05.05.2026 19:37, Bobby Eshleman wrote:
> On Tue, May 05, 2026 at 06:11:13PM +0200, Stefano Garzarella wrote:
>> On Tue, May 05, 2026 at 07:14:36AM -0700, Eric Dumazet wrote:
>>> On Tue, May 5, 2026 at 6:52 AM Stefano Garzarella <[email protected]>
>>> wrote:
>>>>
>>>> On Thu, Apr 30, 2026 at 12:26:52PM +0000, Eric Dumazet wrote:
>>>>> virtio_transport_inc_rx_pkt() checks vvs->rx_bytes + len > vvs->buf_alloc.
>>>>>
>>>>> virtio_transport_recv_enqueue() skips coalescing for packets
>>>>> with VIRTIO_VSOCK_SEQ_EOM.
>>>>>
>>>>> If fed with packets with len == 0 and VIRTIO_VSOCK_SEQ_EOM,
>>>>> a very large number of packets can be queued
>>>>> because vvs->rx_bytes stays at 0.
>>>>>
>>>>> Fix this by estimating the skb metadata size:
>>>>>
>>>>> (Number of skbs in the queue) * SKB_TRUESIZE(0)
>>>>>
>>>>> Fixes: 077706165717 ("virtio/vsock: don't use skbuff state to account
>>>>> credit")
>>>>> Signed-off-by: Eric Dumazet <[email protected]>
>>>>> Cc: Arseniy Krasnov <[email protected]>
>>>>> Cc: Stefan Hajnoczi <[email protected]>
>>>>> Cc: Stefano Garzarella <[email protected]>
>>>>> Cc: "Michael S. Tsirkin" <[email protected]>
>>>>> Cc: Jason Wang <[email protected]>
>>>>> Cc: Xuan Zhuo <[email protected]>
>>>>> Cc: "Eugenio Pérez" <[email protected]>
>>>>> Cc: [email protected]
>>>>> Cc: [email protected]
>>>>> ---
>>>>> net/vmw_vsock/virtio_transport_common.c | 4 +++-
>>>>> 1 file changed, 3 insertions(+), 1 deletion(-)
>>>>>
>>>>> diff --git a/net/vmw_vsock/virtio_transport_common.c
>>>>> b/net/vmw_vsock/virtio_transport_common.c
>>>>> index
>>>>> 416d533f493d7b07e9c77c43f741d28cfcd0953e..9b8014516f4fb1130ae184635fbba4dfee58bd64
>>>>> 100644
>>>>> --- a/net/vmw_vsock/virtio_transport_common.c
>>>>> +++ b/net/vmw_vsock/virtio_transport_common.c
>>>>> @@ -447,7 +447,9 @@ static int virtio_transport_send_pkt_info(struct
>>>>> vsock_sock *vsk,
>>>>> static bool virtio_transport_inc_rx_pkt(struct virtio_vsock_sock *vvs,
>>>>> u32 len)
>>>>> {
>>>>> - if (vvs->buf_used + len > vvs->buf_alloc)
>>>>> + u64 skb_overhead = (skb_queue_len(&vvs->rx_queue) + 1) *
>>>>> SKB_TRUESIZE(0);
>>>>> +
>>>>> + if (skb_overhead + vvs->buf_used + len > vvs->buf_alloc)
>>>>> return false;
>>>>
>>>> I'm not sure about this fix, I mean that maybe this is incomplete.
>>>> In virtio-vsock, there is a credit mechanism between the two peers:
>>>> https://docs.oasis-open.org/virtio/virtio/v1.3/csd01/virtio-v1.3-csd01.html#x1-4850003
>>>>
>>>> This takes only the payload into account, so it’s true that this problem
>>>> exists; however, perhaps we should also inform the other peer of a lower
>>>> credit balance, otherwise the other peer will believe it has much more
>>>> credit than it actually does, send a large payload, and then the packet
>>>> will be discarded and the data lost (there are no retransmissions,
>>>> etc.).
>>>
>>> I dunno, perhaps revert 077706165717 ("virtio/vsock: don't use skbuff
>>> state to account credit")
>>> and find a better fix then?
>>
>> IIRC the same issue was there before the commit fixed by that one (commit
>> 71dc9ec9ac7d ("virtio/vsock: replace virtio_vsock_pkt with sk_buff")), so
>> not sure about reverting it TBH.
>>
>> CCing Arseniy and Bobby.
Thanks!
>>
>>>
>>> There is always a discrepancy between skb->len and skb->truesize.
>>> You will not be able to announce a 1MB window, and accept one milliion
>>> skb of 1-byte each.
>>>
>>> This kind of contract is broken.
>>>
>>
>> Yep, I agree, but before we start discarding data (and losing it), IMHO we
>> should at least inform the other peer that we're out of space.
>>
>> @Stefan, @Michael, do you think we can do something in the spec to avoid
>> this issue and in some way take into account also the metadata in the
>> credit. I mean to avoid the 1-byte packets flooding.
>>
>> Thanks,
>> Stefano
>>
>>
>
> Indeed the old pre-fix skb code would have the same issue.
>
> I can't think of any way around this without extending the spec.
Hi, thanks, agree with Bobby, that accounting metadata (e.g. skb size here) was
not implemented "by
design" in credit logic - another side of data exchange knows nothing about
that. Also the same
situation was before skb implementation was added by Bobby. So looks like need
to update spec may be.
Thanks!
>
> Best,
> Bobby
