[PATCH] vfio: selftests: Fix VLA initialisation in vfio_pci_irq_set()

[mhonap]

From: Manish Honap <mhonap@xxxxxxxxxx>

C does not permit an initialiser expression on a variable-length array
(C99 Section 6.7.9 constraint: "The type of the entity to be initialized
shall not be a variable length array type").

vfio_pci_irq_set() declared:

      u8 buf[sizeof(struct vfio_irq_set) + sizeof(int) * count] = {};

where `count` is a runtime function parameter, making `buf` a VLA.

GCC rejects this with (tried with GCC-9.4.0):

      error: variable-sized object may not be initialized

Fix by removing the `= {}` initialiser and inserting an explicit
memset() immediately after the declaration.  memset() on a VLA is
perfectly legal and achieves the same zero-initialisation on all
conforming C implementations.

Fixes: 19faf6fd969c ("vfio: selftests: Add a helper library for VFIO selftests")
Cc: stable@xxxxxxxxxxxxxxx
Reviewed-by: Dave Jiang <dave.jiang@xxxxxxxxx>
Reviewed-by: David Matlack <dmatlack@xxxxxxxxxx>
Signed-off-by: Manish Honap <mhonap@xxxxxxxxxx>
---

This fix is self-contained: it touches only the existing vfio selftest
helper library and carries no dependency on any other patch.  It was
originally included as PATCH 20/20 in the CXL Type-2 VFIO passthrough
RFC series [1] but belongs on the vfio list independently, as noted by
Dave Jiang.

[1] https://lore.kernel.org/all/20260311203440.752648-1-mhonap@xxxxxxxxxx/

 tools/testing/selftests/vfio/lib/vfio_pci_device.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/vfio/lib/vfio_pci_device.c b/tools/testing/selftests/vfio/lib/vfio_pci_device.c
index fac4c0ecadef..3258e814f450 100644
--- a/tools/testing/selftests/vfio/lib/vfio_pci_device.c
+++ b/tools/testing/selftests/vfio/lib/vfio_pci_device.c
@@ -26,8 +26,10 @@
 static void vfio_pci_irq_set(struct vfio_pci_device *device,
 			     u32 index, u32 vector, u32 count, int *fds)
 {
-	u8 buf[sizeof(struct vfio_irq_set) + sizeof(int) * count] = {};
+	u8 buf[sizeof(struct vfio_irq_set) + sizeof(int) * count];
 	struct vfio_irq_set *irq = (void *)&buf;
+
+	memset(buf, 0, sizeof(buf));
 	int *irq_fds = (void *)&irq->data;

 	irq->argsz = sizeof(buf);
--
2.25.1