ppp: build ifname using unit identifier for rtnl based devices

Userspace programs generally need to know the name of the ppp devices
they create. Both ioctl and rtnl interfaces use the ppp<suffix> sheme
to name them. But although the suffix used by the ioctl interface can
be known by userspace (it's the PPP unit identifier returned by the
PPPIOCGUNIT ioctl), the one used by the rtnl is only known by the
kernel.

This patch brings more consistency between ioctl and rtnl based ppp
devices by generating device names using the PPP unit identifer as
suffix in both cases. This way, userspace can always infer the name of
the devices they create.
Signed-off-by: NGuillaume Nault <g.nault@alphalink.fr>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/ppp/ppp_generic.c

@@ -1103,6 +1103,15 @@ static int ppp_nl_newlink(struct net *src_net, struct net_device *dev,
 	}
 
 	conf.file = file;
+
+	/* Don't use device name generated by the rtnetlink layer when ifname
+	 * isn't specified. Let ppp_dev_configure() set the device name using
+	 * 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])
+		conf.ifname_is_set = false;
+
 	err = ppp_dev_configure(src_net, dev, &conf);
 
 out_unlock: