[PATCH net-next v2 1/3] net: Do not inherit operational state when protodown is on

[Ido Schimmel <idosch@nvidia.com>]

The protodown functionality allows user space to turn off the carrier of
a net device:

 # ip link add name dummy1 up type dummy
 # ip link add name macvlan1 up link dummy1 type macvlan mode bridge
 # ip link set dev macvlan1 protodown on
 $ ip -br link show dev macvlan1
 macvlan1@dummy1  DOWN           0a:5c:a3:05:c7:86 <NO-CARRIER,BROADCAST,MULTICAST,UP>

Different applications can set different protodown reasons, which
prevents an application from turning on the carrier of a net device as
long as others want it down:

 # ip link set dev macvlan1 protodown_reason 1 on
 # ip link set dev macvlan1 protodown_reason 2 on
 # ip link set dev macvlan1 protodown off
 Error: Cannot clear protodown, active reasons.
 # ip link set dev macvlan1 protodown_reason 2 off
 # ip link set dev macvlan1 protodown off
 Error: Cannot clear protodown, active reasons.
 # ip link set dev macvlan1 protodown_reason 1 off
 # ip link set dev macvlan1 protodown off
 $ ip -br link show dev macvlan1
 macvlan1@dummy1  UP             0a:5c:a3:05:c7:86 <BROADCAST,MULTICAST,UP,LOWER_UP>

Unfortunately, this mechanism is not very useful when the carrier of a
net device can be toggled by toggling the carrier of its lower device:

 # ip link set dev macvlan1 protodown on
 $ ip -br link show dev macvlan1
 macvlan1@dummy1  DOWN           0a:5c:a3:05:c7:86 <NO-CARRIER,BROADCAST,MULTICAST,UP>
 # ip link set dev dummy1 carrier off
 # ip link set dev dummy1 carrier on
 $ ip -br link show dev macvlan1
 macvlan1@dummy1  UP             0a:5c:a3:05:c7:86 <BROADCAST,MULTICAST,UP,LOWER_UP>

Obviously, this is not the intended behavior and it is unlikely to be
relied on by anyone. In fact, it is a problem for applications like FRR
that use protodown with macvlan on top of a bridge as part of Virtual
Router Redundancy Protocol (VRRP).

Solve this by preventing a net device configured with protodown on from
inheriting the operational state of its lower device. Note that
READ_ONCE() is not needed as RTNL is held.

Output with the patch:

 # ip link add name dummy1 up type dummy
 # ip link add name macvlan1 up link dummy1 type macvlan mode bridge
 # ip link set dev macvlan1 protodown on
 $ ip -br link show dev macvlan1
 macvlan1@dummy1  DOWN           0a:5c:a3:05:c7:86 <NO-CARRIER,BROADCAST,MULTICAST,UP>
 # ip link set dev dummy1 carrier off
 # ip link set dev dummy1 carrier on
 $ ip -br link show dev macvlan1
 macvlan1@dummy1  DOWN           0a:5c:a3:05:c7:86 <NO-CARRIER,BROADCAST,MULTICAST,UP>
 # ip link set dev macvlan1 protodown off
 $ ip -br link show dev macvlan1
 macvlan1@dummy1  UP             0a:5c:a3:05:c7:86 <BROADCAST,MULTICAST,UP,LOWER_UP>

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
---
 net/core/dev.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/net/core/dev.c b/net/core/dev.c
index 06c195906231..bfb0f297b234 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -11113,6 +11113,9 @@ EXPORT_SYMBOL(netdev_change_features);
 void netif_stacked_transfer_operstate(const struct net_device *rootdev,
 					struct net_device *dev)
 {
+	if (dev->proto_down)
+		return;
+
 	if (rootdev->operstate == IF_OPER_DORMANT)
 		netif_dormant_on(dev);
 	else
-- 
2.54.0