[PATCH 4.19 14/84] ppp: Fix generating ifname when empty IFLA_IFNAME is specified

[Sasha Levin]

From: Pali Rohár <pali@xxxxxxxxxx>

[ Upstream commit 2459dcb96bcba94c08d6861f8a050185ff301672 ]

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@xxxxxxxxxx>
Fixes: bb8082f69138 ("ppp: build ifname using unit identifier for rtnl based devices")
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 drivers/net/ppp/ppp_generic.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/net/ppp/ppp_generic.c b/drivers/net/ppp/ppp_generic.c
index 1af47aaa7ba5..dc9de8731c56 100644
--- a/drivers/net/ppp/ppp_generic.c
+++ b/drivers/net/ppp/ppp_generic.c
@@ -1125,7 +1125,7 @@ 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);
-- 
2.30.2