Re: [PATCH][next] KVM: Avoid a few dozen -Wflex-array-member-not-at-end warnings

["Gustavo A. R. Silva" <gustavo@embeddedor.com>]

Hi all,

Friendly ping: who can take this, please? :)

Thanks!
-Gustavo

On 10/22/25 03:12, 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.
> 
> So, in order to avoid ending up with a flexible-array member in the
> middle of multiple other structs, we use the `__struct_group()` helper
> to separate the flexible array from the rest of the members in the
> flexible structure, and use the tagged `struct kvm_stats_desc_hdr`
> instead of `struct kvm_stats_desc`.
> 
> So, with these changes, fix 51 instances of the following type of
> warning:
> 
> 49 ./include/linux/kvm_host.h:1923:31: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
> 1 .../include/linux/kvm_host.h:1923:31: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
> 1 +./include/linux/kvm_host.h:1923:31: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
> 
> Notice that, before and after the changes, struct sizes and member offsets
> remain unchanged:
> 
> BEFORE
> 
> struct kvm_stats_desc {
>          __u32                      flags;                /*     0     4 */
>          __s16                      exponent;             /*     4     2 */
>          __u16                      size;                 /*     6     2 */
>          __u32                      offset;               /*     8     4 */
>          __u32                      bucket_size;          /*    12     4 */
>          char                       name[];               /*    16     0 */
> 
>          /* size: 16, cachelines: 1, members: 6 */
>          /* last cacheline: 16 bytes */
> };
> 
> struct _kvm_stats_desc {
>          struct kvm_stats_desc      desc;                 /*     0    16 */
>          char                       name[48];             /*    16    48 */
> 
>          /* size: 64, cachelines: 1, members: 2 */
> };
> 
> AFTER:
> 
> struct kvm_stats_desc {
>          union {
>                  struct {
>                          __u32      flags;                /*     0     4 */
>                          __s16      exponent;             /*     4     2 */
>                          __u16      size;                 /*     6     2 */
>                          __u32      offset;               /*     8     4 */
>                          __u32      bucket_size;          /*    12     4 */
>                  };                                       /*     0    16 */
>                  struct kvm_stats_desc_hdr __hdr;         /*     0    16 */
>          };                                               /*     0    16 */
>          char                       name[];               /*    16     0 */
> 
>          /* size: 16, cachelines: 1, members: 2 */
>          /* last cacheline: 16 bytes */
> };
> 
> struct _kvm_stats_desc {
>          struct kvm_stats_desc_hdr  desc;                 /*     0    16 */
>          char                       name[48];             /*    16    48 */
> 
>          /* size: 64, cachelines: 1, members: 2 */
> };
> 
> 
> Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
> ---
>   include/uapi/linux/kvm.h | 21 ++++++++++++++++-----
>   include/linux/kvm_host.h |  2 +-
>   2 files changed, 17 insertions(+), 6 deletions(-)
> 
> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> index 6efa98a57ec1..99d13ebc5e82 100644
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
> @@ -14,6 +14,12 @@
>   #include <linux/ioctl.h>
>   #include <asm/kvm.h>
>   
> +#ifdef __KERNEL__
> +#include <linux/stddef.h>       /* for offsetof */
> +#else
> +#include <stddef.h>             /* for offsetof */
> +#endif
> +
>   #define KVM_API_VERSION 12
>   
>   /*
> @@ -1563,13 +1569,18 @@ struct kvm_stats_header {
>    *        &kvm_stats_header->name_size.
>    */
>   struct kvm_stats_desc {
> -	__u32 flags;
> -	__s16 exponent;
> -	__u16 size;
> -	__u32 offset;
> -	__u32 bucket_size;
> +	/* New members MUST be added within the __struct_group() macro below. */
> +	__struct_group(kvm_stats_desc_hdr, __hdr, /* no attrs */,
> +		__u32 flags;
> +		__s16 exponent;
> +		__u16 size;
> +		__u32 offset;
> +		__u32 bucket_size;
> +	);
>   	char name[];
>   };
> +_Static_assert(offsetof(struct kvm_stats_desc, name) == sizeof(struct kvm_stats_desc_hdr),
> +	       "struct member likely outside of __struct_group()");
>   
>   #define KVM_GET_STATS_FD  _IO(KVMIO,  0xce)
>   
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index fa36e70df088..c630991f72be 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -1920,7 +1920,7 @@ struct kvm_stat_data {
>   };
>   
>   struct _kvm_stats_desc {
> -	struct kvm_stats_desc desc;
> +	struct kvm_stats_desc_hdr desc;
>   	char name[KVM_STATS_NAME_SIZE];
>   };
>