FAILED: patch "[PATCH] KVM: x86: Use __DECLARE_FLEX_ARRAY() for UAPI structures with" failed to apply to 6.18-stable tree

PATCH In review [email protected] 3 participants 5 messages Apr 13 – Apr 15

Subsystems (4)

Stable branch x86 KVM KVM x86

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[email protected]

author maintainer

2026-04-13 12:08 UTC #0 lore raw

The patch below does not apply to the 6.18-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.18.y
git checkout FETCH_HEAD
git cherry-pick -x 2619da73bb2f10d88f7e1087125c40144fdf0987
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2026041317-engaged-onset-3db3@gregkh' --subject-prefix 'PATCH 6.18.y' HEAD^..

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 2619da73bb2f10d88f7e1087125c40144fdf0987 Mon Sep 17 00:00:00 2001
From: David Woodhouse <[email protected]>
Date: Thu, 5 Mar 2026 20:49:55 +0100
Subject: [PATCH] KVM: x86: Use __DECLARE_FLEX_ARRAY() for UAPI structures with
VLAs

Commit 94dfc73e7cf4 ("treewide: uapi: Replace zero-length arrays with flexible-array members") broke the userspace API for C++.

These structures ending in VLAs are typically a *header*, which can be followed by an arbitrary number of entries. Userspace typically creates a larger structure with some non-zero number of entries, for example in QEMU's kvm_arch_get_supported_msr_feature():

    struct {
        struct kvm_msrs info;
        struct kvm_msr_entry entries[1];
    } msr_data = {};

While that works in C, it fails in C++ with an error like:
flexible array member 'kvm_msrs::entries' not at end of 'struct msr_data'

Fix this by using __DECLARE_FLEX_ARRAY() for the VLA, which uses [0] for C++ compilation.

Fixes 94dfc73e7cf4 ("treewide: uapi: Replace zero-length arrays with flexible-array members")

Cc [email protected]

Signed-off-by David Woodhouse <[email protected]>

Link https://patch.msgid.link/[email protected] open here

[sean: tag for stable@]

Signed-off-by Sean Christopherson <[email protected]>

arch/x86/include/uapi/asm/kvm.h +6 -6

index 0d4538fa6c31..5f2b30d0405c 100644
@@ -197,13 +197,13 @@ struct kvm_msrs {
 	__u32 nmsrs; /* number of msrs in entries */
 	__u32 pad;
 
-	struct kvm_msr_entry entries[];
+	__DECLARE_FLEX_ARRAY(struct kvm_msr_entry, entries);
 };
 
 /* for KVM_GET_MSR_INDEX_LIST */
 struct kvm_msr_list {
 	__u32 nmsrs; /* number of msrs in entries */
-	__u32 indices[];
+	__DECLARE_FLEX_ARRAY(__u32, indices);
 };
 
 /* Maximum size of any access bitmap in bytes */

@@ -245,7 +245,7 @@ struct kvm_cpuid_entry {
 struct kvm_cpuid {
 	__u32 nent;
 	__u32 padding;
-	struct kvm_cpuid_entry entries[];
+	__DECLARE_FLEX_ARRAY(struct kvm_cpuid_entry, entries);
 };
 
 struct kvm_cpuid_entry2 {

@@ -267,7 +267,7 @@ struct kvm_cpuid_entry2 {
 struct kvm_cpuid2 {
 	__u32 nent;
 	__u32 padding;
-	struct kvm_cpuid_entry2 entries[];
+	__DECLARE_FLEX_ARRAY(struct kvm_cpuid_entry2, entries);
 };
 
 /* for KVM_GET_PIT and KVM_SET_PIT */

@@ -398,7 +398,7 @@ struct kvm_xsave {
 	 * the contents of CPUID leaf 0xD on the host.
 	 */
 	__u32 region[1024];
-	__u32 extra[];
+	__DECLARE_FLEX_ARRAY(__u32, extra);
 };
 
 #define KVM_MAX_XCRS	16

@@ -566,7 +566,7 @@ struct kvm_pmu_event_filter {
 	__u32 fixed_counter_bitmap;
 	__u32 flags;
 	__u32 pad[4];
-	__u64 events[];
+	__DECLARE_FLEX_ARRAY(__u64, events);
 };
 
 #define KVM_PMU_EVENT_ALLOW 0

include/uapi/linux/kvm.h +6 -5

index 80364d4dbebb..3f0d8d3c3daf 100644
@@ -11,6 +11,7 @@
 #include <linux/const.h>
 #include <linux/types.h>
 #include <linux/compiler.h>
+#include <linux/stddef.h>
 #include <linux/ioctl.h>
 #include <asm/kvm.h>

@@ -542,7 +543,7 @@ struct kvm_coalesced_mmio {
 
 struct kvm_coalesced_mmio_ring {
 	__u32 first, last;
-	struct kvm_coalesced_mmio coalesced_mmio[];
+	__DECLARE_FLEX_ARRAY(struct kvm_coalesced_mmio, coalesced_mmio);
 };
 
 #define KVM_COALESCED_MMIO_MAX \

@@ -592,7 +593,7 @@ struct kvm_clear_dirty_log {
 /* for KVM_SET_SIGNAL_MASK */
 struct kvm_signal_mask {
 	__u32 len;
-	__u8  sigset[];
+	__DECLARE_FLEX_ARRAY(__u8, sigset);
 };
 
 /* for KVM_TPR_ACCESS_REPORTING */

@@ -1051,7 +1052,7 @@ struct kvm_irq_routing_entry {
 struct kvm_irq_routing {
 	__u32 nr;
 	__u32 flags;
-	struct kvm_irq_routing_entry entries[];
+	__DECLARE_FLEX_ARRAY(struct kvm_irq_routing_entry, entries);
 };
 
 #define KVM_IRQFD_FLAG_DEASSIGN (1 << 0)

@@ -1142,7 +1143,7 @@ struct kvm_dirty_tlb {
 
 struct kvm_reg_list {
 	__u64 n; /* number of regs */
-	__u64 reg[];
+	__DECLARE_FLEX_ARRAY(__u64, reg);
 };
 
 struct kvm_one_reg {

@@ -1608,7 +1609,7 @@ struct kvm_stats_desc {
 #ifdef __KERNEL__
 	char name[KVM_STATS_NAME_SIZE];
 #else
-	char name[];
+	__DECLARE_FLEX_ARRAY(char, name);
 #endif
 };

Sasha Levin

maintainer

2026-04-13 13:01 UTC #1 lore raw

PATCH 1/2 KVM: Remove subtle "struct kvm_stats_desc" pseudo-overlay

From: Sean Christopherson <[email protected]>

[ Upstream commit da142f3d373a6ddaca0119615a8db2175ddc4121 ]

Remove KVM's internal pseudo-overlay of kvm_stats_desc, which subtly aliases the flexible name[] in the uAPI definition with a fixed-size array of the same name. The unusual embedded structure results in compiler warnings due to -Wflex-array-member-not-at-end, and also necessitates an extra level of dereferencing in KVM. To avoid the "overlay", define the uAPI structure to have a fixed-size name when building for the kernel.

Opportunistically clean up the indentation for the stats macros, and replace spaces with tabs.

No functional change intended.

Reported-by Gustavo A. R. Silva <[email protected]>

Closes https://lore.kernel.org/all/aPfNKRpLfhmhYqfP@kspp open here

Acked-by Marc Zyngier <[email protected]>

Acked-by Christian Borntraeger <[email protected]>

[..]

Acked-by Anup Patel <[email protected]>

Reviewed-by Bibo Mao <[email protected]>

Acked-by Gustavo A. R. Silva <[email protected]>

Link https://patch.msgid.link/[email protected] open here

Signed-off-by Sean Christopherson <[email protected]>

Stable-dep-of: 2619da73bb2f ("KVM: x86: Use __DECLARE_FLEX_ARRAY() for UAPI structures with VLAs")

Signed-off-by Sasha Levin <[email protected]>

---
 arch/arm64/kvm/guest.c    |  4 +-
 arch/loongarch/kvm/vcpu.c |  2 +-
 arch/loongarch/kvm/vm.c   |  2 +-
 arch/mips/kvm/mips.c      |  4 +-
 arch/powerpc/kvm/book3s.c |  4 +-
 arch/powerpc/kvm/booke.c  |  4 +-
 arch/riscv/kvm/vcpu.c     |  2 +-
 arch/riscv/kvm/vm.c       |  2 +-
 arch/s390/kvm/kvm-s390.c  |  4 +-
 arch/x86/kvm/x86.c        |  4 +-
 include/linux/kvm_host.h  | 83 +++++++++++++++++----------------------
 include/uapi/linux/kvm.h  |  8 ++++
 virt/kvm/binary_stats.c   |  2 +-
 virt/kvm/kvm_main.c       | 20 +++++-----
 14 files changed, 70 insertions(+), 75 deletions(-)

arch/arm64/kvm/guest.c +2 -2

index 1c87699fd886e..332c453b87cf8 100644
@@ -29,7 +29,7 @@
 
 #include "trace.h"
 
-const struct _kvm_stats_desc kvm_vm_stats_desc[] = {
+const struct kvm_stats_desc kvm_vm_stats_desc[] = {
 	KVM_GENERIC_VM_STATS()
 };

@@ -42,7 +42,7 @@ const struct kvm_stats_header kvm_vm_stats_header = {
 		       sizeof(kvm_vm_stats_desc),
 };
 
-const struct _kvm_stats_desc kvm_vcpu_stats_desc[] = {
+const struct kvm_stats_desc kvm_vcpu_stats_desc[] = {
 	KVM_GENERIC_VCPU_STATS(),
 	STATS_DESC_COUNTER(VCPU, hvc_exit_stat),
 	STATS_DESC_COUNTER(VCPU, wfe_exit_stat),

arch/loongarch/kvm/vcpu.c +1 -1

index fbe12a129c604..14bd36597ab33 100644
@@ -13,7 +13,7 @@
 #define CREATE_TRACE_POINTS
 #include "trace.h"
 
-const struct _kvm_stats_desc kvm_vcpu_stats_desc[] = {
+const struct kvm_stats_desc kvm_vcpu_stats_desc[] = {
 	KVM_GENERIC_VCPU_STATS(),
 	STATS_DESC_COUNTER(VCPU, int_exits),
 	STATS_DESC_COUNTER(VCPU, idle_exits),

arch/loongarch/kvm/vm.c +1 -1

index a49b1c1a3dd1f..85246a70d95d3 100644
@@ -9,7 +9,7 @@
 #include <asm/kvm_eiointc.h>
 #include <asm/kvm_pch_pic.h>
 
-const struct _kvm_stats_desc kvm_vm_stats_desc[] = {
+const struct kvm_stats_desc kvm_vm_stats_desc[] = {
 	KVM_GENERIC_VM_STATS(),
 	STATS_DESC_ICOUNTER(VM, pages),
 	STATS_DESC_ICOUNTER(VM, hugepages),

arch/mips/kvm/mips.c +2 -2

index a75587018f447..e0311ca3f2e18 100644
@@ -38,7 +38,7 @@
 #define VECTORSPACING 0x100	/* for EI/VI mode */
 #endif
 
-const struct _kvm_stats_desc kvm_vm_stats_desc[] = {
+const struct kvm_stats_desc kvm_vm_stats_desc[] = {
 	KVM_GENERIC_VM_STATS()
 };

@@ -51,7 +51,7 @@ const struct kvm_stats_header kvm_vm_stats_header = {
 		       sizeof(kvm_vm_stats_desc),
 };
 
-const struct _kvm_stats_desc kvm_vcpu_stats_desc[] = {
+const struct kvm_stats_desc kvm_vcpu_stats_desc[] = {
 	KVM_GENERIC_VCPU_STATS(),
 	STATS_DESC_COUNTER(VCPU, wait_exits),
 	STATS_DESC_COUNTER(VCPU, cache_exits),

arch/powerpc/kvm/book3s.c +2 -2

index d79c5d1098c05..2efbe05caed76 100644
@@ -38,7 +38,7 @@
 
 /* #define EXIT_DEBUG */
 
-const struct _kvm_stats_desc kvm_vm_stats_desc[] = {
+const struct kvm_stats_desc kvm_vm_stats_desc[] = {
 	KVM_GENERIC_VM_STATS(),
 	STATS_DESC_ICOUNTER(VM, num_2M_pages),
 	STATS_DESC_ICOUNTER(VM, num_1G_pages)

@@ -53,7 +53,7 @@ const struct kvm_stats_header kvm_vm_stats_header = {
 		       sizeof(kvm_vm_stats_desc),
 };
 
-const struct _kvm_stats_desc kvm_vcpu_stats_desc[] = {
+const struct kvm_stats_desc kvm_vcpu_stats_desc[] = {
 	KVM_GENERIC_VCPU_STATS(),
 	STATS_DESC_COUNTER(VCPU, sum_exits),
 	STATS_DESC_COUNTER(VCPU, mmio_exits),

arch/powerpc/kvm/booke.c +2 -2

index 3401b96be475e..f3ddb24ece749 100644
@@ -36,7 +36,7 @@
 
 unsigned long kvmppc_booke_handlers;
 
-const struct _kvm_stats_desc kvm_vm_stats_desc[] = {
+const struct kvm_stats_desc kvm_vm_stats_desc[] = {
 	KVM_GENERIC_VM_STATS(),
 	STATS_DESC_ICOUNTER(VM, num_2M_pages),
 	STATS_DESC_ICOUNTER(VM, num_1G_pages)

@@ -51,7 +51,7 @@ const struct kvm_stats_header kvm_vm_stats_header = {
 		       sizeof(kvm_vm_stats_desc),
 };
 
-const struct _kvm_stats_desc kvm_vcpu_stats_desc[] = {
+const struct kvm_stats_desc kvm_vcpu_stats_desc[] = {
 	KVM_GENERIC_VCPU_STATS(),
 	STATS_DESC_COUNTER(VCPU, sum_exits),
 	STATS_DESC_COUNTER(VCPU, mmio_exits),

arch/riscv/kvm/vcpu.c +1 -1

index 5ce35aba60696..d26c4967c20e2 100644
@@ -24,7 +24,7 @@
 #define CREATE_TRACE_POINTS
 #include "trace.h"
 
-const struct _kvm_stats_desc kvm_vcpu_stats_desc[] = {
+const struct kvm_stats_desc kvm_vcpu_stats_desc[] = {
 	KVM_GENERIC_VCPU_STATS(),
 	STATS_DESC_COUNTER(VCPU, ecall_exit_stat),
 	STATS_DESC_COUNTER(VCPU, wfi_exit_stat),

arch/riscv/kvm/vm.c +1 -1

index 66d91ae6e9b2a..715a06ae8c131 100644
@@ -13,7 +13,7 @@
 #include <linux/kvm_host.h>
 #include <asm/kvm_mmu.h>
 
-const struct _kvm_stats_desc kvm_vm_stats_desc[] = {
+const struct kvm_stats_desc kvm_vm_stats_desc[] = {
 	KVM_GENERIC_VM_STATS()
 };
 static_assert(ARRAY_SIZE(kvm_vm_stats_desc) ==

arch/s390/kvm/kvm-s390.c +2 -2

index 16ba04062854a..a0162d03e16b9 100644
@@ -64,7 +64,7 @@
 #define VCPU_IRQS_MAX_BUF (sizeof(struct kvm_s390_irq) * \
 			   (KVM_MAX_VCPUS + LOCAL_IRQS))
 
-const struct _kvm_stats_desc kvm_vm_stats_desc[] = {
+const struct kvm_stats_desc kvm_vm_stats_desc[] = {
 	KVM_GENERIC_VM_STATS(),
 	STATS_DESC_COUNTER(VM, inject_io),
 	STATS_DESC_COUNTER(VM, inject_float_mchk),

@@ -90,7 +90,7 @@ const struct kvm_stats_header kvm_vm_stats_header = {
 		       sizeof(kvm_vm_stats_desc),
 };
 
-const struct _kvm_stats_desc kvm_vcpu_stats_desc[] = {
+const struct kvm_stats_desc kvm_vcpu_stats_desc[] = {
 	KVM_GENERIC_VCPU_STATS(),
 	STATS_DESC_COUNTER(VCPU, exit_userspace),
 	STATS_DESC_COUNTER(VCPU, exit_null),

arch/x86/kvm/x86.c +2 -2

index d15bd078a2d98..4e550400b4e59 100644
@@ -242,7 +242,7 @@ EXPORT_SYMBOL_FOR_KVM_INTERNAL(enable_ipiv);
 bool __read_mostly enable_device_posted_irqs = true;
 EXPORT_SYMBOL_FOR_KVM_INTERNAL(enable_device_posted_irqs);
 
-const struct _kvm_stats_desc kvm_vm_stats_desc[] = {
+const struct kvm_stats_desc kvm_vm_stats_desc[] = {
 	KVM_GENERIC_VM_STATS(),
 	STATS_DESC_COUNTER(VM, mmu_shadow_zapped),
 	STATS_DESC_COUNTER(VM, mmu_pte_write),

@@ -268,7 +268,7 @@ const struct kvm_stats_header kvm_vm_stats_header = {
 		       sizeof(kvm_vm_stats_desc),
 };
 
-const struct _kvm_stats_desc kvm_vcpu_stats_desc[] = {
+const struct kvm_stats_desc kvm_vcpu_stats_desc[] = {
 	KVM_GENERIC_VCPU_STATS(),
 	STATS_DESC_COUNTER(VCPU, pf_taken),
 	STATS_DESC_COUNTER(VCPU, pf_fixed),

include/linux/kvm_host.h +35 ~~-48~~

107 lines not on this page · raw message

include/uapi/linux/kvm.h +8 -0

index 52f6000ab0208..f89e1500962cd 100644
@@ -14,6 +14,10 @@
 #include <linux/ioctl.h>
 #include <asm/kvm.h>
 
+#ifdef __KERNEL__
+#include <linux/kvm_types.h>
+#endif
+
 #define KVM_API_VERSION 12
 
 /*

@@ -1568,7 +1572,11 @@ struct kvm_stats_desc {
 	__u16 size;
 	__u32 offset;
 	__u32 bucket_size;
+#ifdef __KERNEL__
+	char name[KVM_STATS_NAME_SIZE];
+#else
 	char name[];
+#endif
 };
 
 #define KVM_GET_STATS_FD  _IO(KVMIO,  0xce)

virt/kvm/binary_stats.c +1 -1

index eefca6c69f519..76ce697c773bf 100644
@@ -50,7 +50,7 @@
  * Return: the number of bytes that has been successfully read
  */
 ssize_t kvm_stats_read(char *id, const struct kvm_stats_header *header,
-		       const struct _kvm_stats_desc *desc,
+		       const struct kvm_stats_desc *desc,
 		       void *stats, size_t size_stats,
 		       char __user *user_buffer, size_t size, loff_t *offset)
 {

virt/kvm/kvm_main.c +10 ~~-10~~

index 7fea6ba91c1ef..46581554abfb2 100644
@@ -982,9 +982,9 @@ static void kvm_free_memslots(struct kvm *kvm, struct kvm_memslots *slots)
 		kvm_free_memslot(kvm, memslot);
 }
 
-static umode_t kvm_stats_debugfs_mode(const struct _kvm_stats_desc *pdesc)
+static umode_t kvm_stats_debugfs_mode(const struct kvm_stats_desc *desc)
 {
-	switch (pdesc->desc.flags & KVM_STATS_TYPE_MASK) {
+	switch (desc->flags & KVM_STATS_TYPE_MASK) {
 	case KVM_STATS_TYPE_INSTANT:
 		return 0444;
 	case KVM_STATS_TYPE_CUMULATIVE:

@@ -1019,7 +1019,7 @@ static int kvm_create_vm_debugfs(struct kvm *kvm, const char *fdname)
 	struct dentry *dent;
 	char dir_name[ITOA_MAX_LEN * 2];
 	struct kvm_stat_data *stat_data;
-	const struct _kvm_stats_desc *pdesc;
+	const struct kvm_stats_desc *pdesc;
 	int i, ret = -ENOMEM;
 	int kvm_debugfs_num_entries = kvm_vm_stats_header.num_desc +
 				      kvm_vcpu_stats_header.num_desc;

@@ -6160,11 +6160,11 @@ static int kvm_stat_data_get(void *data, u64 *val)
 	switch (stat_data->kind) {
 	case KVM_STAT_VM:
 		r = kvm_get_stat_per_vm(stat_data->kvm,
-					stat_data->desc->desc.offset, val);
+					stat_data->desc->offset, val);
 		break;
 	case KVM_STAT_VCPU:
 		r = kvm_get_stat_per_vcpu(stat_data->kvm,
-					  stat_data->desc->desc.offset, val);
+					  stat_data->desc->offset, val);
 		break;
 	}

@@ -6182,11 +6182,11 @@ static int kvm_stat_data_clear(void *data, u64 val)
 	switch (stat_data->kind) {
 	case KVM_STAT_VM:
 		r = kvm_clear_stat_per_vm(stat_data->kvm,
-					  stat_data->desc->desc.offset);
+					  stat_data->desc->offset);
 		break;
 	case KVM_STAT_VCPU:
 		r = kvm_clear_stat_per_vcpu(stat_data->kvm,
-					    stat_data->desc->desc.offset);
+					    stat_data->desc->offset);
 		break;
 	}

@@ -6334,7 +6334,7 @@ static void kvm_uevent_notify_change(unsigned int type, struct kvm *kvm)
 static void kvm_init_debug(void)
 {
 	const struct file_operations *fops;
-	const struct _kvm_stats_desc *pdesc;
+	const struct kvm_stats_desc *pdesc;
 	int i;
 
 	kvm_debugfs_dir = debugfs_create_dir("kvm", NULL);

@@ -6347,7 +6347,7 @@ static void kvm_init_debug(void)
 			fops = &vm_stat_readonly_fops;
 		debugfs_create_file(pdesc->name, kvm_stats_debugfs_mode(pdesc),
 				kvm_debugfs_dir,
-				(void *)(long)pdesc->desc.offset, fops);
+				(void *)(long)pdesc->offset, fops);
 	}
 
 	for (i = 0; i < kvm_vcpu_stats_header.num_desc; ++i) {

@@ -6358,7 +6358,7 @@ static void kvm_init_debug(void)
 			fops = &vcpu_stat_readonly_fops;
 		debugfs_create_file(pdesc->name, kvm_stats_debugfs_mode(pdesc),
 				kvm_debugfs_dir,
-				(void *)(long)pdesc->desc.offset, fops);
+				(void *)(long)pdesc->offset, fops);
 	}
 }

Signature · 1 line

2.53.0

Sasha Levin

maintainer

2026-04-13 13:01 UTC #2 lore raw

PATCH 2/2 KVM: x86: Use __DECLARE_FLEX_ARRAY() for UAPI structures with VLAs

From: David Woodhouse <[email protected]>

[ Upstream commit 2619da73bb2f10d88f7e1087125c40144fdf0987 ]

Commit 94dfc73e7cf4 ("treewide: uapi: Replace zero-length arrays with flexible-array members") broke the userspace API for C++.

    struct {
        struct kvm_msrs info;
        struct kvm_msr_entry entries[1];
    } msr_data = {};

While that works in C, it fails in C++ with an error like:
flexible array member 'kvm_msrs::entries' not at end of 'struct msr_data'

Fix this by using __DECLARE_FLEX_ARRAY() for the VLA, which uses [0] for C++ compilation.

Fixes 94dfc73e7cf4 ("treewide: uapi: Replace zero-length arrays with flexible-array members")

Cc [email protected]

Signed-off-by David Woodhouse <[email protected]>

Link https://patch.msgid.link/[email protected] open here

[sean: tag for stable@]

Signed-off-by Sean Christopherson <[email protected]>

Signed-off-by Sasha Levin <[email protected]>

---
 arch/x86/include/uapi/asm/kvm.h | 12 ++++++------
 include/uapi/linux/kvm.h        | 11 ++++++-----
 2 files changed, 12 insertions(+), 11 deletions(-)

arch/x86/include/uapi/asm/kvm.h +6 -6

index 42e4e835a7b42..67b3bea44c674 100644
@@ -197,13 +197,13 @@ struct kvm_msrs {
 	__u32 nmsrs; /* number of msrs in entries */
 	__u32 pad;
 
-	struct kvm_msr_entry entries[];
+	__DECLARE_FLEX_ARRAY(struct kvm_msr_entry, entries);
 };
 
 /* for KVM_GET_MSR_INDEX_LIST */
 struct kvm_msr_list {
 	__u32 nmsrs; /* number of msrs in entries */
-	__u32 indices[];
+	__DECLARE_FLEX_ARRAY(__u32, indices);
 };
 
 /* Maximum size of any access bitmap in bytes */

@@ -245,7 +245,7 @@ struct kvm_cpuid_entry {
 struct kvm_cpuid {
 	__u32 nent;
 	__u32 padding;
-	struct kvm_cpuid_entry entries[];
+	__DECLARE_FLEX_ARRAY(struct kvm_cpuid_entry, entries);
 };
 
 struct kvm_cpuid_entry2 {

@@ -267,7 +267,7 @@ struct kvm_cpuid_entry2 {
 struct kvm_cpuid2 {
 	__u32 nent;
 	__u32 padding;
-	struct kvm_cpuid_entry2 entries[];
+	__DECLARE_FLEX_ARRAY(struct kvm_cpuid_entry2, entries);
 };
 
 /* for KVM_GET_PIT and KVM_SET_PIT */

@@ -398,7 +398,7 @@ struct kvm_xsave {
 	 * the contents of CPUID leaf 0xD on the host.
 	 */
 	__u32 region[1024];
-	__u32 extra[];
+	__DECLARE_FLEX_ARRAY(__u32, extra);
 };
 
 #define KVM_MAX_XCRS	16

@@ -564,7 +564,7 @@ struct kvm_pmu_event_filter {
 	__u32 fixed_counter_bitmap;
 	__u32 flags;
 	__u32 pad[4];
-	__u64 events[];
+	__DECLARE_FLEX_ARRAY(__u64, events);
 };
 
 #define KVM_PMU_EVENT_ALLOW 0

include/uapi/linux/kvm.h +6 -5

index f89e1500962cd..954e0511ce91c 100644
@@ -11,6 +11,7 @@
 #include <linux/const.h>
 #include <linux/types.h>
 #include <linux/compiler.h>
+#include <linux/stddef.h>
 #include <linux/ioctl.h>
 #include <asm/kvm.h>

@@ -523,7 +524,7 @@ struct kvm_coalesced_mmio {
 
 struct kvm_coalesced_mmio_ring {
 	__u32 first, last;
-	struct kvm_coalesced_mmio coalesced_mmio[];
+	__DECLARE_FLEX_ARRAY(struct kvm_coalesced_mmio, coalesced_mmio);
 };
 
 #define KVM_COALESCED_MMIO_MAX \

@@ -573,7 +574,7 @@ struct kvm_clear_dirty_log {
 /* for KVM_SET_SIGNAL_MASK */
 struct kvm_signal_mask {
 	__u32 len;
-	__u8  sigset[];
+	__DECLARE_FLEX_ARRAY(__u8, sigset);
 };
 
 /* for KVM_TPR_ACCESS_REPORTING */

@@ -1029,7 +1030,7 @@ struct kvm_irq_routing_entry {
 struct kvm_irq_routing {
 	__u32 nr;
 	__u32 flags;
-	struct kvm_irq_routing_entry entries[];
+	__DECLARE_FLEX_ARRAY(struct kvm_irq_routing_entry, entries);
 };
 
 #define KVM_IRQFD_FLAG_DEASSIGN (1 << 0)

@@ -1120,7 +1121,7 @@ struct kvm_dirty_tlb {
 
 struct kvm_reg_list {
 	__u64 n; /* number of regs */
-	__u64 reg[];
+	__DECLARE_FLEX_ARRAY(__u64, reg);
 };
 
 struct kvm_one_reg {

@@ -1575,7 +1576,7 @@ struct kvm_stats_desc {
 #ifdef __KERNEL__
 	char name[KVM_STATS_NAME_SIZE];
 #else
-	char name[];
+	__DECLARE_FLEX_ARRAY(char, name);
 #endif
 };

Signature · 1 line

2.53.0

Sean Christopherson

maintainer Acked-by on v1 2/2

2026-04-15 22:17 UTC #3 lore raw

Re: [PATCH 6.18.y 2/2] KVM: x86: Use __DECLARE_FLEX_ARRAY() for UAPI structures with VLAs

Quoted 23 lines · Sasha Levin

From: David Woodhouse <[email protected]>
[ Upstream commit 2619da73bb2f10d88f7e1087125c40144fdf0987 ]
Commit 94dfc73e7cf4 ("treewide: uapi: Replace zero-length arrays with flexible-array members") broke the userspace API for C++.
These structures ending in VLAs are typically a *header*, which can be followed by an arbitrary number of entries. Userspace typically creates a larger structure with some non-zero number of entries, for example in QEMU's kvm_arch_get_supported_msr_feature():
    struct {
        struct kvm_msrs info;
        struct kvm_msr_entry entries[1];
    } msr_data = {};
While that works in C, it fails in C++ with an error like:
flexible array member 'kvm_msrs::entries' not at end of 'struct msr_data'
Fix this by using __DECLARE_FLEX_ARRAY() for the VLA, which uses [0] for C++ compilation.

Quoted 8 lines

Fixes 94dfc73e7cf4 ("treewide: uapi: Replace zero-length arrays with flexible-array members")
Cc [email protected]
Signed-off-by David Woodhouse <[email protected]>
Link https://patch.msgid.link/[email protected] open here
[sean: tag for stable@]
Signed-off-by Sean Christopherson <[email protected]>
Signed-off-by Sasha Levin <[email protected]>
---

Acked-by Sean Christopherson <[email protected]>

Sean Christopherson

maintainer Acked-by on v1 1/2

2026-04-15 22:16 UTC #4 lore raw

Re: [PATCH 6.18.y 1/2] KVM: Remove subtle "struct kvm_stats_desc" pseudo-overlay

In reply to Sasha Levin

Quoted 26 lines · Sasha Levin

From: Sean Christopherson <[email protected]>
[ Upstream commit da142f3d373a6ddaca0119615a8db2175ddc4121 ]
Remove KVM's internal pseudo-overlay of kvm_stats_desc, which subtly aliases the flexible name[] in the uAPI definition with a fixed-size array of the same name. The unusual embedded structure results in compiler warnings due to -Wflex-array-member-not-at-end, and also necessitates an extra level of dereferencing in KVM. To avoid the "overlay", define the uAPI structure to have a fixed-size name when building for the kernel.
Opportunistically clean up the indentation for the stats macros, and replace spaces with tabs.
No functional change intended.
Reported-by Gustavo A. R. Silva <[email protected]>
Closes https://lore.kernel.org/all/aPfNKRpLfhmhYqfP@kspp open here
Acked-by Marc Zyngier <[email protected]>
Acked-by Christian Borntraeger <[email protected]>
[..]
Acked-by Anup Patel <[email protected]>
Reviewed-by Bibo Mao <[email protected]>
Acked-by Gustavo A. R. Silva <[email protected]>
Link https://patch.msgid.link/[email protected] open here
Signed-off-by Sean Christopherson <[email protected]>

Quoted 3 lines

Stable-dep-of: 2619da73bb2f ("KVM: x86: Use __DECLARE_FLEX_ARRAY() for UAPI structures with VLAs")
Signed-off-by Sasha Levin <[email protected]>
---

Acked-by Sean Christopherson <[email protected]>