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

From: Bobby Eshleman <[email-masked]>
Date: Wed, 26 Jul 2023 17:55:04 +0000

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 <[email-masked]>
> > ---
> >  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.

> 
> > +	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 <[email-masked]> 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