* FAILED: patch "[PATCH] KVM: x86: Use __DECLARE_FLEX_ARRAY() for UAPI structures with" failed to apply to 6.1-stable tree
@ 2026-04-13 12:08 gregkh
2026-04-13 15:20 ` [PATCH 6.1.y] KVM: x86: Use __DECLARE_FLEX_ARRAY() for UAPI structures with VLAs Sasha Levin
0 siblings, 1 reply; 3+ messages in thread
From: gregkh @ 2026-04-13 12:08 UTC (permalink / raw)
To: dwmw, seanjc; +Cc: stable
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 <stable@vger.kernel.org>.
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 '<stable@vger.kernel.org>' --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 <dwmw@amazon.co.uk>
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: stable@vger.kernel.org
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Link: https://patch.msgid.link/3abaf6aefd6e5efeff3b860ac38421d9dec908db.camel@infradead.org
[sean: tag for stable@]
Signed-off-by: Sean Christopherson <seanjc@google.com>
diff --git a/arch/x86/include/uapi/asm/kvm.h b/arch/x86/include/uapi/asm/kvm.h
index 0d4538fa6c31..5f2b30d0405c 100644
--- a/arch/x86/include/uapi/asm/kvm.h
+++ b/arch/x86/include/uapi/asm/kvm.h
@@ -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
diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index 80364d4dbebb..3f0d8d3c3daf 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -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
};
^ permalink raw reply related [flat|nested] 3+ messages in thread
* [PATCH 6.1.y] KVM: x86: Use __DECLARE_FLEX_ARRAY() for UAPI structures with VLAs
2026-04-13 12:08 FAILED: patch "[PATCH] KVM: x86: Use __DECLARE_FLEX_ARRAY() for UAPI structures with" failed to apply to 6.1-stable tree gregkh
@ 2026-04-13 15:20 ` Sasha Levin
2026-04-15 21:58 ` Sean Christopherson
0 siblings, 1 reply; 3+ messages in thread
From: Sasha Levin @ 2026-04-13 15:20 UTC (permalink / raw)
To: stable; +Cc: David Woodhouse, Sean Christopherson, Sasha Levin
From: David Woodhouse <dwmw@amazon.co.uk>
[ 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: stable@vger.kernel.org
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Link: https://patch.msgid.link/3abaf6aefd6e5efeff3b860ac38421d9dec908db.camel@infradead.org
[sean: tag for stable@]
Signed-off-by: Sean Christopherson <seanjc@google.com>
[ applied `__DECLARE_FLEX_ARRAY(char, name)` change directly instead of inside missing `#ifdef __KERNEL__` else branch ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
arch/x86/include/uapi/asm/kvm.h | 12 ++++++------
include/uapi/linux/kvm.h | 11 ++++++-----
2 files changed, 12 insertions(+), 11 deletions(-)
diff --git a/arch/x86/include/uapi/asm/kvm.h b/arch/x86/include/uapi/asm/kvm.h
index 46de10a809ecb..c4ca3fb749e01 100644
--- a/arch/x86/include/uapi/asm/kvm.h
+++ b/arch/x86/include/uapi/asm/kvm.h
@@ -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
diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index 0d5d4419139ae..79b4c194b262f 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -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)
--
2.53.0
^ permalink raw reply related [flat|nested] 3+ messages in thread
* Re: [PATCH 6.1.y] KVM: x86: Use __DECLARE_FLEX_ARRAY() for UAPI structures with VLAs
2026-04-13 15:20 ` [PATCH 6.1.y] KVM: x86: Use __DECLARE_FLEX_ARRAY() for UAPI structures with VLAs Sasha Levin
@ 2026-04-15 21:58 ` Sean Christopherson
0 siblings, 0 replies; 3+ messages in thread
From: Sean Christopherson @ 2026-04-15 21:58 UTC (permalink / raw)
To: Sasha Levin; +Cc: stable, David Woodhouse
On Mon, Apr 13, 2026, Sasha Levin wrote:
> From: David Woodhouse <dwmw@amazon.co.uk>
>
> [ 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: stable@vger.kernel.org
> Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
> Link: https://patch.msgid.link/3abaf6aefd6e5efeff3b860ac38421d9dec908db.camel@infradead.org
> [sean: tag for stable@]
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> [ applied `__DECLARE_FLEX_ARRAY(char, name)` change directly instead of inside missing `#ifdef __KERNEL__` else branch ]
> Signed-off-by: Sasha Levin <sashal@kernel.org>
> ---
Acked-by: Sean Christopherson <seanjc@google.com>
^ permalink raw reply [flat|nested] 3+ messages in thread
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2026-04-13 12:08 FAILED: patch "[PATCH] KVM: x86: Use __DECLARE_FLEX_ARRAY() for UAPI structures with" failed to apply to 6.1-stable tree gregkh
2026-04-13 15:20 ` [PATCH 6.1.y] KVM: x86: Use __DECLARE_FLEX_ARRAY() for UAPI structures with VLAs Sasha Levin
2026-04-15 21:58 ` Sean Christopherson
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