Re: [RFC] Saner typechecking for closures

[Kees Cook <keescook@chromium.org>]

On Tue, Apr 23, 2024 at 05:38:41AM +0100, Matthew Wilcox wrote:
> What would you think to this?
> 
> +++ b/include/linux/delayed_call.h
> @@ -14,13 +14,12 @@ struct delayed_call {
> 
>  #define DEFINE_DELAYED_CALL(name) struct delayed_call name = {NULL, NULL}
> 
> -/* I really wish we had closures with sane typechecking... */
> -static inline void set_delayed_call(struct delayed_call *call,
> -               void (*fn)(void *), void *arg)
> -{
> -       call->fn = fn;
> -       call->arg = arg;
> -}
> +/* Typecheck the arg is appropriate for the function */
> +#define set_delayed_call(call, _fn, _arg) do {                         \
> +       (void)sizeof(_fn(_arg));                                        \
> +       (call)->fn = (void (*)(void *))(_fn);                           \
> +       (call)->arg = (_arg);                                           \
> +} while (0)
> 
>  static inline void do_delayed_call(struct delayed_call *call)
>  {
> 
> 
> That should give us the possibility of passing any pointer
> to the function, but gets us away from void pointers.  I did this as a

This just means KCFI will freak out now, since it will see a mismatch
between call->fn's type and the target function's type. :(

And instead of args like this, can't we use the regular container_of()
tricks to get at the pointer we want? i.e. make "call" a member of the
strut doing the delayed call?

-Kees

> followup:
> 
> -extern void page_put_link(void *);
> +extern void page_put_link(struct folio *);
> 
> ...
> 
> -void page_put_link(void *arg)
> +void page_put_link(struct folio *folio)
>  {
> -       put_page(arg);
> +       folio_put(folio);
>  }
>  EXPORT_SYMBOL(page_put_link);
> 
> and similar changes to the three callers.
> 
> Or is there something newer and shinier we should be using instead of
> delayed_call?

-- 
Kees Cook