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>

1 files changed, 13 insertions, 0 deletions

diff --git a/scripts/kconfig/preprocess.c b/scripts/kconfig/preprocess.c
index 0574039238c6..65da87fce907 100644
--- a/scripts/kconfig/preprocess.c
+++ b/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);
         }