netfilter: nf_conntrack: make nf_ct_zone_dflt built-in

Fengguang reported, that some randconfig generated the following linker
issue with nf_ct_zone_dflt object involved:

  [...]
  CC      init/version.o
  LD      init/built-in.o
  net/built-in.o: In function `ipv4_conntrack_defrag':
  nf_defrag_ipv4.c:(.text+0x93e95): undefined reference to `nf_ct_zone_dflt'
  net/built-in.o: In function `ipv6_defrag':
  nf_defrag_ipv6_hooks.c:(.text+0xe3ffe): undefined reference to `nf_ct_zone_dflt'
  make: *** [vmlinux] Error 1

Given that configurations exist where we have a built-in part, which is
accessing nf_ct_zone_dflt such as the two handlers nf_ct_defrag_user()
and nf_ct6_defrag_user(), and a part that configures nf_conntrack as a
module, we must move nf_ct_zone_dflt into a fixed, guaranteed built-in
area when netfilter is configured in general.

Therefore, split the more generic parts into a common header under
include/linux/netfilter/ and move nf_ct_zone_dflt into the built-in
section that already holds parts related to CONFIG_NF_CONNTRACK in the
netfilter core. This fixes the issue on my side.

Fixes: 308ac9143ee2 ("netfilter: nf_conntrack: push zone object into functions")
Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/netfilter.h

@@ -368,6 +368,8 @@ nf_nat_decode_session(struct sk_buff *skb, struct flowi *fl, u_int8_t family)
 #endif /*CONFIG_NETFILTER*/
 
 #if defined(CONFIG_NF_CONNTRACK) || defined(CONFIG_NF_CONNTRACK_MODULE)
+#include <linux/netfilter/nf_conntrack_zones_common.h>
+
 extern void (*ip_ct_attach)(struct sk_buff *, const struct sk_buff *) __rcu;
 void nf_ct_attach(struct sk_buff *, const struct sk_buff *);
 extern void (*nf_ct_destroy)(struct nf_conntrack *) __rcu;

include/linux/netfilter/nf_conntrack_zones_common.h

@@ -0,0 +1,23 @@
+#ifndef _NF_CONNTRACK_ZONES_COMMON_H
+#define _NF_CONNTRACK_ZONES_COMMON_H
+
+#include <uapi/linux/netfilter/nf_conntrack_tuple_common.h>
+
+#define NF_CT_DEFAULT_ZONE_ID	0
+
+#define NF_CT_ZONE_DIR_ORIG	(1 << IP_CT_DIR_ORIGINAL)
+#define NF_CT_ZONE_DIR_REPL	(1 << IP_CT_DIR_REPLY)
+
+#define NF_CT_DEFAULT_ZONE_DIR	(NF_CT_ZONE_DIR_ORIG | NF_CT_ZONE_DIR_REPL)
+
+#define NF_CT_FLAG_MARK		1
+
+struct nf_conntrack_zone {
+	u16	id;
+	u8	flags;
+	u8	dir;
+};
+
+extern const struct nf_conntrack_zone nf_ct_zone_dflt;
+
+#endif /* _NF_CONNTRACK_ZONES_COMMON_H */

include/net/netfilter/nf_conntrack_zones.h

@@ -1,24 +1,7 @@
 #ifndef _NF_CONNTRACK_ZONES_H
 #define _NF_CONNTRACK_ZONES_H
 
-#include <linux/netfilter/nf_conntrack_tuple_common.h>
-
-#define NF_CT_DEFAULT_ZONE_ID	0
-
-#define NF_CT_ZONE_DIR_ORIG	(1 << IP_CT_DIR_ORIGINAL)
-#define NF_CT_ZONE_DIR_REPL	(1 << IP_CT_DIR_REPLY)
-
-#define NF_CT_DEFAULT_ZONE_DIR	(NF_CT_ZONE_DIR_ORIG | NF_CT_ZONE_DIR_REPL)
-
-#define NF_CT_FLAG_MARK		1
-
-struct nf_conntrack_zone {
-	u16	id;
-	u8	flags;
-	u8	dir;
-};
-
-extern const struct nf_conntrack_zone nf_ct_zone_dflt;
+#include <linux/netfilter/nf_conntrack_zones_common.h>
 
 #if IS_ENABLED(CONFIG_NF_CONNTRACK)
 #include <net/netfilter/nf_conntrack_extend.h>

net/netfilter/core.c

@@ -388,6 +388,12 @@ EXPORT_SYMBOL(nf_conntrack_destroy);
 struct nfq_ct_hook __rcu *nfq_ct_hook __read_mostly;
 EXPORT_SYMBOL_GPL(nfq_ct_hook);
 
+/* Built-in default zone used e.g. by modules. */
+const struct nf_conntrack_zone nf_ct_zone_dflt = {
+	.id	= NF_CT_DEFAULT_ZONE_ID,
+	.dir	= NF_CT_DEFAULT_ZONE_DIR,
+};
+EXPORT_SYMBOL_GPL(nf_ct_zone_dflt);
 #endif /* CONFIG_NF_CONNTRACK */
 
 #ifdef CONFIG_NF_NAT_NEEDED

net/netfilter/nf_conntrack_core.c

@@ -1286,13 +1286,6 @@ bool __nf_ct_kill_acct(struct nf_conn *ct,
 }
 EXPORT_SYMBOL_GPL(__nf_ct_kill_acct);
 
-/* Built-in default zone used e.g. by modules. */
-const struct nf_conntrack_zone nf_ct_zone_dflt = {
-	.id	= NF_CT_DEFAULT_ZONE_ID,
-	.dir	= NF_CT_DEFAULT_ZONE_DIR,
-};
-EXPORT_SYMBOL_GPL(nf_ct_zone_dflt);
-
 #ifdef CONFIG_NF_CONNTRACK_ZONES
 static struct nf_ct_ext_type nf_ct_zone_extend __read_mostly = {
 	.len	= sizeof(struct nf_conntrack_zone),