Re: [PATCH 6.6.y] KVM: x86: Use __DECLARE_FLEX_ARRAY() for UAPI structures with VLAs

[Sean Christopherson <seanjc@google.com>]

On Mon, Apr 13, 2026, Sasha Levin wrote:
> From: David Woodhouse <dwmw@amazon.co.uk>
> 
> [ Upstream commit 2619da73bb2f10d88f7e1087125c40144fdf0987 ]
> 
> Commit 94dfc73e7cf4 ("treewide: uapi: Replace zero-length arrays with
> flexible-array members") broke the userspace API for C++.
> 
> These structures ending in VLAs are typically a *header*, which can be
> followed by an arbitrary number of entries. Userspace typically creates
> a larger structure with some non-zero number of entries, for example in
> QEMU's kvm_arch_get_supported_msr_feature():
> 
>     struct {
>         struct kvm_msrs info;
>         struct kvm_msr_entry entries[1];
>     } msr_data = {};
> 
> While that works in C, it fails in C++ with an error like:
>  flexible array member 'kvm_msrs::entries' not at end of 'struct msr_data'
> 
> Fix this by using __DECLARE_FLEX_ARRAY() for the VLA, which uses [0]
> for C++ compilation.
> 
> Fixes: 94dfc73e7cf4 ("treewide: uapi: Replace zero-length arrays with flexible-array members")
> Cc: stable@vger.kernel.org
> Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
> Link: https://patch.msgid.link/3abaf6aefd6e5efeff3b860ac38421d9dec908db.camel@infradead.org
> [sean: tag for stable@]
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> [ applied `__DECLARE_FLEX_ARRAY(char, name)` change directly instead of inside missing `#ifdef __KERNEL__` else branch ]
> Signed-off-by: Sasha Levin <sashal@kernel.org>
> ---

Acked-by: Sean Christopherson <seanjc@google.com>