Re: [PATCH net-next v2] virtio_net: Introduce skb_vnet_common_hdr to avoid typecasting

[Feng Liu <feliu@nvidia.com>]

On 2023-08-17 p.m.6:11, Willem de Bruijn wrote:

>>> You mean virtio_net_common_hdr?
>>>
>> It is a typo, will correct it.
>>
will do

>>> I'm not sure I follow the reasoning. Because then hdr_len might be
>>> sizeof(virtio_net_hdr_mrg_rxbuf), but sizeof(virtio_net_common_hdr) is
>>> larger. So the same issue remains?
>>>
>> static int virtnet_probe(struct virtio_device *vdev)
>> {
>> [...]
>>          if (vi->has_rss_hash_report) {
>>                  vi->hdr_len = sizeof(struct virtio_net_hdr_v1_hash); /* hdr_len will
>> be 20 bytes */
>>          }
>>          else if (virtio_has_feature(vdev, VIRTIO_NET_F_MRG_RXBUF) ||
>>                   virtio_has_feature(vdev, VIRTIO_F_VERSION_1))
>>                  vi->hdr_len = sizeof(struct virtio_net_hdr_mrg_rxbuf);
>>          else
>>                  vi->hdr_len = sizeof(struct virtio_net_hdr);
>> [...]
>> }
>>
>> When VIRTIO_NET_F_HASH_REPORT is enabled, hdr_len = 20 (as above); and
>> the size of virtio_net_hdr_mrg_rxbuf is 12, so virtio_net_hdr_mrg_rxbuf
>> is wrong, should use struct virtio_net_common_hdr here.
> 
> I understand in this specific instance. I'm just saying that using sizeof can
> be wrong both in the new and old case.
> 
> This does not fix a real bug, as memcpy just uses hdr_len, which is correct.
> 
Already have used hdr_len in the patch instead of sizeof(*hdr)

>>> Indeed, everywhere this patches replaces the one with the other, you
>>> have to verify that nothing was using sizeof(*hdr). Which would not be
>>> visible from the truncated patch contents itself.
>>>
>> Have checked. Nothing is using sizeof(*hdr).
> 
> Thanks.
:-D

>>
>>>>
>>>> Change log
>>>> v1->v2
>>>> feedback from Willem de Bruijn <willemdebruijn.kernel@gmail.com>
>>>> feedback from Simon Horman <horms@kernel.org>
>>>> 1. change to use net-next tree.
>>>> 2. move skb_vnet_common_hdr inside kernel file instead of the UAPI header.
>>>>
>>>> Signed-off-by: Feng Liu <feliu@nvidia.com>
>>>> Reviewed-by: Jiri Pirko <jiri@nvidia.com>
>>>> ---
>>>>    drivers/net/virtio_net.c | 36 +++++++++++++++++++++++-------------
>>>>    1 file changed, 23 insertions(+), 13 deletions(-)
>>>>
>>>> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
>>>> index 1270c8d23463..03cf744de512 100644
>>>> --- a/drivers/net/virtio_net.c
>>>> +++ b/drivers/net/virtio_net.c
>>>> @@ -303,6 +303,13 @@ struct padded_vnet_hdr {
>>>>           char padding[12];
>>>>    };
>>>>
>>>> +struct virtio_net_common_hdr {
>>>> +       union {
>>>> +               struct virtio_net_hdr_mrg_rxbuf mrg_hdr;
>>>> +               struct virtio_net_hdr_v1_hash hash_v1_hdr;
>>>> +       };
>>>> +};
>>>
>>> Perhaps even add in struct virtio_net_hdr. As that is the original of
>>> the three structs, and all the initial fields overlap.
>>>
>>
>> But I didn't use virtio_net_hdr in this patch, is it redundant to put it
>> here？ what do you think?
> 
> That's true. But if we're going to add a helper to bind together alll the
> virtio variants, then I think it should be there?
> 
> No strong opinion. Leave out if you prefer and no one else speaks up.
> 
will do

>>>> @@ -1577,7 +1585,8 @@ static void receive_buf(struct virtnet_info *vi, struct receive_queue *rq,
>>>>    {
>>>>           struct net_device *dev = vi->dev;
>>>>           struct sk_buff *skb;
>>>> -       struct virtio_net_hdr_mrg_rxbuf *hdr;
>>>> +       struct virtio_net_common_hdr *common_hdr;
>>>> +       struct virtio_net_hdr_mrg_rxbuf *mrg_hdr;
>>>
>>> No more need for this second struct now that we have the union. That's
>>> its whole purpose?
>>
>> Yes, struct virtio_net_hdr_mrg_rxbuf *mrg_hdr is not needed. Writing
>> mrg_hdr here is just to make the code look more concise, such as
>> mrg_hdr->hdr.flags, if mrg_hdr is not used, it should be written as
>> common_hdr->mrg_hdr.hdr.flags, I think it looks too long. what you think?
> 
> If we're going to continue to assign to different structs, then I'm honestly
> not sure how much this patch buys us.
> 
> Adding virtio_net_hdr to the union also shortens the code btw. Then it
> can be common_hdr->hdr.flags
> 
> Also, just a shorter variable name than common_hdr. Fine to call it hdr.
> 
will do

>>
>>>>
>>>>           if (unlikely(len < vi->hdr_len + ETH_HLEN)) {
>>>>                   pr_debug("%s: short packet %i\n", dev->name, len);
>>>> @@ -1597,18 +1606,19 @@ static void receive_buf(struct virtnet_info *vi, struct receive_queue *rq,
>>>>           if (unlikely(!skb))
>>>>                   return;
>>>>
>>>> -       hdr = skb_vnet_hdr(skb);
>>>> +       common_hdr = skb_vnet_common_hdr(skb);
>>>>           if (dev->features & NETIF_F_RXHASH && vi->has_rss_hash_report)
>>>> -               virtio_skb_set_hash((const struct virtio_net_hdr_v1_hash *)hdr, skb);
>>>> +               virtio_skb_set_hash(&common_hdr->hash_v1_hdr, skb);
>>>>
>>>> -       if (hdr->hdr.flags & VIRTIO_NET_HDR_F_DATA_VALID)
>>>> +       mrg_hdr = &common_hdr->mrg_hdr;
>>>> +       if (mrg_hdr->hdr.flags & VIRTIO_NET_HDR_F_DATA_VALID)
>>>>                   skb->ip_summed = CHECKSUM_UNNECESSARY;
>>>>
>>>> -       if (virtio_net_hdr_to_skb(skb, &hdr->hdr,
>>>> +       if (virtio_net_hdr_to_skb(skb, &mrg_hdr->hdr,
>>>>                                     virtio_is_little_endian(vi->vdev))) {
>>>>                   net_warn_ratelimited("%s: bad gso: type: %u, size: %u\n",
>>>> -                                    dev->name, hdr->hdr.gso_type,
>>>> -                                    hdr->hdr.gso_size);
>>>> +                                    dev->name, mrg_hdr->hdr.gso_type,
>>>> +                                    mrg_hdr->hdr.gso_size);
>>>>                   goto frame_err;
>>>>           }
>>>>
>>>> @@ -2105,7 +2115,7 @@ static int xmit_skb(struct send_queue *sq, struct sk_buff *skb)
>>>>           if (can_push)
>>>>                   hdr = (struct virtio_net_hdr_mrg_rxbuf *)(skb->data - hdr_len);
>>>>           else
>>>> -               hdr = skb_vnet_hdr(skb);
>>>> +               hdr = &skb_vnet_common_hdr(skb)->mrg_hdr;
>>>>
>>>>           if (virtio_net_hdr_from_skb(skb, &hdr->hdr,
>>>>                                       virtio_is_little_endian(vi->vdev), false,
>>>> --
>>>> 2.37.1 (Apple Git-137.1)
>>>>