linux-kernel - Re: [RFC PATCH v1 2/4] virtio/vsock: support MSG_PEEK for SOCK

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Date:   Tue, 27 Jun 2023 07:34:29 +0300
From:   Arseniy Krasnov <avkrasnov@...rdevices.ru>
To:     Stefano Garzarella <sgarzare@...hat.com>
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>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...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 v1 2/4] virtio/vsock: support MSG_PEEK for
 SOCK_SEQPACKET



On 26.06.2023 19:28, Stefano Garzarella wrote:
> On Sun, Jun 18, 2023 at 09:24:49AM +0300, Arseniy Krasnov wrote:
>> This adds support of MSG_PEEK flag for SOCK_SEQPACKET type of socket.
>> Difference with SOCK_STREAM is that this callback returns either length
>> of the message or error.
>>
>> Signed-off-by: Arseniy Krasnov <AVKrasnov@...rdevices.ru>
>> ---
>> net/vmw_vsock/virtio_transport_common.c | 63 +++++++++++++++++++++++--
>> 1 file changed, 60 insertions(+), 3 deletions(-)
>>
>> diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
>> index 2ee40574c339..352d042b130b 100644
>> --- a/net/vmw_vsock/virtio_transport_common.c
>> +++ b/net/vmw_vsock/virtio_transport_common.c
>> @@ -460,6 +460,63 @@ virtio_transport_stream_do_dequeue(struct vsock_sock *vsk,
>>     return err;
>> }
>>
>> +static ssize_t
>> +virtio_transport_seqpacket_do_peek(struct vsock_sock *vsk,
>> +                   struct msghdr *msg)
>> +{
>> +    struct virtio_vsock_sock *vvs = vsk->trans;
>> +    struct sk_buff *skb;
>> +    size_t total, len;
>> +
>> +    spin_lock_bh(&vvs->rx_lock);
>> +
>> +    if (!vvs->msg_count) {
>> +        spin_unlock_bh(&vvs->rx_lock);
>> +        return 0;
>> +    }
>> +
>> +    total = 0;
>> +    len = msg_data_left(msg);
>> +
>> +    skb_queue_walk(&vvs->rx_queue, skb) {
>> +        struct virtio_vsock_hdr *hdr;
>> +
>> +        if (total < len) {
>> +            size_t bytes;
>> +            int err;
>> +
>> +            bytes = len - total;
>> +            if (bytes > skb->len)
>> +                bytes = skb->len;
>> +
>> +            spin_unlock_bh(&vvs->rx_lock);
>> +
>> +            /* sk_lock is held by caller so no one else can dequeue.
>> +             * Unlock rx_lock since memcpy_to_msg() may sleep.
>> +             */
>> +            err = memcpy_to_msg(msg, skb->data, bytes);
>> +            if (err)
>> +                return err;
>> +
>> +            spin_lock_bh(&vvs->rx_lock);
>> +        }
>> +
>> +        total += skb->len;
>> +        hdr = virtio_vsock_hdr(skb);
>> +
>> +        if (le32_to_cpu(hdr->flags) & VIRTIO_VSOCK_SEQ_EOM) {
>> +            if (le32_to_cpu(hdr->flags) & VIRTIO_VSOCK_SEQ_EOR)
>> +                msg->msg_flags |= MSG_EOR;
>> +
>> +            break;
>> +        }
>> +    }
>> +
>> +    spin_unlock_bh(&vvs->rx_lock);
>> +
>> +    return total;
> 
> Should we return the minimum between total and len?

I guess no, because seqpacket dequeue callback always returns length of message,
then, in af_vsock.c we return either number of bytes read or length of message
depending on MSG_TRUNC flags.

Thanks, Arseniy

> 
> Thanks,
> Stefano
> 
>> +}
>> +
>> static int virtio_transport_seqpacket_do_dequeue(struct vsock_sock *vsk,
>>                          struct msghdr *msg,
>>                          int flags)
>> @@ -554,9 +611,9 @@ virtio_transport_seqpacket_dequeue(struct vsock_sock *vsk,
>>                    int flags)
>> {
>>     if (flags & MSG_PEEK)
>> -        return -EOPNOTSUPP;
>> -
>> -    return virtio_transport_seqpacket_do_dequeue(vsk, msg, flags);
>> +        return virtio_transport_seqpacket_do_peek(vsk, msg);
>> +    else
>> +        return virtio_transport_seqpacket_do_dequeue(vsk, msg, flags);
>> }
>> EXPORT_SYMBOL_GPL(virtio_transport_seqpacket_dequeue);
>>
>> -- 
>> 2.25.1
>>
>