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

[patchbot@xxxxxxx]

commit 8899a280a493d6717afc7a709f7d1e842b4eb2d6
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Mon Jul 27 19:21:09 2020 +0100
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Tue Jul 28 19:33:28 2020 +0100

    public/domctl: Fix the struct xen_domctl ABI in 32bit builds
    
    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@xxxxxxxxxx>
    Acked-by: Julien Grall <jgrall@xxxxxxxxxx>
---
 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;
--
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