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, 5 deletions

diff --git a/src/parser_bison.y b/src/parser_bison.y
index 58bc6805..ee6729f1 100644
--- a/src/parser_bison.y
+++ b/src/parser_bison.y
@@ -771,8 +771,6 @@ common_block		:	INCLUDE		QUOTED_STRING	stmt_separator
 			{
 				struct scope *scope = current_scope(state);
 
-				/* ignore missing identifier */
-				symbol_unbind(scope, $2);
 				symbol_bind(scope, $2, $4);
 				xfree($2);
 			}
@@ -2584,16 +2582,17 @@ match_stmt		:	relational_expr
 variable_expr		:	'$'	identifier
 			{
 				struct scope *scope = current_scope(state);
+				struct symbol *sym;
 
-				if (symbol_lookup(scope, $2) == NULL) {
+				sym = symbol_get(scope, $2);
+				if (!sym) {
 					erec_queue(error(&@2, "unknown identifier '%s'", $2),
 						   state->msgs);
 					xfree($2);
 					YYERROR;
 				}
 
-				$$ = symbol_expr_alloc(&@$, SYMBOL_DEFINE,
-						       scope, $2);
+				$$ = variable_expr_alloc(&@$, scope, sym);
 				xfree($2);
 			}
 			;