Re: [PATCH] guestcopy: evaluate {,__}copy{,_field}_to_guest*() arguments just once

[Julien Grall <julien@xxxxxxx>]

Hi,

On 01/04/2020 15:29, Jan Beulich wrote:
> There's nothing wrong with having e.g.
>
>      copy_to_guest(uarg, ptr++, 1);
>
> yet until now this would increment "ptr" twice.
Is there such use in Xen today? I guess not as we would have noticed any 
issue.
> Also drop a pair of unneeded parentheses from every instance at this
> occasion.
>
> Fixes: b7954cc59831 ("Enhance guest memory accessor macros so that source operands 
> can be")
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> ---
> Arm side untested so far, as I don't have all the tool chain pieces
> available at home.
I will have a look.

> ---
> This goes on top of the assumed v2 of Julien's "xen/guest_access: Harden
> copy_to_guest_offset to prevent const dest operand".
>
> --- a/xen/include/asm-arm/guest_access.h
> +++ b/xen/include/asm-arm/guest_access.h
> @@ -79,7 +79,7 @@ int access_guest_memory_by_ipa(struct do
>       const typeof(*(ptr)) *_s = (ptr);                   \
>       char (*_d)[sizeof(*_s)] = (void *)(hnd).p;          \
>       void *__maybe_unused _t = (hnd).p;                  \
> -    ((void)((hnd).p == (ptr)));                         \
> +    (void)((hnd).p == _s);                              \
May I ask why this is a problem with 'ptr' but not 'hnd'? Wouldn't it 
theorically possible to have an array of handle?
Cheers,

--
Julien Grall