FAILED: patch "[PATCH] KVM: x86: Use __DECLARE_FLEX_ARRAY() for UAPI structures with" failed to apply to 6.1-stable tree

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

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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 6.1-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-6.1.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 '2026041319-perceive-bok-f424@gregkh' --subject-prefix 'PATCH 6.1.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 15:20 UTC #1 lore raw

PATCH 1/1 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]>

[ applied `__DECLARE_FLEX_ARRAY(char, name)` change directly instead of inside missing `#ifdef __KERNEL__` else branch ]

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 46de10a809ecb..c4ca3fb749e01 100644
@@ -198,13 +198,13 @@ 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 */

@@ -241,7 +241,7 @@ 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 {

@@ -263,7 +263,7 @@ 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 */

@@ -394,7 +394,7 @@ 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

@@ -522,7 +522,7 @@ 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 0d5d4419139ae..79b4c194b262f 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>

@@ -556,7 +557,7 @@ 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 \

@@ -635,7 +636,7 @@ 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 */

@@ -1242,7 +1243,7 @@ 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);
 };
 
 #endif

@@ -1362,7 +1363,7 @@ struct kvm_dirty_tlb {
 
 struct kvm_reg_list {
 	__u64 n; /* number of regs */
-	__u64 reg[];
+	__DECLARE_FLEX_ARRAY(__u64, reg);
 };
 
 struct kvm_one_reg {

@@ -2183,7 +2184,7 @@ struct kvm_stats_desc {
 	__u16 size;
 	__u32 offset;
 	__u32 bucket_size;
-	char name[];
+	__DECLARE_FLEX_ARRAY(char, name);
 };
 
 #define KVM_GET_STATS_FD  _IO(KVMIO,  0xce)

Signature · 1 line

2.53.0

Sean Christopherson

maintainer Acked-by on v1 1/1

2026-04-15 21:58 UTC #2 lore raw

Re: [PATCH 6.1.y] KVM: x86: Use __DECLARE_FLEX_ARRAY() for UAPI structures with VLAs

Quoted 31 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.
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]>
[ applied `__DECLARE_FLEX_ARRAY(char, name)` change directly instead of inside missing `#ifdef __KERNEL__` else branch ]
Signed-off-by Sasha Levin <[email protected]>

Quoted 1 line

---

Acked-by Sean Christopherson <[email protected]>