kconfig: error out if a recursive variable references itself

When using a recursively expanded variable, it is a common mistake
to make circular reference.

For example, Make terminates the following code:

  X = $(X)
  Y := $(X)

Let's detect the circular expansion in Kconfig, too.

On the other hand, a function that recurses itself is a commonly-used
programming technique.  So, Make does not check recursion in the
reference with 'call'.  For example, the following code continues
running eternally:

  X = $(call X)
  Y := $(X)

Kconfig allows circular expansion if one or more arguments are given,
but terminates when the same function is recursively invoked 1000 times,
assuming it is a programming mistake.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

scripts/kconfig/preprocess.c

@@ -229,6 +229,7 @@ struct variable {
 	char *name;
 	char *value;
 	enum variable_flavor flavor;
+	int exp_count;
 	struct list_head node;
 };
 
@@ -253,11 +254,22 @@ static char *variable_expand(const char *name, int argc, char *argv[])
 	if (!v)
 		return NULL;
 
+	if (argc == 0 && v->exp_count)
+		pperror("Recursive variable '%s' references itself (eventually)",
+			name);
+
+	if (v->exp_count > 1000)
+		pperror("Too deep recursive expansion");
+
+	v->exp_count++;
+
 	if (v->flavor == VAR_RECURSIVE)
 		res = expand_string_with_args(v->value, argc, argv);
 	else
 		res = xstrdup(v->value);
 
+	v->exp_count--;
+
 	return res;
 }
 
@@ -284,6 +296,7 @@ void variable_add(const char *name, const char *value,
 
 		v = xmalloc(sizeof(*v));
 		v->name = xstrdup(name);
+		v->exp_count = 0;
 		list_add_tail(&v->node, &variable_list);
 	}