src: add variable expression and use it to allow redefinitions

Add new variable expression that we can use to attach symbols in
runtime, this allows us to redefine variables via new keyword, eg.

 table ip x {
        chain y {
                define address = { 1.1.1.1, 2.2.2.2 }
                ip saddr $address
                redefine address = { 3.3.3.3 }
                ip saddr $address
        }
 }

 # nft list ruleset
 table ip x {
        chain y {
                ip saddr { 1.1.1.1, 2.2.2.2 }
                ip saddr { 3.3.3.3 }
        }
 }

Note that redefinition just places a new symbol version before the
existing one, so symbol lookups always find the latest version. The
undefine keyword decrements the reference counter and removes the symbol
from the list, so it cannot be used anymore. Still, previous references
to this symbol via variable expression are still valid.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

1 files changed, 4 insertions, 1 deletions

diff --git a/include/rule.h b/include/rule.h
index 531222ce..58c4aeef 100644
--- a/include/rule.h
+++ b/include/rule.h
@@ -73,18 +73,21 @@ extern void scope_release(const struct scope *scope);
  * @list:	scope symbol list node
  * @identifier:	identifier
  * @expr:	initializer
+ * @refcnt:	reference counter
  */
 struct symbol {
 	struct list_head	list;
 	const char		*identifier;
 	struct expr		*expr;
+	int			refcnt;
 };
 
 extern void symbol_bind(struct scope *scope, const char *identifier,
 			struct expr *expr);
-extern int symbol_unbind(struct scope *scope, const char *identifier);
+extern int symbol_unbind(const struct scope *scope, const char *identifier);
 extern struct symbol *symbol_lookup(const struct scope *scope,
 				    const char *identifier);
+struct symbol *symbol_get(const struct scope *scope, const char *identifier);
 
 enum table_flags {
 	TABLE_F_DORMANT		= (1 << 0),