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

[Alex Williamson <alex@shazbot.org>]

On Wed, 18 Mar 2026 14:53:23 -0600
Alex Williamson <alwilliamson@nvidia.com> wrote:

> 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;

Applied with this fix to vfio next branch for v7.1.  Thanks,

Alex