[PATCH] public/domctl: Fix the struct xen_domctl ABI in 32bit builds

[Andrew Cooper <andrew.cooper3@citrix.com>]

The Xen domctl ABI currently relies on the union containing a field with
alignment of 8.

32bit projects which only copy the used subset of functionality end up with an
ABI breakage if they don't have at least one uint64_aligned_t field copied.

Insert explicit padding, and some build assertions to ensure it never changes
moving forwards.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
---
CC: George Dunlap <George.Dunlap@eu.citrix.com>
CC: Ian Jackson <ian.jackson@citrix.com>
CC: Jan Beulich <JBeulich@suse.com>
CC: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
CC: Stefano Stabellini <sstabellini@kernel.org>
CC: Wei Liu <wl@xen.org>
CC: Julien Grall <julien@xen.org>

Further proof that C isn't an appropriate way to desribe an ABI...
---
 xen/common/domctl.c         | 8 ++++++++
 xen/include/public/domctl.h | 1 +
 2 files changed, 9 insertions(+)

diff --git a/xen/common/domctl.c b/xen/common/domctl.c
index a69b3b59a8..20ef8399bd 100644
--- a/xen/common/domctl.c
+++ b/xen/common/domctl.c
@@ -959,6 +959,14 @@ long do_domctl(XEN_GUEST_HANDLE_PARAM(xen_domctl_t) u_domctl)
     return ret;
 }
 
+static void __init __maybe_unused build_assertions(void)
+{
+    struct xen_domctl d;
+
+    BUILD_BUG_ON(sizeof(d) != 16 /* header */ + 128 /* union */);
+    BUILD_BUG_ON(offsetof(typeof(d), u) != 16);
+}
+
 /*
  * Local variables:
  * mode: C
diff --git a/xen/include/public/domctl.h b/xen/include/public/domctl.h
index 59bdc28c89..9464a9058a 100644
--- a/xen/include/public/domctl.h
+++ b/xen/include/public/domctl.h
@@ -1222,6 +1222,7 @@ struct xen_domctl {
 #define XEN_DOMCTL_gdbsx_domstatus             1003
     uint32_t interface_version; /* XEN_DOMCTL_INTERFACE_VERSION */
     domid_t  domain;
+    uint16_t _pad[3];
     union {
         struct xen_domctl_createdomain      createdomain;
         struct xen_domctl_getdomaininfo     getdomaininfo;
-- 
2.11.0