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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 14.06.12 at 16:08, Jean Guyader <jean.guyader@xxxxxxxxxx> wrote:
> On 31/05 04:47, Jan Beulich wrote:
>> >>> On 31.05.12 at 17:07, Jean Guyader <jean.guyader@xxxxxxxxxx> 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?

Depends on what pfn_list_hnd's type is. Maybe going through
XEN_GUEST_HANDLE(void) would do (perhaps you could even
replace uint8_t with void in your code above, making use of
gcc's extension allowing void pointer arithmetic).

If that doesn't help, maybe you could post a complete example,
compilable with your modification, and I could look into making
this work with what is there already.

Jan


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