netdev - Re: [PATCH] vsock/virtio: add support for MSG

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Date:   Thu, 26 Sep 2019 12:33:36 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Matias Ezequiel Vara Larsen <matiasevara@...il.com>,
        stefanha@...hat.com
Cc:     davem@...emloft.net, kvm@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, sgarzare@...hat.com
Subject: Re: [PATCH] vsock/virtio: add support for MSG_PEEK



On 9/26/19 11:23 AM, Matias Ezequiel Vara Larsen wrote:
> This patch adds support for MSG_PEEK. In such a case, packets are not
> removed from the rx_queue and credit updates are not sent.
> 
> Signed-off-by: Matias Ezequiel Vara Larsen <matiasevara@...il.com>
> ---
>  net/vmw_vsock/virtio_transport_common.c | 50 +++++++++++++++++++++++++++++++--
>  1 file changed, 47 insertions(+), 3 deletions(-)
> 
> diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
> index 94cc0fa..938f2ed 100644
> --- a/net/vmw_vsock/virtio_transport_common.c
> +++ b/net/vmw_vsock/virtio_transport_common.c
> @@ -264,6 +264,50 @@ static int virtio_transport_send_credit_update(struct vsock_sock *vsk,
>  }
>  
>  static ssize_t
> +virtio_transport_stream_do_peek(struct vsock_sock *vsk,
> +				struct msghdr *msg,
> +				size_t len)
> +{
> +	struct virtio_vsock_sock *vvs = vsk->trans;
> +	struct virtio_vsock_pkt *pkt;
> +	size_t bytes, total = 0;
> +	int err = -EFAULT;
> +
> +	spin_lock_bh(&vvs->rx_lock);
> +
> +	list_for_each_entry(pkt, &vvs->rx_queue, list) {
> +		if (total == len)
> +			break;
> +
> +		bytes = len - total;
> +		if (bytes > pkt->len - pkt->off)
> +			bytes = pkt->len - pkt->off;
> +
> +		/* sk_lock is held by caller so no one else can dequeue.
> +		 * Unlock rx_lock since memcpy_to_msg() may sleep.
> +		 */
> +		spin_unlock_bh(&vvs->rx_lock);
> +
> +		err = memcpy_to_msg(msg, pkt->buf + pkt->off, bytes);
> +		if (err)
> +			goto out;
> +
> +		spin_lock_bh(&vvs->rx_lock);
> +
> +		total += bytes;
> +	}
> +
> +	spin_unlock_bh(&vvs->rx_lock);
> +
> +	return total;
> +
> +out:
> +	if (total)
> +		err = total;
> +	return err;
> +}
>

This seems buggy to me.

virtio_transport_recv_enqueue() seems to be able to add payload to the last packet in the queue.

The loop you wrote here would miss newly added chunks while the vvs->rx_lock spinlock has been released.

virtio_transport_stream_do_dequeue() is ok, because it makes sure pkt->off == pkt->len
before cleaning the packet from the queue.