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Subject[PATCH v8 10/19] vhost: force spec specified alignment on types
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@redhat.com>
---
drivers/vhost/vhost.h | 6 +++---
include/linux/virtio_ring.h | 24 +++++++++++++++++++++---
2 files changed, 24 insertions(+), 6 deletions(-)

diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
index f8403bd46b85..60cab4c78229 100644
--- a/drivers/vhost/vhost.h
+++ b/drivers/vhost/vhost.h
@@ -67,9 +67,9 @@ 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 file *kick;
struct eventfd_ctx *call_ctx;
diff --git a/include/linux/virtio_ring.h b/include/linux/virtio_ring.h
index 11680e74761a..c3f9ca054250 100644
--- a/include/linux/virtio_ring.h
+++ b/include/linux/virtio_ring.h
@@ -60,14 +60,32 @@ static inline void virtio_store_mb(bool weak_barriers,
struct virtio_device;
struct virtqueue;

+/*
+ * 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 __aligned(VRING_DESC_ALIGN_SIZE) vring_desc_t;
+typedef struct vring_avail __aligned(VRING_AVAIL_ALIGN_SIZE) vring_avail_t;
+typedef struct vring_used __aligned(VRING_USED_ALIGN_SIZE) vring_used_t;
+
struct vring {
unsigned int num;

- struct vring_desc *desc;
+ vring_desc_t *desc;

- struct vring_avail *avail;
+ vring_avail_t *avail;

- struct vring_used *used;
+ vring_used_t *used;
};

/*
--
MST
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