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

[Sasha Levin <sashal@kernel.org>]

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