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

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Message-ID:
 <SN6PR02MB4157D4C3A66E0563A9071DF1D43AA@SN6PR02MB4157.namprd02.prod.outlook.com>
Date: Fri, 29 Aug 2025 16:07:12 +0000
From: Michael Kelley <mhklinux@...look.com>
To: "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

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?

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
>