Re: [PATCH][next] treewide: uapi: Replace zero-length arrays with flexible-array members

[David Woodhouse]

On Mon, 2026-02-23 at 12:38 +0900, Gustavo A. R. Silva wrote:
> > These changes broke userspace compilation, because a *lot* of these
> > structures ending with VLAs are very much used as headers.
> > 
> > QEMU does it, for example...
> > 
> > uint64_t kvm_arch_get_supported_msr_feature(KVMState *s, uint32_t index)
> > {
> >      struct {
> >          struct kvm_msrs info;
> >          struct kvm_msr_entry entries[1];
> >      } msr_data = {};
> > 
> > 
> > ... which works in C but not in C++, which gives an error I can't work
> > out how to avoid.
> > 
> > Am I missing something, or did we break our userspace headers for C++
> > and make them C-only?
> > 
> >   $ cat test_kvm.cpp
> > #include <cstdlib>
> > #include <asm/kvm.h>
> > 
> > struct msr_data {
> > 	struct kvm_msrs info;
> > 	struct kvm_msr_entry entries[1];
> > };
> 
> Something like this should eventually land:
> 
> diff --git a/tools/arch/x86/include/uapi/asm/kvm.h b/tools/arch/x86/include/uapi/asm/kvm.h
> index 7ceff6583652..295b2a54b25d 100644
> --- a/tools/arch/x86/include/uapi/asm/kvm.h
> +++ b/tools/arch/x86/include/uapi/asm/kvm.h
> @@ -192,11 +192,14 @@ struct kvm_msr_entry {
>          __u64 data;
>   };
> 
> -/* for KVM_GET_MSRS and KVM_SET_MSRS */
> -struct kvm_msrs {
> +struct kvm_msrs_hdr {
>          __u32 nmsrs; /* number of msrs in entries */
>          __u32 pad;
> +};
> 
> +/* for KVM_GET_MSRS and KVM_SET_MSRS */
> +struct kvm_msrs {
> +       struct kvm_msrs_hdr;
>          struct kvm_msr_entry entries[];
>   };

I guess that would work, although it's still breaking the userspace
API.

I was thinking of something along the lines of

 #ifdef __cplusplus
 #define __KERNEL_VLA 0
 #else
 #define __KERNEL_VLA
 #endif

 struct kvm_msrs {
         __u32 nmsrs; /* number of msrs in entries */
         __u32 pad;
         struct kvm_msr_entry entries[__KERNEL_VLA];
 };

I suppose it could depend on __KERNEL__ instead of __cplusplus too?

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