Re: [PATCH] KVM: x86: Fix C++ user API for structures with variable length arrays

[Kees Cook <kees@kernel.org>]

On February 27, 2026 12:29:11 AM PST, David Woodhouse <dwmw2@infradead.org> wrote:
>On Thu, 2026-02-26 at 11:02 -0800, Kees Cook wrote:
>> 
>> > Also put the header fields into a struct_group() to provide (in C) a
>> > separate struct (e.g 'struct kvm_msrs_hdr') without the trailing VLA.
>> 
>> Right, my only worry is if C++ would want those header structs too. In
>> that case, you'd probably want to use a macro to include them (since not
>> all compilers are supporting transparent struct members yet):
>> 
>> #define __kvm_msrs_hdr	\
>> 	__u32 nmsrs; /* number of msrs in entries */	\
>> 	__u32 pad
>> 
>> struct kvm_msrs_hdr {
>> 	__kvm_msrs_hdr;
>> };
>> 
>> struct kvm_msrs {
>> 	__kvm_msrs_hdr;
>> 	__DECLARE_FLEX_ARRAY(struct kvm_msr_entry, entries);
>> };
>
>Hm, does the struct_group() not also depend on the compiler supporting
>anonymous struct members? Is there a distinction I'm missing?

Sorry, I should have been mor clear: the _tag_ that struct_group creates isn't supported for all C++ compilers (so is commented out for C++) so if a C++ project wanted to use the struct by its tag ("struct kvm_msrs_hdr"), you'd need the construct I wrote above.

-Kees

-- 
Kees Cook