netns: enable to inherit devconf from current netns

The goal is to be able to inherit the initial devconf parameters from the
current netns, ie the netns where this new netns has been created.

This is useful in a containers environment where /proc/sys is read only.
For example, if a pod is created with specifics devconf parameters and has
the capability to create netns, the user expects to get the same parameters
than his 'init_net', which is not the real init_net in this case.
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/admin-guide/sysctl/net.rst

@@ -339,7 +339,9 @@ settings from init_net and for IPv6 we reset all settings to default.
 
 If set to 1, both IPv4 and IPv6 settings are forced to inherit from
 current ones in init_net. If set to 2, both IPv4 and IPv6 settings are
-forced to reset to their default values.
+forced to reset to their default values. If set to 3, both IPv4 and IPv6
+settings are forced to inherit from current ones in the netns where this
+new netns has been created.
 
 Default : 0  (for compatibility reasons)
 

net/core/sysctl_net_core.c

@@ -23,6 +23,7 @@
 #include <net/pkt_sched.h>
 
 static int two __maybe_unused = 2;
+static int three = 3;
 static int min_sndbuf = SOCK_MIN_SNDBUF;
 static int min_rcvbuf = SOCK_MIN_RCVBUF;
 static int max_skb_frags = MAX_SKB_FRAGS;
@@ -39,6 +40,7 @@ EXPORT_SYMBOL(sysctl_fb_tunnels_only_for_init_net);
  *     IPv6: reset all settings to default
  * 1 - Both inherit all current settings from init_net
  * 2 - Both reset all settings to default
+ * 3 - Both inherit all settings from current netns
  */
 int sysctl_devconf_inherit_init_net __read_mostly;
 EXPORT_SYMBOL(sysctl_devconf_inherit_init_net);
@@ -553,7 +555,7 @@ static struct ctl_table net_core_table[] = {
 		.mode		= 0644,
 		.proc_handler	= proc_dointvec_minmax,
 		.extra1		= SYSCTL_ZERO,
-		.extra2		= &two,
+		.extra2		= &three,
 	},
 	{
 		.procname	= "high_order_alloc_disable",

net/ipv4/devinet.c

@@ -2666,11 +2666,24 @@ static __net_init int devinet_init_net(struct net *net)
 	tbl[0].extra2 = net;
 #endif
 
-	if ((!IS_ENABLED(CONFIG_SYSCTL) ||
-	     sysctl_devconf_inherit_init_net != 2) &&
-	    !net_eq(net, &init_net)) {
-		memcpy(all, init_net.ipv4.devconf_all, sizeof(ipv4_devconf));
-		memcpy(dflt, init_net.ipv4.devconf_dflt, sizeof(ipv4_devconf_dflt));
+	if (!net_eq(net, &init_net)) {
+		if (IS_ENABLED(CONFIG_SYSCTL) &&
+		    sysctl_devconf_inherit_init_net == 3) {
+			/* copy from the current netns */
+			memcpy(all, current->nsproxy->net_ns->ipv4.devconf_all,
+			       sizeof(ipv4_devconf));
+			memcpy(dflt,
+			       current->nsproxy->net_ns->ipv4.devconf_dflt,
+			       sizeof(ipv4_devconf_dflt));
+		} else if (!IS_ENABLED(CONFIG_SYSCTL) ||
+			   sysctl_devconf_inherit_init_net != 2) {
+			/* inherit == 0 or 1: copy from init_net */
+			memcpy(all, init_net.ipv4.devconf_all,
+			       sizeof(ipv4_devconf));
+			memcpy(dflt, init_net.ipv4.devconf_dflt,
+			       sizeof(ipv4_devconf_dflt));
+		}
+		/* else inherit == 2: use compiled values */
 	}
 
 #ifdef CONFIG_SYSCTL

net/ipv6/addrconf.c

@@ -6991,9 +6991,26 @@ static int __net_init addrconf_init_net(struct net *net)
 		goto err_alloc_dflt;
 
 	if (IS_ENABLED(CONFIG_SYSCTL) &&
-	    sysctl_devconf_inherit_init_net == 1 && !net_eq(net, &init_net)) {
-		memcpy(all, init_net.ipv6.devconf_all, sizeof(ipv6_devconf));
-		memcpy(dflt, init_net.ipv6.devconf_dflt, sizeof(ipv6_devconf_dflt));
+	    !net_eq(net, &init_net)) {
+		switch (sysctl_devconf_inherit_init_net) {
+		case 1:  /* copy from init_net */
+			memcpy(all, init_net.ipv6.devconf_all,
+			       sizeof(ipv6_devconf));
+			memcpy(dflt, init_net.ipv6.devconf_dflt,
+			       sizeof(ipv6_devconf_dflt));
+			break;
+		case 3: /* copy from the current netns */
+			memcpy(all, current->nsproxy->net_ns->ipv6.devconf_all,
+			       sizeof(ipv6_devconf));
+			memcpy(dflt,
+			       current->nsproxy->net_ns->ipv6.devconf_dflt,
+			       sizeof(ipv6_devconf_dflt));
+			break;
+		case 0:
+		case 2:
+			/* use compiled values */
+			break;
+		}
 	}
 
 	/* these will be inherited by all namespaces */