linux-kernel - Re: [RFC PATCH v2 2/4] virtio/vsock: remove all data from sk

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Message-ID: <20230306161852.4s7qf4qm3fnwjck7@sgarzare-redhat>
Date:   Mon, 6 Mar 2023 17:18:52 +0100
From:   Stefano Garzarella <sgarzare@...hat.com>
To:     Arseniy Krasnov <avkrasnov@...rdevices.ru>
Cc:     Stefan Hajnoczi <stefanha@...hat.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Bobby Eshleman <bobby.eshleman@...edance.com>,
        kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel@...rdevices.ru, oxffffaa@...il.com
Subject: Re: [RFC PATCH v2 2/4] virtio/vsock: remove all data from sk_buff

On Mon, Mar 06, 2023 at 07:00:10PM +0300, Arseniy Krasnov wrote:
>
>
>On 06.03.2023 18:51, Stefano Garzarella wrote:
>> On Mon, Mar 06, 2023 at 06:31:22PM +0300, Arseniy Krasnov wrote:
>>>
>>>
>>> On 06.03.2023 15:08, Stefano Garzarella wrote:
>>>> On Sun, Mar 05, 2023 at 11:07:37PM +0300, Arseniy Krasnov wrote:
>>>>> In case of SOCK_SEQPACKET all sk_buffs are used once - after read some
>>>>> data from it, it will be removed, so user will never read rest of the
>>>>> data. Thus we need to update credit parameters of the socket like whole
>>>>> sk_buff is read - so call 'skb_pull()' for the whole buffer.
>>>>>
>>>>> Fixes: 71dc9ec9ac7d ("virtio/vsock: replace virtio_vsock_pkt with sk_buff")
>>>>> Signed-off-by: Arseniy Krasnov <AVKrasnov@...rdevices.ru>
>>>>> ---
>>>>> net/vmw_vsock/virtio_transport_common.c | 2 +-
>>>>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>>>
>>>> Maybe we could avoid this patch if we directly use pkt_len as I
>>>> suggested in the previous patch.
>>> Hm, may be we can avoid calling 'skb_pull()' here if 'virtio_transport_dec_rx_pkt()'
>>> will use integer argument?
>>
>> Yep, exactly!
>>
>>> Just call 'virtio_transport_dec_rx_pkt(skb->len)'. skb
>>
>> It depends on how we call virtio_transport_inc_rx_pkt(). If we use
>> hdr->len there I would use the same to avoid confusion. Plus that's the
>> value the other peer sent us, so definitely the right value to increase
>> fwd_cnt with. But if skb->len always reflects it, then that's fine.
>i've checked 'virtio_transport_rx_work()', it calls 'virtio_vsock_skb_rx_put()' which
>sets 'skb->len'. Value is used from header, so seems 'skb->len' == 'hdr->len' in this
>case.

Thank you for checking it.

However, I still think it is better to use `hdr->len` (we have to assign 
it to `pkt_len` anyway, as in the proposal I sent for patch 1), 
otherwise we have to go every time to check if skb_* functions touch 
skb->len.

E.g. skb_pull() decrease skb->len, so I'm not sure we can call 
virtio_transport_dec_rx_pkt(skb->len) if we don't remove `skb_pull(skb, 
bytes_to_copy);` inside the loop.

Thanks,
Stefano