[PATCH] vhost: force spec specified alignment on types

[Michael S. Tsirkin]

The ring element addresses are passed between components with different
alignments assumptions. Thus, if guest/userspace selects a pointer and
host then gets and dereferences it, we might need to decrease the
compiler-selected alignment to prevent compiler on the host from
assuming pointer is aligned.

This actually triggers on ARM with -mabi=apcs-gnu - which is a
deprecated configuration, but it seems safer to handle this
generally.

I verified that the produced binary is exactly identical on x86.

Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
---

This is my preferred way to handle the ARM incompatibility issues
(in preference to kconfig hacks).
I will push this into next now.
Comments?

 drivers/vhost/vhost.h            |  6 ++---
 include/uapi/linux/virtio_ring.h | 41 ++++++++++++++++++++++++--------
 2 files changed, 34 insertions(+), 13 deletions(-)

diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
index cc82918158d2..a67bda9792ec 100644
--- a/drivers/vhost/vhost.h
+++ b/drivers/vhost/vhost.h
@@ -74,9 +74,9 @@ struct vhost_virtqueue {
 	/* The actual ring of buffers. */
 	struct mutex mutex;
 	unsigned int num;
-	struct vring_desc __user *desc;
-	struct vring_avail __user *avail;
-	struct vring_used __user *used;
+	vring_desc_t __user *desc;
+	vring_avail_t __user *avail;
+	vring_used_t __user *used;
 	const struct vhost_iotlb_map *meta_iotlb[VHOST_NUM_ADDRS];
 
 	struct vhost_desc *descs;
diff --git a/include/uapi/linux/virtio_ring.h b/include/uapi/linux/virtio_ring.h
index 559f42e73315..cd6e0b2eaf2f 100644
--- a/include/uapi/linux/virtio_ring.h
+++ b/include/uapi/linux/virtio_ring.h
@@ -118,16 +118,6 @@ struct vring_used {
 	struct vring_used_elem ring[];
 };
 
-struct vring {
-	unsigned int num;
-
-	struct vring_desc *desc;
-
-	struct vring_avail *avail;
-
-	struct vring_used *used;
-};
-
 /* Alignment requirements for vring elements.
  * When using pre-virtio 1.0 layout, these fall out naturally.
  */
@@ -164,6 +154,37 @@ struct vring {
 #define vring_used_event(vr) ((vr)->avail->ring[(vr)->num])
 #define vring_avail_event(vr) (*(__virtio16 *)&(vr)->used->ring[(vr)->num])
 
+/*
+ * The ring element addresses are passed between components with different
+ * alignments assumptions. Thus, we might need to decrease the compiler-selected
+ * alignment, and so must use a typedef to make sure the __aligned attribute
+ * actually takes hold:
+ *
+ * https://gcc.gnu.org/onlinedocs//gcc/Common-Type-Attributes.html#Common-Type-Attributes
+ *
+ * When used on a struct, or struct member, the aligned attribute can only
+ * increase the alignment; in order to decrease it, the packed attribute must
+ * be specified as well. When used as part of a typedef, the aligned attribute
+ * can both increase and decrease alignment, and specifying the packed
+ * attribute generates a warning.
+ */
+typedef struct vring_desc __attribute__((aligned(VRING_DESC_ALIGN_SIZE)))
+	vring_desc_t;
+typedef struct vring_avail __attribute__((aligned(VRING_AVAIL_ALIGN_SIZE)))
+	vring_avail_t;
+typedef struct vring_used __attribute__((aligned(VRING_USED_ALIGN_SIZE)))
+	vring_used_t;
+
+struct vring {
+	unsigned int num;
+
+	vring_desc_t *desc;
+
+	vring_avail_t *avail;
+
+	vring_used_t *used;
+};
+
 static inline void vring_init(struct vring *vr, unsigned int num, void *p,
 			      unsigned long align)
 {
-- 
MST