Re: [PATCH 1/8] xen/guest_access: Harden copy_to_guest_offset to prevent const dest operand

[Jan Beulich <jbeulich@suse.com>]

On 31.03.2020 21:13, Julien Grall wrote:
> I am not aware of any way before C11 to check if a variable is
> const or not. If we wanted to keep allow void type the handle
> then a possible approach would be:
> 
> #define copy_to_guest_offset(hnd, off, ptr, nr) ({              \
>     const typeof(*(ptr)) *_s = (ptr);                           \
>     typeof(*((hnd).p)) *_d = (hnd).p;                           \
>     size_t mul = (sizeof(*(hnd).p) > 1) ? 1 : sizeof (*_s);     \
>     ((void)((hnd).p == (ptr)));                                 \
>     raw_copy_to_guest(_d + (off) * mul, _s, sizeof(*_s)*(nr));  \
> })
> 
> I don't particularly like it but I could not come up with better so far.

Having looked at how in particular copy_field_to_guest() (which
doesn't have this issue afaict) works, here's an imo much better
alternative:

@@ -87,6 +87,7 @@
 #define copy_to_guest_offset(hnd, off, ptr, nr) ({      \
     const typeof(*(ptr)) *_s = (ptr);                   \
     char (*_d)[sizeof(*_s)] = (void *)(hnd).p;          \
+    void *__maybe_unused _t = (hnd).p;                  \
     ((void)((hnd).p == (ptr)));                         \
     raw_copy_to_guest(_d+(off), _s, sizeof(*_s)*(nr));  \
 })
@@ -143,6 +144,7 @@ static inline void put_guest_handle(void
 #define __copy_to_guest_offset(hnd, off, ptr, nr) ({    \
     const typeof(*(ptr)) *_s = (ptr);                   \
     char (*_d)[sizeof(*_s)] = (void *)(hnd).p;          \
+    void *__maybe_unused _t = (hnd).p;                  \
     ((void)((hnd).p == (ptr)));                         \
     __raw_copy_to_guest(_d+(off), _s, sizeof(*_s)*(nr));\
 })

Jan