selftests/vfio: Fix VLA initialisation in vfio_pci_irq_set()

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

Posted by mhonap@nvidia.com 3 weeks ago

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.

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/

Fixes: 19faf6fd969c ("vfio: selftests: Add a helper library for VFIO selftests")
Cc: stable@vger.kernel.org
Suggested-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Manish Honap <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

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

Posted by David Matlack 3 weeks ago

On Mon, Mar 16, 2026 at 11:21 AM <mhonap@nvidia.com> wrote:
>
> From: Manish Honap <mhonap@nvidia.com>

Please use "vfio: selftests: " instead of "selftests/vfio:" to match
all the other changes to tools/testing/selftests/vfio.

  $ git log --oneline tools/testing/selftests/vfio

> 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.
>
> 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.

This should go after the "---" below. It does not need to be in the
commit message.

> [1] https://lore.kernel.org/all/20260311203440.752648-1-mhonap@nvidia.com/

I see you also included a new selftest in this series. I'm glad to see
more usage of VFIO selftests!

Can you please Cc me on any future changes that touch
tools/testing/selftests/vfio? Or better yet run
scripts/get_maintainer.pl and it should add me automatically. Thanks.

> Fixes: 19faf6fd969c ("vfio: selftests: Add a helper library for VFIO selftests")
> Cc: stable@vger.kernel.org
> Suggested-by: Dave Jiang <dave.jiang@intel.com>
> Signed-off-by: Manish Honap <mhonap@nvidia.com>

Aside from the commit message nits above:

  Reviewed-by: David Matlack <dmatlack@google.com>

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

Posted by Dave Jiang 3 weeks ago


On 3/16/26 11:20 AM, 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.
> 
> 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/
> 
> Fixes: 19faf6fd969c ("vfio: selftests: Add a helper library for VFIO selftests")
> Cc: stable@vger.kernel.org
> Suggested-by: Dave Jiang <dave.jiang@intel.com>

You can drop this tag. I didn't suggest the changes. It's your patch. :)
 
> Signed-off-by: Manish Honap <mhonap@nvidia.com>

Reviewed-by: Dave Jiang <dave.jiang@intel.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
>