[XEN PATCH v2] xen/include: avoid undefined behavior.

[Nicola Vetrini <nicola.vetrini@xxxxxxxxxxx>]

Redefine BUILD_BUG_ON_ZERO to fully comply with C99 avoiding
undefined behavior 58 ("A structure or union is defined as
containing no named members (6.7.2.1)."

The chosen ill-formed construct is a negative bitwidth in a
bitfield within a struct containing at least one named member,
which prevents the UB while keeping the semantics of the construct
for any memory layout of the struct (this motivates the
"sizeof(unsigned) * 8" in the definition of the macro).

Signed-off-by: Nicola Vetrini <nicola.vetrini@xxxxxxxxxxx>
---
Changes in V2:
- Avoid using a VLA as the compile-time assertion
- Do not drop _Static_assert
---
 xen/include/xen/lib.h | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/xen/include/xen/lib.h b/xen/include/xen/lib.h
index 67fc7c1d7e..e57d272772 100644
--- a/xen/include/xen/lib.h
+++ b/xen/include/xen/lib.h
@@ -51,9 +51,10 @@
    e.g. in a structure initializer (or where-ever else comma expressions
    aren't permitted). */
 #define BUILD_BUG_ON_ZERO(cond) \
-    sizeof(struct { _Static_assert(!(cond), "!(" #cond ")"); })
+    (sizeof(struct { char c; _Static_assert(!(cond), "!(" #cond ")"); }) - 1U)
 #else
-#define BUILD_BUG_ON_ZERO(cond) sizeof(struct { int:-!!(cond); })
+#define BUILD_BUG_ON_ZERO(cond) \
+    (sizeof(struct { unsigned u : (cond) ? -1 : sizeof(unsigned) * 8; }) - 
sizeof(unsigned))
 #define BUILD_BUG_ON(cond) ((void)BUILD_BUG_ON_ZERO(cond))
 #endif
 
-- 
2.34.1