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

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Message-ID: <8aa5928f-4338-4ab5-a144-e6ab3b9644d6@embeddedor.com>
Date: Tue, 16 Dec 2025 11:59:50 +0900
From: "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To: Wei Liu <wei.liu@...nel.org>, "Gustavo A. R. Silva"
 <gustavoars@...nel.org>
Cc: "K. Y. Srinivasan" <kys@...rosoft.com>,
 Haiyang Zhang <haiyangz@...rosoft.com>, 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 12/16/25 03:47, Wei Liu wrote:
> 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.

I'm using the following patch (which is the one I'll actually submit
upstream once all these warnings are fixed).

diff --git a/Makefile b/Makefile
index e404e4767944..0d6a0d8f791b 100644
--- a/Makefile
+++ b/Makefile
@@ -1086,6 +1086,8 @@ KBUILD_CFLAGS += $(call cc-option, -fstrict-flex-arrays=3)
  # Allow including a tagged struct or union anonymously in another struct/union.
  KBUILD_CFLAGS += -fms-extensions

+KBUILD_CFLAGS += $(call cc-option, -Wflex-array-member-not-at-end)
+
  # disable invalid "can't wrap" optimizations for signed / pointers
  KBUILD_CFLAGS  += -fno-strict-overflow


Thanks
-Gustavo

> 
> 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
>>