rtnetlink: do not depend on RTNL in rtnl_fill_proto_down()

Change dev_change_proto_down() and dev_change_proto_down_reason()
to write once on dev->proto_down and dev->proto_down_reason.

Then rtnl_fill_proto_down() can use READ_ONCE() annotations
and run locklessly.

rtnl_proto_down_size() should assume worst case,
because readng dev->proto_down_reason multiple
times would be racy without RTNL in the future.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

net/core/dev.c

@@ -9223,7 +9223,7 @@ int dev_change_proto_down(struct net_device *dev, bool proto_down)
 		netif_carrier_off(dev);
 	else
 		netif_carrier_on(dev);
-	dev->proto_down = proto_down;
+	WRITE_ONCE(dev->proto_down, proto_down);
 	return 0;
 }
 
@@ -9237,18 +9237,21 @@ int dev_change_proto_down(struct net_device *dev, bool proto_down)
 void dev_change_proto_down_reason(struct net_device *dev, unsigned long mask,
 				  u32 value)
 {
+	u32 proto_down_reason;
 	int b;
 
 	if (!mask) {
-		dev->proto_down_reason = value;
+		proto_down_reason = value;
 	} else {
+		proto_down_reason = dev->proto_down_reason;
 		for_each_set_bit(b, &mask, 32) {
 			if (value & (1 << b))
-				dev->proto_down_reason |= BIT(b);
+				proto_down_reason |= BIT(b);
 			else
-				dev->proto_down_reason &= ~BIT(b);
+				proto_down_reason &= ~BIT(b);
 		}
 	}
+	WRITE_ONCE(dev->proto_down_reason, proto_down_reason);
 }
 
 struct bpf_xdp_link {

net/core/rtnetlink.c

@@ -1036,7 +1036,7 @@ static size_t rtnl_proto_down_size(const struct net_device *dev)
 {
 	size_t size = nla_total_size(1);
 
-	if (dev->proto_down_reason)
+	/* Assume dev->proto_down_reason is not zero. */
 	size += nla_total_size(0) + nla_total_size(4);
 
 	return size;
@@ -1737,10 +1737,10 @@ static int rtnl_fill_proto_down(struct sk_buff *skb,
 	struct nlattr *pr;
 	u32 preason;
 
-	if (nla_put_u8(skb, IFLA_PROTO_DOWN, dev->proto_down))
+	if (nla_put_u8(skb, IFLA_PROTO_DOWN, READ_ONCE(dev->proto_down)))
 		goto nla_put_failure;
 
-	preason = dev->proto_down_reason;
+	preason = READ_ONCE(dev->proto_down_reason);
 	if (!preason)
 		return 0;