Re: [PATCH 1/3] virtio_ring: always warn when descriptor chain exceeds queue size

[Jason Wang]

在 2021/3/18 下午9:52, Connor Kuehl 写道:
 From section 2.6.5.3.1 (Driver Requirements: Indirect Descriptors)
of the virtio spec:

   "A driver MUST NOT create a descriptor chain longer than the Queue
   Size of the device."

This text suggests that the warning should trigger even if
indirect descriptors are in use.


So I think at least the commit log needs some tweak.

For split virtqueue. We had:

2.6.5.2 Driver Requirements: The Virtqueue Descriptor Table

Drivers MUST NOT add a descriptor chain longer than 2^32 bytes in total; 
this implies that loops in the descriptor chain are forbidden!

2.6.5.3.1 Driver Requirements: Indirect Descriptors

A driver MUST NOT create a descriptor chain longer than the Queue Size 
of the device.

If I understand the spec correctly, the check is only needed for a 
single indirect descriptor table?

For packed virtqueue. We had:

2.7.17 Driver Requirements: Scatter-Gather Support

A driver MUST NOT create a descriptor list longer than allowed by the 
device.

A driver MUST NOT create a descriptor list longer than the Queue Size.

2.7.19 Driver Requirements: Indirect Descriptors

A driver MUST NOT create a descriptor chain longer than allowed by the 
device.

So it looks to me the packed part is fine.

Note that if I understand the spec correctly 2.7.17 implies 2.7.19.

Thanks



Reported-by: Stefan Hajnoczi <stefa...@redhat.com>
Signed-off-by: Connor Kuehl <cku...@redhat.com>
---
  drivers/virtio/virtio_ring.c | 7 ++++---
  1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index 71e16b53e9c1..1bc290f9ba13 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -444,11 +444,12 @@ static inline int virtqueue_add_split(struct virtqueue 
*_vq,
  
  	head = vq->free_head;
  
+	WARN_ON_ONCE(total_sg > vq->split.vring.num);
+
        if (virtqueue_use_indirect(_vq, total_sg))
                desc = alloc_indirect_split(_vq, total_sg, gfp);
        else {
                desc = NULL;
-               WARN_ON_ONCE(total_sg > vq->split.vring.num && !vq->indirect);
        }
  
  	if (desc) {
@@ -1118,6 +1119,8 @@ static inline int virtqueue_add_packed(struct virtqueue 
*_vq,
  
  	BUG_ON(total_sg == 0);
  
+	WARN_ON_ONCE(total_sg > vq->packed.vring.num);
+
        if (virtqueue_use_indirect(_vq, total_sg))
                return virtqueue_add_indirect_packed(vq, sgs, total_sg,
                                out_sgs, in_sgs, data, gfp);
@@ -1125,8 +1128,6 @@ static inline int virtqueue_add_packed(struct virtqueue 
*_vq,
        head = vq->packed.next_avail_idx;
        avail_used_flags = vq->packed.avail_used_flags;
  
-	WARN_ON_ONCE(total_sg > vq->packed.vring.num && !vq->indirect);
-
        desc = vq->packed.vring.desc;
        i = head;
        descs_used = total_sg;