Linux Virtualization - Aug 2023 - [PATCH net-next v5 4/4] vsock/virtio: MSG_ZEROCOPY flag support

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...
> +		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

???

If so it looks like skb can go through __zerocopy_sg_from_iter() even
without a prior skb_set_owner_w()...


Cheers,

Paolo