From: Gustavo A. R. Silva <[email protected]> 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 <[email protected]>
> --
> 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
>
