netdev - Re: [PATCH net-next v5 4/4] vsock/virtio: MSG

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Date: Tue, 1 Aug 2023 17:06:56 +0300
From: Arseniy Krasnov <avkrasnov@...rdevices.ru>
To: Paolo Abeni <pabeni@...hat.com>, Stefan Hajnoczi <stefanha@...hat.com>,
	Stefano Garzarella <sgarzare@...hat.com>, "David S. Miller"
	<davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski
	<kuba@...nel.org>, "Michael S. Tsirkin" <mst@...hat.com>, Jason Wang
	<jasowang@...hat.com>, Bobby Eshleman <bobby.eshleman@...edance.com>
CC: <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: [PATCH net-next v5 4/4] vsock/virtio: MSG_ZEROCOPY flag support



On 01.08.2023 17:04, Paolo Abeni wrote:
> On Tue, 2023-08-01 at 16:36 +0300, Arseniy Krasnov wrote:
>>
>> On 01.08.2023 16:34, Paolo Abeni wrote:
>>> On Sun, 2023-07-30 at 11:59 +0300, Arseniy Krasnov wrote:
>>>> +static int virtio_transport_fill_skb(struct sk_buff *skb,
>>>> +				     struct virtio_vsock_pkt_info *info,
>>>> +				     size_t len,
>>>> +				     bool zcopy)
>>>> +{
>>>> +	if (zcopy) {
>>>> +		return __zerocopy_sg_from_iter(info->msg, NULL, skb,
>>>> +					      &info->msg->msg_iter,
>>>> +					      len);
>>>> +	} else {
>>>
>>>
>>> No need for an else statement after 'return'
>>>
>>>> +		void *payload;
>>>> +		int err;
>>>> +
>>>> +		payload = skb_put(skb, len);
>>>> +		err = memcpy_from_msg(payload, info->msg, len);
>>>> +		if (err)
>>>> +			return -1;
>>>> +
>>>> +		if (msg_data_left(info->msg))
>>>> +			return 0;
>>>> +
>>>
>>> This path does not update truesize, evem if it increases the skb len...
>>
>> Thanks, I'll fix it.
>>
>>>
>>>> +		return 0;
>>>> +	}
>>>> +}
>>>
>>> [...]
>>>
>>>> @@ -214,6 +251,70 @@ static u16 virtio_transport_get_type(struct sock *sk)
>>>>  		return VIRTIO_VSOCK_TYPE_SEQPACKET;
>>>>  }
>>>>  
>>>> +static struct sk_buff *virtio_transport_alloc_skb(struct vsock_sock *vsk,
>>>> +						  struct virtio_vsock_pkt_info *info,
>>>> +						  size_t payload_len,
>>>> +						  bool zcopy,
>>>> +						  u32 src_cid,
>>>> +						  u32 src_port,
>>>> +						  u32 dst_cid,
>>>> +						  u32 dst_port)
>>>> +{
>>>> +	struct sk_buff *skb;
>>>> +	size_t skb_len;
>>>> +
>>>> +	skb_len = VIRTIO_VSOCK_SKB_HEADROOM;
>>>> +
>>>> +	if (!zcopy)
>>>> +		skb_len += payload_len;
>>>> +
>>>> +	skb = virtio_vsock_alloc_skb(skb_len, GFP_KERNEL);
>>>> +	if (!skb)
>>>> +		return NULL;
>>>> +
>>>> +	virtio_transport_init_hdr(skb, info, src_cid, src_port,
>>>> +				  dst_cid, dst_port,
>>>> +				  payload_len);
>>>> +
>>>> +	/* Set owner here, because '__zerocopy_sg_from_iter()' uses
>>>> +	 * owner of skb without check to update 'sk_wmem_alloc'.
>>>> +	 */
>>>> +	if (vsk)
>>>> +		skb_set_owner_w(skb, sk_vsock(vsk));
>>>
>>> ... which can lead to bad things(TM) if the skb goes trough some later
>>> non trivial processing, due to the above skb_set_owner_w().
>>>
>>> Additionally can be the following condition be true:
>>>
>>> 	vsk == NULL && (info->msg && payload_len > 0) && zcopy
>>>
>>> ???
>>
>> No, vsk == NULL only when we reset connection, in that case both info->msg == NULL and payload_len == 0,
>> as this is control message without any data.
> 
> Perhaps a comment with possibly even a WARN_ON_ONCE(!<the above>) could
> help ;)

Ack

Thanks, Arseniy

> 
> Thanks!
> 
> Paolo
>