net: introduce a knob to control whether to inherit devconf config

There have been many people complaining about the inconsistent
behaviors of IPv4 and IPv6 devconf when creating new network
namespaces.  Currently, for IPv4, we inherit all current settings
from init_net, but for IPv6 we reset all setting to default.

This patch introduces a new /proc file
/proc/sys/net/core/devconf_inherit_init_net to control the
behavior of whether to inhert sysctl current settings from init_net.
This file itself is only available in init_net.

As demonstrated below:

Initial setup in init_net:
 # cat /proc/sys/net/ipv4/conf/all/rp_filter
 2
 # cat /proc/sys/net/ipv6/conf/all/accept_dad
 1

Default value 0 (current behavior):
 # ip netns del test
 # ip netns add test
 # ip netns exec test cat /proc/sys/net/ipv4/conf/all/rp_filter
 2
 # ip netns exec test cat /proc/sys/net/ipv6/conf/all/accept_dad
 0

Set to 1 (inherit from init_net):
 # echo 1 > /proc/sys/net/core/devconf_inherit_init_net
 # ip netns del test
 # ip netns add test
 # ip netns exec test cat /proc/sys/net/ipv4/conf/all/rp_filter
 2
 # ip netns exec test cat /proc/sys/net/ipv6/conf/all/accept_dad
 1

Set to 2 (reset to default):
 # echo 2 > /proc/sys/net/core/devconf_inherit_init_net
 # ip netns del test
 # ip netns add test
 # ip netns exec test cat /proc/sys/net/ipv4/conf/all/rp_filter
 0
 # ip netns exec test cat /proc/sys/net/ipv6/conf/all/accept_dad
 0

Set to a value out of range (invalid):
 # echo 3 > /proc/sys/net/core/devconf_inherit_init_net
 -bash: echo: write error: Invalid argument
 # echo -1 > /proc/sys/net/core/devconf_inherit_init_net
 -bash: echo: write error: Invalid argument

Reported-by: Zhu Yanjun <Yanjun.Zhu@windriver.com>
Reported-by: Tonghao Zhang <xiangxia.m.yue@gmail.com>
Cc: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Acked-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Acked-by: Tonghao Zhang <xiangxia.m.yue@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 18 insertions, 0 deletions

diff --git a/net/core/sysctl_net_core.c b/net/core/sysctl_net_core.c
index d67ec17f2cc8..84bf2861f45f 100644
--- a/net/core/sysctl_net_core.c
+++ b/net/core/sysctl_net_core.c
@@ -36,6 +36,15 @@ 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 - Keep current behavior:
+ *     IPv4: inherit all current settings from init_net
+ *     IPv6: reset all settings to default
+ * 1 - Both inherit all current settings from init_net
+ * 2 - Both reset all settings to default
+ */
+int sysctl_devconf_inherit_init_net __read_mostly;
+EXPORT_SYMBOL(sysctl_devconf_inherit_init_net);
+
 #ifdef CONFIG_RPS
 static int rps_sock_flow_sysctl(struct ctl_table *table, int write,
 				void __user *buffer, size_t *lenp, loff_t *ppos)
@@ -544,6 +553,15 @@ static struct ctl_table net_core_table[] = {
 		.extra1		= &zero,
 		.extra2		= &one,
 	},
+	{
+		.procname	= "devconf_inherit_init_net",
+		.data		= &sysctl_devconf_inherit_init_net,
+		.maxlen		= sizeof(int),
+		.mode		= 0644,
+		.proc_handler	= proc_dointvec_minmax,
+		.extra1		= &zero,
+		.extra2		= &two,
+	},
 	{ }
 };