net-sysfs: restore behavior for not running devices

[ Upstream commit 75bc3da ]

modprobe dummy dumdummies=1

Old behavior :

$ cat /sys/class/net/dummy0/carrier
cat: /sys/class/net/dummy0/carrier: Invalid argument

After blamed commit, an empty string is reported.

$ cat /sys/class/net/dummy0/carrier
$

In this commit, I restore the old behavior for carrier,
speed and duplex attributes.

Fixes: 79c6189 ("net-sysfs: remove rtnl_trylock from device attributes")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Marco Leogrande <leogrande@google.com>
Reviewed-by: Antoine Tenart <atenart@kernel.org>
Link: https://patch.msgid.link/20250221051223.576726-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

Author: edumazet

net/core/net-sysfs.c

@@ -232,11 +232,12 @@ static ssize_t carrier_show(struct device *dev,
 			    struct device_attribute *attr, char *buf)
 {
 	struct net_device *netdev = to_net_dev(dev);
-	int ret = -EINVAL;
+	int ret;
 
 	if (!rtnl_trylock())
 		return restart_syscall();
 
+	ret = -EINVAL;
 	if (netif_running(netdev)) {
 		/* Synchronize carrier state with link watch,
 		 * see also rtnl_getlink().
@@ -266,6 +267,7 @@ static ssize_t speed_show(struct device *dev,
 	if (!rtnl_trylock())
 		return restart_syscall();
 
+	ret = -EINVAL;
 	if (netif_running(netdev)) {
 		struct ethtool_link_ksettings cmd;
 
@@ -292,6 +294,7 @@ static ssize_t duplex_show(struct device *dev,
 	if (!rtnl_trylock())
 		return restart_syscall();
 
+	ret = -EINVAL;
 	if (netif_running(netdev)) {
 		struct ethtool_link_ksettings cmd;