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, 39 insertions, 0 deletions

diff --git a/src/expression.c b/src/expression.c
index 393c1b2b..ee75284e 100644
--- a/src/expression.c
+++ b/src/expression.c
@@ -254,6 +254,45 @@ struct expr *symbol_expr_alloc(const struct location *loc,
 	return expr;
 }
 
+static void variable_expr_print(const struct expr *expr,
+				struct output_ctx *octx)
+{
+	nft_print(octx, "$%s", expr->sym->identifier);
+}
+
+static void variable_expr_clone(struct expr *new, const struct expr *expr)
+{
+	new->scope      = expr->scope;
+	new->sym	= expr->sym;
+
+	expr->sym->refcnt++;
+}
+
+static void variable_expr_destroy(struct expr *expr)
+{
+	expr->sym->refcnt--;
+}
+
+static const struct expr_ops variable_expr_ops = {
+	.type		= EXPR_VARIABLE,
+	.name		= "variable",
+	.print		= variable_expr_print,
+	.clone		= variable_expr_clone,
+	.destroy	= variable_expr_destroy,
+};
+
+struct expr *variable_expr_alloc(const struct location *loc,
+				 struct scope *scope, struct symbol *sym)
+{
+	struct expr *expr;
+
+	expr = expr_alloc(loc, &variable_expr_ops, &invalid_type,
+			  BYTEORDER_INVALID, 0);
+	expr->scope	 = scope;
+	expr->sym	 = sym;
+	return expr;
+}
+
 static void constant_expr_print(const struct expr *expr,
 				 struct output_ctx *octx)
 {