ppp: Fix generating ifname when empty IFLA_IFNAME is specified

IFLA_IFNAME is nul-term string which means that IFLA_IFNAME buffer can be
larger than length of string which contains.

Function __rtnl_newlink() generates new own ifname if either IFLA_IFNAME
was not specified at all or userspace passed empty nul-term string.

It is expected that if userspace does not specify ifname for new ppp netdev
then kernel generates one in format "ppp<id>" where id matches to the ppp
unit id which can be later obtained by PPPIOCGUNIT ioctl.

And it works in this way if IFLA_IFNAME is not specified at all. But it
does not work when IFLA_IFNAME is specified with empty string.

So fix this logic also for empty IFLA_IFNAME in ppp_nl_newlink() function
and correctly generates ifname based on ppp unit identifier if userspace
did not provided preferred ifname.

Without this patch when IFLA_IFNAME was specified with empty string then
kernel created a new ppp interface in format "ppp<id>" but id did not
match ppp unit id returned by PPPIOCGUNIT ioctl. In this case id was some
number generated by __rtnl_newlink() function.
Signed-off-by: Pali Rohár <pali@kernel.org>
Fixes: bb8082f6 ("ppp: build ifname using unit identifier for rtnl based devices")
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ppp/ppp_generic.c

@@ -1306,7 +1306,7 @@ static int ppp_nl_newlink(struct net *src_net, struct net_device *dev,
 	 * the PPP unit identifer as suffix (i.e. ppp<unit_id>). This allows
 	 * userspace to infer the device name using to the PPPIOCGUNIT ioctl.
 	 */
-	if (!tb[IFLA_IFNAME])
+	if (!tb[IFLA_IFNAME] || !nla_len(tb[IFLA_IFNAME]) || !*(char *)nla_data(tb[IFLA_IFNAME]))
 		conf.ifname_is_set = false;
 
 	err = ppp_dev_configure(src_net, dev, &conf);