Re: [PATCH] x86/altcall: further refine clang workaround

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Thu, Jul 25, 2024 at 05:00:22PM +0200, Jan Beulich wrote:
> On 25.07.2024 16:54, Roger Pau Monné wrote:
> > On Thu, Jul 25, 2024 at 03:18:29PM +0200, Jan Beulich wrote:
> >> On 25.07.2024 12:56, Roger Pau Monne wrote:
> >>> --- a/xen/arch/x86/include/asm/alternative.h
> >>> +++ b/xen/arch/x86/include/asm/alternative.h
> >>> @@ -184,11 +184,11 @@ extern void alternative_branches(void);
> >>>   * https://github.com/llvm/llvm-project/issues/82598
> >>>   */
> >>>  #define ALT_CALL_ARG(arg, n)                                            \
> >>> -    register union {                                                    \
> >>> -        typeof(arg) e[sizeof(long) / sizeof(arg)];                      \
> >>> -        unsigned long r;                                                \
> >>> +    register struct {                                                   \
> >>> +        typeof(arg) e;                                                  \
> >>> +        char pad[sizeof(void *) - sizeof(arg)];                         \
> >>
> >> One thing that occurred to me only after our discussion, and I then forgot
> >> to mention this before you would send a patch: What if sizeof(void *) ==
> >> sizeof(arg)? Zero-sized arrays are explicitly something we're trying to
> >> get rid of.
> > 
> > I wondered about this, but I though it was only [] that we were trying
> > to get rid of, not [0].
> 
> Sadly (here) it's actually the other way around, aiui.

The only other option I have in mind is using an oversized array on
the union, like:

#define ALT_CALL_ARG(arg, n)                                            \
    union {                                                             \
        typeof(arg) e[(sizeof(long) + sizeof(arg) - 1) / sizeof(arg)];  \
        unsigned long r;                                                \
    } a ## n ## __  = {                                                 \
        .e[0] = ({ BUILD_BUG_ON(sizeof(arg) > sizeof(void *)); (arg); })\
    };                                                                  \
    register unsigned long a ## n ## _ asm ( ALT_CALL_arg ## n ) =      \
        a ## n ## __.r

Regards, Roger.