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Message-ID: <127bd529-45a0-47bd-8364-5e99144cb773@embeddedor.com>
Date: Wed, 3 Dec 2025 14:10:19 +0900
From: "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To: "Gustavo A. R. Silva" <gustavoars@...nel.org>,
Paolo Bonzini <pbonzini@...hat.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
linux-hardening@...r.kernel.org
Subject: Re: [PATCH][next] KVM: Avoid a few dozen
-Wflex-array-member-not-at-end warnings
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@...nel.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];
> };
>
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