linux-kernel - [PATCH 1/2] s390/vfio-ap: make mdev_types not look like a fake flex array

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Message-ID: <20250217100614.3043620-2-pasic@linux.ibm.com>
Date: Mon, 17 Feb 2025 11:06:13 +0100
From: Halil Pasic <pasic@...ux.ibm.com>
To: Eric Farman <farman@...ux.ibm.com>,
        Matthew Rosato <mjrosato@...ux.ibm.com>,
        Halil Pasic <pasic@...ux.ibm.com>,
        Vineeth Vijayan <vneethv@...ux.ibm.com>,
        Peter Oberparleiter <oberpar@...ux.ibm.com>,
        Heiko Carstens <hca@...ux.ibm.com>, Vasily Gorbik <gor@...ux.ibm.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Sven Schnelle <svens@...ux.ibm.com>,
        Tony Krowiak <akrowiak@...ux.ibm.com>,
        Jason Herne <jjherne@...ux.ibm.com>,
        Harald Freudenberger <freude@...ux.ibm.com>,
        Holger Dengler <dengler@...ux.ibm.com>, linux-s390@...r.kernel.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Cc: "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        Thorsten Blum <thorsten.blum@...ux.dev>
Subject: [PATCH 1/2] s390/vfio-ap: make mdev_types not look like a fake flex array

The vfio-ap driver and the vfio parent device provided by it
(matrix_dev) support just a single mdev_type, and this is not likely to
change any time soon.  Despite that matrix_dev->mdev_types started out
as a C99 flexible array presumably as a typo, and since the typo messed
up the allocation, commit e2c8cee9f489 ("s390/vfio-ap: Fix memory
allocation for mdev_types array") changed it to an array of size 1. And
to make things worse mdev_types happens to be the last member of struct
ap_matrix_dev.

Now the problem with that is that before C99 the usual way to get
something similar to a flexible array member was to use a trailing array of
size 0 or 1. This is what I called fake flex array. For a while now the
community is trying to get rid of fake flex arrays. And while mdev_types
is not a fake flex array but an array of size one (to match the mdev
interfaces nicer), it can easily be and was mistaken for a fake flex
array.

So, let us make mdev_types a pointer to struct mdev_type and pass in the
address of that pointer as the 4th formal parameter of
mdev_register_parent().

Signed-off-by: Halil Pasic <pasic@...ux.ibm.com>
Reviewed-by: Anthony Krowiak <akrowiak@...ux.ibm.com>
Reviewed-by: Matthew Rosato <mjrosato@...ux.ibm.com>
Reviewed-by: Eric Farman <farman@...ux.ibm.com>
Tested-by: Anthony Krowiak <akrowiak@...ux.ibm.com>

---

I've also considered switching up the order in which the members
mdev_types and mdev_type are defined in  struct ap_matrix_dev but
decided against that because that could look to somebody like
well known mistake that can be made when using fake flex arrays.
---
 drivers/s390/crypto/vfio_ap_ops.c     | 4 ++--
 drivers/s390/crypto/vfio_ap_private.h | 2 +-
 2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/s390/crypto/vfio_ap_ops.c b/drivers/s390/crypto/vfio_ap_ops.c
index a52c2690933f..5212b3863aff 100644
--- a/drivers/s390/crypto/vfio_ap_ops.c
+++ b/drivers/s390/crypto/vfio_ap_ops.c
@@ -2316,10 +2316,10 @@ int vfio_ap_mdev_register(void)
 
 	matrix_dev->mdev_type.sysfs_name = VFIO_AP_MDEV_TYPE_HWVIRT;
 	matrix_dev->mdev_type.pretty_name = VFIO_AP_MDEV_NAME_HWVIRT;
-	matrix_dev->mdev_types[0] = &matrix_dev->mdev_type;
+	matrix_dev->mdev_types = &matrix_dev->mdev_type;
 	ret = mdev_register_parent(&matrix_dev->parent, &matrix_dev->device,
 				   &vfio_ap_matrix_driver,
-				   matrix_dev->mdev_types, 1);
+				   &matrix_dev->mdev_types, 1);
 	if (ret)
 		goto err_driver;
 	return 0;
diff --git a/drivers/s390/crypto/vfio_ap_private.h b/drivers/s390/crypto/vfio_ap_private.h
index 437a161c8659..9d16321777c8 100644
--- a/drivers/s390/crypto/vfio_ap_private.h
+++ b/drivers/s390/crypto/vfio_ap_private.h
@@ -53,7 +53,7 @@ struct ap_matrix_dev {
 	struct mutex guests_lock; /* serializes access to each KVM guest */
 	struct mdev_parent parent;
 	struct mdev_type mdev_type;
-	struct mdev_type *mdev_types[1];
+	struct mdev_type *mdev_types;
 };
 
 extern struct ap_matrix_dev *matrix_dev;
-- 
2.45.2