net-loopback: allow lo dev initial state to be controlled

Traditionally loopback devices comes up with initial state as DOWN for
any new network-namespace. This would mean that anyone needing this
device (which is mostly true except sandboxes where networking in not
needed at all), would have to bring this UP by issuing something like
'ip link set lo up' which can be avoided if the initial state can be set
as UP. Also ICMP error propagation needs loopback to be UP.

The default value for this sysctl is set to ZERO which will preserve the
backward compatible behavior for the root-netns while changing the
sysctl will only alter the behavior of the newer network namespaces.

Signed-off-by: Mahesh Bandewar <maheshb@google.com>
Signed-off-by: Jian Yang <jianyang@google.com>

Documentation/admin-guide/sysctl/net.rst

@@ -268,6 +268,17 @@ Maximum number of microseconds in one NAPI polling cycle. Polling
 will exit when either netdev_budget_usecs have elapsed during the
 poll cycle or the number of packets processed reaches netdev_budget.
 
+netdev_loopback_state
+---------------------
+
+Controls the loopback device initial state for any new network namespaces. By
+default, we keep the initial state as DOWN.
+
+If set to 1, the loopback device will be brought UP during namespace creation.
+This will only apply to all new network namespaces.
+
+Default : 0  (for compatibility reasons)
+
 netdev_max_backlog
 ------------------
 

drivers/net/loopback.c

@@ -219,6 +219,13 @@ static __net_init int loopback_net_init(struct net *net)
 
 	BUG_ON(dev->ifindex != LOOPBACK_IFINDEX);
 	net->loopback_dev = dev;
+
+	if (sysctl_netdev_loopback_state) {
+		/* Bring loopback device UP */
+		rtnl_lock();
+		dev_open(dev, NULL);
+		rtnl_unlock();
+	}
 	return 0;
 
 out_free_netdev:

include/linux/netdevice.h

@@ -625,6 +625,7 @@ struct netdev_queue {
 
 extern int sysctl_fb_tunnels_only_for_init_net;
 extern int sysctl_devconf_inherit_init_net;
+extern int sysctl_netdev_loopback_state;
 
 /*
  * sysctl_fb_tunnels_only_for_init_net == 0 : For all netns

net/core/sysctl_net_core.c

@@ -35,6 +35,11 @@ static int net_msg_warn;	/* Unused, but still a sysctl */
 int sysctl_fb_tunnels_only_for_init_net __read_mostly = 0;
 EXPORT_SYMBOL(sysctl_fb_tunnels_only_for_init_net);
 
+/* 0 - default (backward compatible) state: DOWN by default
+ * 1 - UP by default (for all new network namespaces)
+ */
+int sysctl_netdev_loopback_state __read_mostly;
+
 /* 0 - Keep current behavior:
  *     IPv4: inherit all current settings from init_net
  *     IPv6: reset all settings to default
@@ -507,6 +512,15 @@ static struct ctl_table net_core_table[] = {
 		.proc_handler	= set_default_qdisc
 	},
 #endif
+	{
+		.procname	= "netdev_loopback_state",
+		.data		= &sysctl_netdev_loopback_state,
+		.maxlen		= sizeof(int),
+		.mode		= 0644,
+		.proc_handler	= proc_dointvec_minmax,
+		.extra1		= SYSCTL_ZERO,
+		.extra2		= SYSCTL_ONE
+	},
 #endif /* CONFIG_NET */
 	{
 		.procname	= "netdev_budget",