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

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

[Gustavo A. R. Silva]

-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@kernel.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;
-	struct vmbus_channel_close_channel msg;
+	TRAILING_OVERLAP(struct vmbus_channel_msginfo, info, msg,
+		struct vmbus_channel_close_channel msg;
+	);
 };
 
 enum vmbus_device_type {
@@ -800,8 +801,6 @@ struct vmbus_channel {
 	struct hv_ring_buffer_info outbound;	/* send to parent */
 	struct hv_ring_buffer_info inbound;	/* receive from parent */
 
-	struct vmbus_close_msg close_msg;
-
 	/* Statistics */
 	u64	interrupts;	/* Host to Guest interrupts */
 	u64	sig_events;	/* Guest to Host events */
@@ -1008,6 +1007,9 @@ struct vmbus_channel {
 
 	/* boolean to control visibility of sysfs for ring buffer */
 	bool ring_sysfs_visible;
+
+	/* Must be last --ends in a flexible-array member. */
+	struct vmbus_close_msg close_msg;
 };
 
 #define lock_requestor(channel, flags)					\
-- 
2.43.0

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

[Michael Kelley]

From: Gustavo A. R. Silva <gustavoars@kernel.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@kernel.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?

Michael


> -	struct vmbus_channel_close_channel msg;
> +	TRAILING_OVERLAP(struct vmbus_channel_msginfo, info, msg,
> +		struct vmbus_channel_close_channel msg;
> +	);
>  };
> 
>  enum vmbus_device_type {
> @@ -800,8 +801,6 @@ struct vmbus_channel {
>  	struct hv_ring_buffer_info outbound;	/* send to parent */
>  	struct hv_ring_buffer_info inbound;	/* receive from parent */
> 
> -	struct vmbus_close_msg close_msg;
> -
>  	/* Statistics */
>  	u64	interrupts;	/* Host to Guest interrupts */
>  	u64	sig_events;	/* Guest to Host events */
> @@ -1008,6 +1007,9 @@ struct vmbus_channel {
> 
>  	/* boolean to control visibility of sysfs for ring buffer */
>  	bool ring_sysfs_visible;
> +
> +	/* Must be last --ends in a flexible-array member. */
> +	struct vmbus_close_msg close_msg;
>  };
> 
>  #define lock_requestor(channel, flags)					\
> --
> 2.43.0
> 

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

[Gustavo A. R. Silva]

On 8/29/25 18:07, Michael Kelley wrote:
> From: Gustavo A. R. Silva <gustavoars@kernel.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@kernel.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