Re: [PATCH RFC net-next v5 11/14] vhost/vsock: implement datagram support

[Arseniy Krasnov]

On 26.07.2023 20:55, Bobby Eshleman wrote:
> On Sat, Jul 22, 2023 at 11:42:38AM +0300, Arseniy Krasnov wrote:
>>
>>
>> On 19.07.2023 03:50, Bobby Eshleman wrote:
>>> This commit implements datagram support for vhost/vsock by teaching
>>> vhost to use the common virtio transport datagram functions.
>>>
>>> If the virtio RX buffer is too small, then the transmission is
>>> abandoned, the packet dropped, and EHOSTUNREACH is added to the socket's
>>> error queue.
>>>
>>> Signed-off-by: Bobby Eshleman <bobby.eshleman@xxxxxxxxxxxxx>
>>> ---
>>>  drivers/vhost/vsock.c    | 62 +++++++++++++++++++++++++++++++++++++++++++++---
>>>  net/vmw_vsock/af_vsock.c |  5 +++-
>>>  2 files changed, 63 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/drivers/vhost/vsock.c b/drivers/vhost/vsock.c
>>> index d5d6a3c3f273..da14260c6654 100644
>>> --- a/drivers/vhost/vsock.c
>>> +++ b/drivers/vhost/vsock.c
>>> @@ -8,6 +8,7 @@
>>>   */
>>>  #include <linux/miscdevice.h>
>>>  #include <linux/atomic.h>
>>> +#include <linux/errqueue.h>
>>>  #include <linux/module.h>
>>>  #include <linux/mutex.h>
>>>  #include <linux/vmalloc.h>
>>> @@ -32,7 +33,8 @@
>>>  enum {
>>>  	VHOST_VSOCK_FEATURES = VHOST_FEATURES |
>>>  			       (1ULL << VIRTIO_F_ACCESS_PLATFORM) |
>>> -			       (1ULL << VIRTIO_VSOCK_F_SEQPACKET)
>>> +			       (1ULL << VIRTIO_VSOCK_F_SEQPACKET) |
>>> +			       (1ULL << VIRTIO_VSOCK_F_DGRAM)
>>>  };
>>>  
>>>  enum {
>>> @@ -56,6 +58,7 @@ struct vhost_vsock {
>>>  	atomic_t queued_replies;
>>>  
>>>  	u32 guest_cid;
>>> +	bool dgram_allow;
>>>  	bool seqpacket_allow;
>>>  };
>>>  
>>> @@ -86,6 +89,32 @@ static struct vhost_vsock *vhost_vsock_get(u32 guest_cid)
>>>  	return NULL;
>>>  }
>>>  
>>> +/* Claims ownership of the skb, do not free the skb after calling! */
>>> +static void
>>> +vhost_transport_error(struct sk_buff *skb, int err)
>>> +{
>>> +	struct sock_exterr_skb *serr;
>>> +	struct sock *sk = skb->sk;
>>> +	struct sk_buff *clone;
>>> +
>>> +	serr = SKB_EXT_ERR(skb);
>>> +	memset(serr, 0, sizeof(*serr));
>>> +	serr->ee.ee_errno = err;
>>> +	serr->ee.ee_origin = SO_EE_ORIGIN_NONE;
>>> +
>>> +	clone = skb_clone(skb, GFP_KERNEL);
>>
>> May for skb which is error carrier we can use 'sock_omalloc()', not 'skb_clone()' ? TCP uses skb
>> allocated by this function as carriers of error structure. I guess 'skb_clone()' also clones data of origin,
>> but i think that there is no need in data as we insert it to error queue of the socket.
>>
>> What do You think?
> 
> IIUC skb_clone() is often used in this scenario so that the user can
> retrieve the error-causing packet from the error queue.  Is there some
> reason we shouldn't do this?
> 
> I'm seeing that the serr bits need to occur on the clone here, not the
> original. I didn't realize the SKB_EXT_ERR() is a skb->cb cast. I'm not
> actually sure how this passes the test case since ->cb isn't cloned.

Ah yes, sorry, You are right, I just confused this case with zerocopy completion
handling - there we allocate "empty" skb which carries completion metadata in its
'cb' field.

Hm, but can't we just reinsert current skb (update it's 'cb' as 'sock_exterr_skb')
to error queue of the socket without cloning it ?

Thanks, Arseniy

> 
>>
>>> +	if (!clone)
>>> +		return;
>>
>> What will happen here 'if (!clone)' ? skb will leak as it was removed from queue?
>>
> 
> Ah yes, true.
> 
>>> +
>>> +	if (sock_queue_err_skb(sk, clone))
>>> +		kfree_skb(clone);
>>> +
>>> +	sk->sk_err = err;
>>> +	sk_error_report(sk);
>>> +
>>> +	kfree_skb(skb);
>>> +}
>>> +
>>>  static void
>>>  vhost_transport_do_send_pkt(struct vhost_vsock *vsock,
>>>  			    struct vhost_virtqueue *vq)
>>> @@ -160,9 +189,15 @@ vhost_transport_do_send_pkt(struct vhost_vsock *vsock,
>>>  		hdr = virtio_vsock_hdr(skb);
>>>  
>>>  		/* If the packet is greater than the space available in the
>>> -		 * buffer, we split it using multiple buffers.
>>> +		 * buffer, we split it using multiple buffers for connectible
>>> +		 * sockets and drop the packet for datagram sockets.
>>>  		 */
>>>  		if (payload_len > iov_len - sizeof(*hdr)) {
>>> +			if (le16_to_cpu(hdr->type) == VIRTIO_VSOCK_TYPE_DGRAM) {
>>> +				vhost_transport_error(skb, EHOSTUNREACH);
>>> +				continue;
>>> +			}
>>> +
>>>  			payload_len = iov_len - sizeof(*hdr);
>>>  
>>>  			/* As we are copying pieces of large packet's buffer to
>>> @@ -394,6 +429,7 @@ static bool vhost_vsock_more_replies(struct vhost_vsock *vsock)
>>>  	return val < vq->num;
>>>  }
>>>  
>>> +static bool vhost_transport_dgram_allow(u32 cid, u32 port);
>>>  static bool vhost_transport_seqpacket_allow(u32 remote_cid);
>>>  
>>>  static struct virtio_transport vhost_transport = {
>>> @@ -410,7 +446,8 @@ static struct virtio_transport vhost_transport = {
>>>  		.cancel_pkt               = vhost_transport_cancel_pkt,
>>>  
>>>  		.dgram_enqueue            = virtio_transport_dgram_enqueue,
>>> -		.dgram_allow              = virtio_transport_dgram_allow,
>>> +		.dgram_allow              = vhost_transport_dgram_allow,
>>> +		.dgram_addr_init          = virtio_transport_dgram_addr_init,
>>>  
>>>  		.stream_enqueue           = virtio_transport_stream_enqueue,
>>>  		.stream_dequeue           = virtio_transport_stream_dequeue,
>>> @@ -443,6 +480,22 @@ static struct virtio_transport vhost_transport = {
>>>  	.send_pkt = vhost_transport_send_pkt,
>>>  };
>>>  
>>> +static bool vhost_transport_dgram_allow(u32 cid, u32 port)
>>> +{
>>> +	struct vhost_vsock *vsock;
>>> +	bool dgram_allow = false;
>>> +
>>> +	rcu_read_lock();
>>> +	vsock = vhost_vsock_get(cid);
>>> +
>>> +	if (vsock)
>>> +		dgram_allow = vsock->dgram_allow;
>>> +
>>> +	rcu_read_unlock();
>>> +
>>> +	return dgram_allow;
>>> +}
>>> +
>>>  static bool vhost_transport_seqpacket_allow(u32 remote_cid)
>>>  {
>>>  	struct vhost_vsock *vsock;
>>> @@ -799,6 +852,9 @@ static int vhost_vsock_set_features(struct vhost_vsock *vsock, u64 features)
>>>  	if (features & (1ULL << VIRTIO_VSOCK_F_SEQPACKET))
>>>  		vsock->seqpacket_allow = true;
>>>  
>>> +	if (features & (1ULL << VIRTIO_VSOCK_F_DGRAM))
>>> +		vsock->dgram_allow = true;
>>> +
>>>  	for (i = 0; i < ARRAY_SIZE(vsock->vqs); i++) {
>>>  		vq = &vsock->vqs[i];
>>>  		mutex_lock(&vq->mutex);
>>> diff --git a/net/vmw_vsock/af_vsock.c b/net/vmw_vsock/af_vsock.c
>>> index e73f3b2c52f1..449ed63ac2b0 100644
>>> --- a/net/vmw_vsock/af_vsock.c
>>> +++ b/net/vmw_vsock/af_vsock.c
>>> @@ -1427,9 +1427,12 @@ int vsock_dgram_recvmsg(struct socket *sock, struct msghdr *msg,
>>>  		return prot->recvmsg(sk, msg, len, flags, NULL);
>>>  #endif
>>>  
>>> -	if (flags & MSG_OOB || flags & MSG_ERRQUEUE)
>>> +	if (unlikely(flags & MSG_OOB))
>>>  		return -EOPNOTSUPP;
>>>  
>>> +	if (unlikely(flags & MSG_ERRQUEUE))
>>> +		return sock_recv_errqueue(sk, msg, len, SOL_VSOCK, 0);
>>> +
>>
>> Sorry, but I get build error here, because SOL_VSOCK in undefined. I think it should be added to
>> include/linux/socket.h and to uapi files also for future use in userspace.
>>
> 
> Strange, I built each patch individually without issue. My base is
> netdev/main with your SOL_VSOCK patch applied. I will look today and see
> if I'm missing something.
> 
>> Also Stefano Garzarella <sgarzare@xxxxxxxxxx> suggested to add define something like VSOCK_RECVERR,
>> in the same way as IP_RECVERR, and use it as last parameter of 'sock_recv_errqueue()'.
>>
> 
> Got it, thanks.
> 
>>>  	transport = vsk->transport;
>>>  
>>>  	/* Retrieve the head sk_buff from the socket's receive queue. */
>>>
>>
>> Thanks, Arseniy
> 
> Thanks,
> Bobby