Re: [PATCH 4/5] xen: Enforce casting for guest_handle_cast

[Jean Guyader <jean.guyader@citrix.com>]

On 31/05 04:47, Jan Beulich wrote:
> >>> On 31.05.12 at 17:07, Jean Guyader <jean.guyader@citrix.com> wrote:
> >--- a/xen/include/asm-x86/guest_access.h
> >+++ b/xen/include/asm-x86/guest_access.h
> >@@ -47,7 +47,7 @@
> > 
> > /* Cast a guest handle to the specified type of handle. */
> > #define guest_handle_cast(hnd, type) ({         \
> >-    type *_x = (hnd).p;                         \
> >+    type *_x = (type *)(hnd).p;                 \
> 
> 
> You would have to explain how this is safe: Without the cast, we
> get compiler warnings (and hence build failures due to -Werror)
> if "type *" and typeof((hnd).p) are incompatible. Adding an
> explicit cast removes that intentional check.
> 

I can't realy explain how this is safe because I agree it make
this function less safe.

Maybe I should put here the reason that led me to do something
like that. Here is what I'm trying to do:

        XEN_GUEST_HANDLE (uint8_t) slop_hnd =
                guest_handle_cast (pfn_list_hnd, uint8_t);
        guest_handle_add_offset (slop_hnd, sizeof (v4v_pfn_list_t));
        pfn_hnd = guest_handle_cast (slop_hnd, v4v_pfn_t);

I need to cast to uint8_t first to get the add_offset to behave
correctly. Maybe what I need would need a new macro that would
do those two operations.

What would be the proper way to doing something like this?

Thanks,
Jean