linux-kernel - Re: [PATCH][next] hyperv: Avoid -Wflex-array-member-not-at-end warning

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Message-ID: <20251215184759.GA654575@liuwe-devbox-debian-v2.local>
Date: Mon, 15 Dec 2025 18:47:59 +0000
From: Wei Liu <wei.liu@...nel.org>
To: "Gustavo A. R. Silva" <gustavoars@...nel.org>
Cc: "K. Y. Srinivasan" <kys@...rosoft.com>,
	Haiyang Zhang <haiyangz@...rosoft.com>,
	Wei Liu <wei.liu@...nel.org>, Dexuan Cui <decui@...rosoft.com>,
	Long Li <longli@...rosoft.com>, linux-hyperv@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH][next] hyperv: Avoid -Wflex-array-member-not-at-end
 warning

On Fri, Dec 12, 2025 at 03:44:50PM +0900, Gustavo A. R. Silva wrote:
> -Wflex-array-member-not-at-end was introduced in GCC-14, and we are
> getting ready to enable it, globally.
> 
> Use the new __TRAILING_OVERLAP() helper to fix the following warning:
> 
> include/hyperv/hvgdk_mini.h:581:25: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
> 
> This helper creates a union between a flexible-array member (FAM) and a
> set of MEMBERS that would otherwise follow it.
> 
> This overlays the trailing MEMBER u64 gva_list[]; onto the FAM
> struct hv_tlb_flush_ex::hv_vp_set.bank_contents[], while keeping
> the FAM and the start of MEMBER aligned.
> 
> The static_assert() ensures this alignment remains, and it's
> intentionally placed inmediately after the related structure --no
> blank line in between.
> 
> Signed-off-by: Gustavo A. R. Silva <gustavoars@...nel.org>

Gustavo, what is your build command? I would like to incorporate that
into my own build tests.

Thanks,
Wei

> ---
>  include/hyperv/hvgdk_mini.h | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/include/hyperv/hvgdk_mini.h b/include/hyperv/hvgdk_mini.h
> index 04b18d0e37af..30fbbde81c5c 100644
> --- a/include/hyperv/hvgdk_mini.h
> +++ b/include/hyperv/hvgdk_mini.h
> @@ -578,9 +578,12 @@ struct hv_tlb_flush {	 /* HV_INPUT_FLUSH_VIRTUAL_ADDRESS_LIST */
>  struct hv_tlb_flush_ex {
>  	u64 address_space;
>  	u64 flags;
> -	struct hv_vpset hv_vp_set;
> -	u64 gva_list[];
> +	__TRAILING_OVERLAP(struct hv_vpset, hv_vp_set, bank_contents, __packed,
> +		u64 gva_list[];
> +	);
>  } __packed;
> +static_assert(offsetof(struct hv_tlb_flush_ex, hv_vp_set.bank_contents) ==
> +	      offsetof(struct hv_tlb_flush_ex, gva_list));
>  
>  struct ms_hyperv_tsc_page {	 /* HV_REFERENCE_TSC_PAGE */
>  	volatile u32 tsc_sequence;
> -- 
> 2.43.0
>