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    Subject[PATCH v5 18/31] kconfig: error out if a recursive variable references itself
    Date
    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>
    ---

    Changes in v5: None
    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 0574039..65da87f 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);
    }

    --
    2.7.4
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