[PATCH v4 18/31] kconfig: error out if a recursive variable references itself

[Masahiro Yamada]

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@xxxxxxxxxxxxx>
---

Changes in v4:
 - Newly added

Changes in v3: None
Changes in v2: None

 scripts/kconfig/preprocess.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/scripts/kconfig/preprocess.c b/scripts/kconfig/preprocess.c
index c39e30e..246f879 100644
--- a/scripts/kconfig/preprocess.c
+++ b/scripts/kconfig/preprocess.c
@@ -276,6 +276,7 @@ struct variable {
 	char *name;
 	char *value;
 	enum variable_flavor flavor;
+	int exp_count;
 	struct list_head node;
 };
 
@@ -306,8 +307,19 @@ static char *variable_expand(const char *name, int argc, char *argv[],
 		expanded_argv[i] = expand_string_with_args(argv[i],
 							   old_argc, old_argv);
 
+	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++;
+
 	res = expand_string_with_args(v->value, argc, expanded_argv);
 
+	v->exp_count--;
+
 	for (i = 0; i < argc; i++)
 		free(expanded_argv[i]);
 
@@ -337,6 +349,7 @@ 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);
 	}
 
-- 
2.7.4