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Message-ID: <aPfNKRpLfhmhYqfP@kspp>
Date: Tue, 21 Oct 2025 19:12:57 +0100
From: "Gustavo A. R. Silva" <gustavoars@...nel.org>
To: Paolo Bonzini <pbonzini@...hat.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
"Gustavo A. R. Silva" <gustavoars@...nel.org>,
linux-hardening@...r.kernel.org
Subject: [PATCH][next] KVM: Avoid a few dozen -Wflex-array-member-not-at-end
warnings
-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];
};
--
2.43.0
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