Re: [Xen-devel] [PATCH 4/4] xen/public: arm: rework the macro set_xen_guest_handle_raw

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Fri, 30 Oct 2015, Julien Grall wrote:
> The current implementation of set_xen_guest_handle_raw is using the
> keyword "typeof" which is not part of C spec.
> 
> Furthermore, when the guest handle is defined in ARM32 guest, the
> pointer will always be smaller than the handle. Based on the C99 spec
> [1] 6.2.6.1#7, the bits that do not correspond to the member written
> will take unspecified value.
> 
> Finally, based on the defect report #283 [2], the behavior of writing
> from one member and reading from another is not clear.

In that case, with set_xen_guest_handle_raw implemented as:

> +#define set_xen_guest_handle_raw(hnd, val)                              \
> +    do {                                                                \
> +        /* Check if the handle is 64-bit (i.e 8-byte) */                \
> +        (void) sizeof(struct { int : -!!(sizeof (hnd) != 8); });        \
> +        /* Check if the type of val is compatible with the handle */    \
> +        (void) sizeof((val) != (hnd).p);                                \
> +        (hnd).q = (uint64_t)(uintptr_t)(val);                           \
>      } while ( 0 )
>  #define set_xen_guest_handle(hnd, val) set_xen_guest_handle_raw(hnd, val)

will get_xen_guest_handle, which access (hnd).p, have undefined behaviour?

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