On 2020/4/6 下午9:55, Michael S. Tsirkin wrote:
On Mon, Apr 06, 2020 at 09:34:00PM +0800, Jason Wang wrote:
On 2020/4/6 下午8:50, Michael S. Tsirkin wrote:
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?
I'm not sure if it's too late to fix. It would still be still problematic
for the userspace that is using old uapi headers?
Thanks
It's not a problem in userspace. The problem is when
userspace/guest uses 2 byte alignment and passes it to kernel
assuming 8 byte alignment. The fix is for host not to
make these assumptions.
Yes, but I meant when userspace is complied with apcs-gnu, then it still
assumes 8 byte alignment?
Thanks
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)
{
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