decnet: Fix disappearing sysctl entries

When decnet is built as a module a simple:
echo 0.0 >/proc/sys/net/decnet/node_address

results in most of the sysctl entries under /proc/sys/net/decnet and
/proc/sys/net/decnet/conf disappearing.

For more details see http://www.spinics.net/lists/netdev/msg226123.html.

This change applies the same workaround used in
net/core/sysctl_net_core.c and net/ipv6/sysctl_net_ipv6.c of creating
a skeleton of decnet sysctl entries before doing anything else.

The problem first appeared in kernel 2.6.27.  The later rewrite of
sysctl in kernel 3.4 restored the previous behavior and eliminated the
need for this workaround.

This patch was heavily inspired by a similar but more complex patch by
Larry Baker.

Reported-by: Larry Baker <baker@usgs.gov>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Acked-by: David Miller <davem@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

2 files changed, 32 insertions, 0 deletions

diff --git a/net/decnet/af_decnet.c b/net/decnet/af_decnet.c
index ea3b6ee21fc9..d71f0d2b8dd4 100644
--- a/net/decnet/af_decnet.c
+++ b/net/decnet/af_decnet.c
@@ -2355,6 +2355,8 @@ static const struct proto_ops dn_proto_ops = {
 	.sendpage =	sock_no_sendpage,
 };
 
+void dn_register_sysctl_skeleton(void);
+void dn_unregister_sysctl_skeleton(void);
 void dn_register_sysctl(void);
 void dn_unregister_sysctl(void);
 
@@ -2375,6 +2377,7 @@ static int __init decnet_init(void)
 	if (rc != 0)
 		goto out;
 
+	dn_register_sysctl_skeleton();
 	dn_neigh_init();
 	dn_dev_init();
 	dn_route_init();
@@ -2414,6 +2417,7 @@ static void __exit decnet_exit(void)
 	dn_fib_cleanup();
 
 	proc_net_remove(&init_net, "decnet");
+	dn_unregister_sysctl_skeleton();
 
 	proto_unregister(&dn_proto);
 
diff --git a/net/decnet/sysctl_net_decnet.c b/net/decnet/sysctl_net_decnet.c
index 28f8b5e5f73b..d1cc2fd72838 100644
--- a/net/decnet/sysctl_net_decnet.c
+++ b/net/decnet/sysctl_net_decnet.c
@@ -55,6 +55,7 @@ static int max_decnet_no_fc_max_cwnd[] = { NSP_MAX_WINDOW };
 static char node_name[7] = "???";
 
 static struct ctl_table_header *dn_table_header = NULL;
+static struct ctl_table_header *dn_skeleton_table_header = NULL;
 
 /*
  * ctype.h :-)
@@ -356,6 +357,27 @@ static struct ctl_path dn_path[] = {
 	{ }
 };
 
+static struct ctl_table empty[1];
+
+static struct ctl_table dn_skeleton[] = {
+	{
+		.procname = "conf",
+		.mode = 0555,
+		.child = empty,
+	},
+	{ }
+};
+
+void dn_register_sysctl_skeleton(void)
+{
+	dn_skeleton_table_header = register_sysctl_paths(dn_path, dn_skeleton);
+}
+
+void dn_unregister_sysctl_skeleton(void)
+{
+	unregister_sysctl_table(dn_skeleton_table_header);
+}
+
 void dn_register_sysctl(void)
 {
 	dn_table_header = register_sysctl_paths(dn_path, dn_table);
@@ -367,6 +389,12 @@ void dn_unregister_sysctl(void)
 }
 
 #else  /* CONFIG_SYSCTL */
+void dn_register_sysctl_skeleton(void)
+{
+}
+void dn_unregister_sysctl_skeleton(void)
+{
+}
 void dn_unregister_sysctl(void)
 {
 }