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

On 28.07.2020 12:15, Andrew Cooper wrote:
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.

I'm entirely find with the change, but I'm struggling with this justification:
How can _any_ change to the header in our tree help people using custom
derivations (i.e. including those defining only some of the union members)?

Further proof that C isn't an appropriate way to desribe an ABI...

Further proof that it requires more careful writing in C to serve as an ABI
description, I would say.

--- a/xen/common/domctl.c
+++ b/xen/common/domctl.c
@@ -959,6 +959,14 @@ long do_domctl(XEN_GUEST_HANDLE_PARAM(xen_domctl_t) 
      return ret;
+static void __init __maybe_unused build_assertions(void)
+    struct xen_domctl d;

Doesn't this also need __maybe_unused? Afaik ...

+    BUILD_BUG_ON(sizeof(d) != 16 /* header */ + 128 /* union */);
+    BUILD_BUG_ON(offsetof(typeof(d), u) != 16);

... neither sizeof() nor typeof() count as actual uses.




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