Re: [PATCH 3/6] x86/mem-paging: use guest handle for XENMEM_paging_op_prep

[Jan Beulich <jbeulich@xxxxxxxx>]

On 20.04.2020 14:08, Julien Grall wrote:
> On 20/04/2020 08:26, Jan Beulich wrote:
>> On 17.04.2020 19:13, Julien Grall wrote:
>>> FWIW, the different matters on Arm. Although, it looks like the
>>> compiler will not warn you if you are using the wrong handler :(.
>>
>> I find this highly suspicious, but can't check myself until back
>> in the office - these are distinct compound types after all, so
>> this shouldn't just be a warning, but an error. Or did you merely
>> mean there's no warning on x86?
> 
> I mean on Arm 32-bit. I have changed one of the function to use 
> XEN_GUEST_HANDLE_PARAM() rather than XEN_GUEST_HANDLE() but not changing the 
> caller.
> 
> It is probably because they are both defined using an union. Interestly, the 
> type will also not be checked, so the code a function will happily accept a 
> XEN_GUEST_HANDLE_PARAM(uint8) even if the prototype requested 
> XEN_GUEST_HANDLE_PARAM(uint64).
> 
> This looks rather messy, maybe we should use a structure (and some alignment) 
> to add more safety.

Are the unions plain ones? I could see room for behavior like
the one you describe with transparent unions, albeit still
not quite like you describe it. Getting handle types to be
properly type-checked by the compiler is pretty imperative imo.

Jan