> >On platforms which support it (only Linux currently) nbdkit can act as
> >a vsock server.  Guests running on the host see a raw NBD socket which
> >it can connect to by opening an AF_VSOCK connection.  (Although only
> >libnbd supports this).
> >
> >The current limitations are:
> >
> >  * nbdkit can only act as a host (cid == VMADDR_CID_HOST == 2).
> 
> How hard would it be to allow a different cid to run a server on the
> guest and client on the host?

Actually thinking about it, my code is wrong anyway.  I believe it
should use VMADDR_CID_ANY as the cid (like INADDR_ANY), in which case
it can be used from a guest.  Stefan is that the way to go?

> >  * There is no access control.  Any guest which has vsock enabled can
> >    open the socket.
> 
> Well, there's TLS if you need it.

Also this is controlled to some extent by the hypervisor administrator
since they can turn vsock off and on (as a whole feature) per guest.

> >+If you see the error C<unable to open vhost-vsock device> then you may
> >+have to unload the VMCI transport:
> >+
> >+ modprobe -r vmw_vsock_vmci_transport
> 
> Is that in the host or in the guest?

Host.  Actually this is a bug which Stefan is looking into, so maybe
we can just link to the BZ if there is one.

> >@@ -247,6 +252,74 @@ bind_tcpip_socket (size_t *nr_socks)
> >    return socks;
> >  }
> >+int *
> >+bind_vsock (size_t *nr_socks)
> >+{
> >+#ifdef AF_VSOCK
> >+  uint32_t vsock_port;
> >+  int sock;
> >+  int *ret;
> >+  struct sockaddr_vm addr;
> >+
> >+  if (port == NULL)
> >+    vsock_port = 10809;
> >+  else {
> >+    /* --port parameter must be numeric for vsock, unless
> >+     * /etc/services is extended but that seems unlikely. XXX
> >+     */
> >+    if (nbdkit_parse_uint32_t ("port", port, &vsock_port) == -1)
> >+      exit (EXIT_FAILURE);
> >+  }
> >+
> >+#ifdef SOCK_CLOEXEC
> >+  sock = socket (AF_VSOCK, SOCK_STREAM|SOCK_CLOEXEC, 0);
> >+#else
> >+  /* Fortunately, this code is only run at startup, so there is no
> >+   * risk of the fd leaking to a plugin's fork()
> >+   */
> >+  sock = set_cloexec (socket (AF_VSOCK, SOCK_STREAM, 0));
> 
> Even better, all known platforms with AF_VSOCK have SOCK_CLOEXEC.
> Make this #else just be an #error.

Very true!  I will fix this one.

> >+#endif
> >+  if (sock == -1) {
> >+    perror ("bind_unix_socket: socket");
> >+    exit (EXIT_FAILURE);
> >+  }
> 
> Also, it wouldn't be that much harder for a followup patch exposing:
> 
> nbdkit nbd vsock_cid=... vsock_port=...
> 
> to have our passthrough wrapper call into libnbd's
> nbd_connect_vsock, and let nbdkit transparently convert from vsock
> server to TCP/Unix client.

Yup.

Rich.

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