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>
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>

diff --git a/config/preprocess.c b/config/preprocess.c
index 0574039238c65cf1df6bb1dba86b9fff8c989375..65da87fce907ad2bc7b52adba4651dc2c32786be 100644 (file)
--- a/config/preprocess.c
+++ b/config/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);
        }