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

[Alex Williamson <alwilliamson@nvidia.com>]

On Tue, 17 Mar 2026 10:44:02 +0530
<mhonap@nvidia.com> wrote:

> From: Manish Honap <mhonap@nvidia.com>
> 
> 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@vger.kernel.org
> Reviewed-by: Dave Jiang <dave.jiang@intel.com>
> Reviewed-by: David Matlack <dmatlack@google.com>
> Signed-off-by: Manish Honap <mhonap@nvidia.com>
> ---
> 
> 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@nvidia.com/
> 
>  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
> 

This unnecessarily split the declaration block.  Without objection,
I'll commit this with the following change:

diff --git a/tools/testing/selftests/vfio/lib/vfio_pci_device.c b/tools/testing/selftests/vfio/lib/vfio_pci_device.c
index d306ab81123a..fc75e04ef010 100644
--- a/tools/testing/selftests/vfio/lib/vfio_pci_device.c
+++ b/tools/testing/selftests/vfio/lib/vfio_pci_device.c
@@ -32,9 +32,9 @@ static void vfio_pci_irq_set(struct vfio_pci_device *device,
 {
        u8 buf[sizeof(struct vfio_irq_set) + sizeof(int) * count];
        struct vfio_irq_set *irq = (void *)&buf;
+       int *irq_fds = (void *)&irq->data;
 
        memset(buf, 0, sizeof(buf));
-       int *irq_fds = (void *)&irq->data;
 
        irq->argsz = sizeof(buf);
        irq->flags = VFIO_IRQ_SET_ACTION_TRIGGER;

Thanks,
Alex