linux-kernel - Re: [PATCH v2][next] hyperv: Avoid a hundred -Wflex-array-member-not-at-end warnings

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Message-ID: <dc494864-22c9-42a6-ba3b-f64ae753ae53@embeddedor.com>
Date: Fri, 29 Aug 2025 18:17:03 +0200
From: "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To: Michael Kelley <mhklinux@...look.com>,
 "Gustavo A. R. Silva" <gustavoars@...nel.org>,
 "K. Y. Srinivasan" <kys@...rosoft.com>,
 Haiyang Zhang <haiyangz@...rosoft.com>, Wei Liu <wei.liu@...nel.org>,
 Dexuan Cui <decui@...rosoft.com>
Cc: "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 "linux-hardening@...r.kernel.org" <linux-hardening@...r.kernel.org>
Subject: Re: [PATCH v2][next] hyperv: Avoid a hundred
 -Wflex-array-member-not-at-end warnings



On 8/29/25 18:07, Michael Kelley wrote:
> From: Gustavo A. R. Silva <gustavoars@...nel.org> Sent: Friday, August 29, 2025 4:44 AM
>>
>> -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 159 of the following type
>> of warnings:
>>
>>      159 ./include/linux/hyperv.h:711:38: 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 members onto the FAM while preserving the original
>> memory layout.
>>
>> Also, move `struct vmbus_close_msg close_msg;` at the end of
>> `struct vmbus_channel`, as `struct vmbus_channel_msginfo,` ends
>> in a flexible array member.
>>
>> Signed-off-by: Gustavo A. R. Silva <gustavoars@...nel.org>
>> --
>> Changes in v2:
>>   - Fix subject line.
>>
>> v1:
>>   - Link: https://lore.kernel.org/linux-hardening/aLGSDpi4xDjUUYVm@kspp/
>>
>>   include/linux/hyperv.h | 10 ++++++----
>>   1 file changed, 6 insertions(+), 4 deletions(-)
>>
>> diff --git a/include/linux/hyperv.h b/include/linux/hyperv.h
>> index a59c5c3e95fb..efdd570669fa 100644
>> --- a/include/linux/hyperv.h
>> +++ b/include/linux/hyperv.h
>> @@ -708,8 +708,9 @@ struct vmbus_channel_msginfo {
>>   };
>>
>>   struct vmbus_close_msg {
>> -	struct vmbus_channel_msginfo info;
> 
> It turns out that this field of struct vmbus_close_msg is never used.
> It dates back to 2011, so maybe somewhere along the way it stopped
> being used, but struct vmbus_close_msg was left unchanged.
> 
> So a better solution to the "flex-array-member-not-at-end" issue is
> to eliminate this structure entirely, and use struct
> vmbus_channel_close_channel directly in the one place where
> struct vmbus_close_msg is currently used. I've done a quick test of
> this change and I don't see any problems.
> 
> I'll submit a separate patch with my proposed change, and this
> patch can be dropped. Does that work?
Yes, please. Go for it. :)

Thanks!
-Gustavo