Re: [PATCH RFC net-next v5 03/14] af_vsock: support multi-transport datagrams

[Bobby Eshleman]

On Sun, Jul 23, 2023 at 12:53:15AM +0300, Arseniy Krasnov wrote:
> 
> 
> On 19.07.2023 03:50, Bobby Eshleman wrote:
> > This patch adds support for multi-transport datagrams.
> > 
> > This includes:
> > - Per-packet lookup of transports when using sendto(sockaddr_vm)
> > - Selecting H2G or G2H transport using VMADDR_FLAG_TO_HOST and CID in
> >   sockaddr_vm
> > - rename VSOCK_TRANSPORT_F_DGRAM to VSOCK_TRANSPORT_F_DGRAM_FALLBACK
> > - connect() now assigns the transport for (similar to connectible
> >   sockets)
> > 
> > To preserve backwards compatibility with VMCI, some important changes
> > are made. The "transport_dgram" / VSOCK_TRANSPORT_F_DGRAM is changed to
> > be used for dgrams only if there is not yet a g2h or h2g transport that
> > has been registered that can transmit the packet. If there is a g2h/h2g
> > transport for that remote address, then that transport will be used and
> > not "transport_dgram". This essentially makes "transport_dgram" a
> > fallback transport for when h2g/g2h has not yet gone online, and so it
> > is renamed "transport_dgram_fallback". VMCI implements this transport.
> > 
> > The logic around "transport_dgram" needs to be retained to prevent
> > breaking VMCI:
> > 
> > 1) VMCI datagrams existed prior to h2g/g2h and so operate under a
> >    different paradigm. When the vmci transport comes online, it registers
> >    itself with the DGRAM feature, but not H2G/G2H. Only later when the
> >    transport has more information about its environment does it register
> >    H2G or G2H.  In the case that a datagram socket is created after
> >    VSOCK_TRANSPORT_F_DGRAM registration but before G2H/H2G registration,
> >    the "transport_dgram" transport is the only registered transport and so
> >    needs to be used.
> > 
> > 2) VMCI seems to require a special message be sent by the transport when a
> >    datagram socket calls bind(). Under the h2g/g2h model, the transport
> >    is selected using the remote_addr which is set by connect(). At
> >    bind time there is no remote_addr because often no connect() has been
> >    called yet: the transport is null. Therefore, with a null transport
> >    there doesn't seem to be any good way for a datagram socket to tell the
> >    VMCI transport that it has just had bind() called upon it.
> > 
> > With the new fallback logic, after H2G/G2H comes online the socket layer
> > will access the VMCI transport via transport_{h2g,g2h}. Prior to H2G/G2H
> > coming online, the socket layer will access the VMCI transport via
> > "transport_dgram_fallback".
> > 
> > Only transports with a special datagram fallback use-case such as VMCI
> > need to register VSOCK_TRANSPORT_F_DGRAM_FALLBACK.
> > 
> > Signed-off-by: Bobby Eshleman <bobby.eshleman@xxxxxxxxxxxxx>
> > ---
> >  drivers/vhost/vsock.c                   |  1 -
> >  include/linux/virtio_vsock.h            |  2 --
> >  include/net/af_vsock.h                  | 10 +++---
> >  net/vmw_vsock/af_vsock.c                | 64 ++++++++++++++++++++++++++-------
> >  net/vmw_vsock/hyperv_transport.c        |  6 ----
> >  net/vmw_vsock/virtio_transport.c        |  1 -
> >  net/vmw_vsock/virtio_transport_common.c |  7 ----
> >  net/vmw_vsock/vmci_transport.c          |  2 +-
> >  net/vmw_vsock/vsock_loopback.c          |  1 -
> >  9 files changed, 58 insertions(+), 36 deletions(-)
> > 
> > diff --git a/drivers/vhost/vsock.c b/drivers/vhost/vsock.c
> > index ae8891598a48..d5d6a3c3f273 100644
> > --- a/drivers/vhost/vsock.c
> > +++ b/drivers/vhost/vsock.c
> > @@ -410,7 +410,6 @@ static struct virtio_transport vhost_transport = {
> >  		.cancel_pkt               = vhost_transport_cancel_pkt,
> >  
> >  		.dgram_enqueue            = virtio_transport_dgram_enqueue,
> > -		.dgram_bind               = virtio_transport_dgram_bind,
> >  		.dgram_allow              = virtio_transport_dgram_allow,
> >  
> >  		.stream_enqueue           = virtio_transport_stream_enqueue,
> > diff --git a/include/linux/virtio_vsock.h b/include/linux/virtio_vsock.h
> > index 18cbe8d37fca..7632552bee58 100644
> > --- a/include/linux/virtio_vsock.h
> > +++ b/include/linux/virtio_vsock.h
> > @@ -211,8 +211,6 @@ void virtio_transport_notify_buffer_size(struct vsock_sock *vsk, u64 *val);
> >  u64 virtio_transport_stream_rcvhiwat(struct vsock_sock *vsk);
> >  bool virtio_transport_stream_is_active(struct vsock_sock *vsk);
> >  bool virtio_transport_stream_allow(u32 cid, u32 port);
> > -int virtio_transport_dgram_bind(struct vsock_sock *vsk,
> > -				struct sockaddr_vm *addr);
> >  bool virtio_transport_dgram_allow(u32 cid, u32 port);
> >  
> >  int virtio_transport_connect(struct vsock_sock *vsk);
> > diff --git a/include/net/af_vsock.h b/include/net/af_vsock.h
> > index 305d57502e89..f6a0ca9d7c3e 100644
> > --- a/include/net/af_vsock.h
> > +++ b/include/net/af_vsock.h
> > @@ -96,13 +96,13 @@ struct vsock_transport_send_notify_data {
> >  
> >  /* Transport features flags */
> >  /* Transport provides host->guest communication */
> > -#define VSOCK_TRANSPORT_F_H2G		0x00000001
> > +#define VSOCK_TRANSPORT_F_H2G			0x00000001
> >  /* Transport provides guest->host communication */
> > -#define VSOCK_TRANSPORT_F_G2H		0x00000002
> > -/* Transport provides DGRAM communication */
> > -#define VSOCK_TRANSPORT_F_DGRAM		0x00000004
> > +#define VSOCK_TRANSPORT_F_G2H			0x00000002
> > +/* Transport provides fallback for DGRAM communication */
> > +#define VSOCK_TRANSPORT_F_DGRAM_FALLBACK	0x00000004
> >  /* Transport provides local (loopback) communication */
> > -#define VSOCK_TRANSPORT_F_LOCAL		0x00000008
> > +#define VSOCK_TRANSPORT_F_LOCAL			0x00000008
> >  
> >  struct vsock_transport {
> >  	struct module *module;
> > diff --git a/net/vmw_vsock/af_vsock.c b/net/vmw_vsock/af_vsock.c
> > index ae5ac5531d96..26c97b33d55a 100644
> > --- a/net/vmw_vsock/af_vsock.c
> > +++ b/net/vmw_vsock/af_vsock.c
> > @@ -139,8 +139,8 @@ struct proto vsock_proto = {
> >  static const struct vsock_transport *transport_h2g;
> >  /* Transport used for guest->host communication */
> >  static const struct vsock_transport *transport_g2h;
> > -/* Transport used for DGRAM communication */
> > -static const struct vsock_transport *transport_dgram;
> > +/* Transport used as a fallback for DGRAM communication */
> > +static const struct vsock_transport *transport_dgram_fallback;
> >  /* Transport used for local communication */
> >  static const struct vsock_transport *transport_local;
> >  static DEFINE_MUTEX(vsock_register_mutex);
> > @@ -439,6 +439,18 @@ vsock_connectible_lookup_transport(unsigned int cid, __u8 flags)
> >  	return transport;
> >  }
> >  
> > +static const struct vsock_transport *
> > +vsock_dgram_lookup_transport(unsigned int cid, __u8 flags)
> > +{
> > +	const struct vsock_transport *transport;
> > +
> > +	transport = vsock_connectible_lookup_transport(cid, flags);
> > +	if (transport)
> > +		return transport;
> > +
> > +	return transport_dgram_fallback;
> > +}
> > +
> >  /* Assign a transport to a socket and call the .init transport callback.
> >   *
> >   * Note: for connection oriented socket this must be called when vsk->remote_addr
> > @@ -475,7 +487,8 @@ int vsock_assign_transport(struct vsock_sock *vsk, struct vsock_sock *psk)
> >  
> >  	switch (sk->sk_type) {
> >  	case SOCK_DGRAM:
> > -		new_transport = transport_dgram;
> > +		new_transport = vsock_dgram_lookup_transport(remote_cid,
> > +							     remote_flags);
> 
> I'm a little bit confused about this: 
> 1) Let's create SOCK_DGRAM socket using vsock_create()
> 2) for SOCK_DGRAM it calls 'vsock_assign_transport()' and we go here, remote_cid == -1
> 3) I guess 'vsock_dgram_lookup_transport()' calls logic from 0002 and returns h2g for such remote cid, which is not
>    correct I think...
> 
> Please correct me if i'm wrong
> 
> Thanks, Arseniy
> 

As I understand, for the VMCI case, if transport_h2g != NULL, then
transport_h2g == transport_dgram_fallback. In either case,
vsk->transport == transport_dgram_fallback.

For the virtio/vhost case, temporarily vsk->transport == transport_h2g,
but it is unused because vsk->transport->dgram_bind == NULL.

Until SS_CONNECTED is set by connect() and vsk->transport is set
correctly, the send path is barred from using the bad transport.

I guess the recvmsg() path is a little more sketchy, and probably only
works in my test cases because h2g/g2h in the vhost/virtio case have
identical dgram_addr_init() implementations.

I think a cleaner solution is maybe checking in vsock_create() if
dgram_bind is implemented. If it is not, then vsk->transport should be
reset to NULL and a comment added explaining why VMCI requires this.

Then the other calls can begin explicitly checking for vsk->transport ==
NULL.

Thoughts?

> >  		break;
> >  	case SOCK_STREAM:
> >  	case SOCK_SEQPACKET:
> > @@ -692,6 +705,9 @@ static int __vsock_bind_connectible(struct vsock_sock *vsk,
> >  static int __vsock_bind_dgram(struct vsock_sock *vsk,
> >  			      struct sockaddr_vm *addr)
> >  {
> > +	if (!vsk->transport || !vsk->transport->dgram_bind)
> > +		return -EINVAL;
> > +
> >  	return vsk->transport->dgram_bind(vsk, addr);
> >  }
> >  
> > @@ -1162,6 +1178,7 @@ static int vsock_dgram_sendmsg(struct socket *sock, struct msghdr *msg,
> >  	struct vsock_sock *vsk;
> >  	struct sockaddr_vm *remote_addr;
> >  	const struct vsock_transport *transport;
> > +	bool module_got = false;
> >  
> >  	if (msg->msg_flags & MSG_OOB)
> >  		return -EOPNOTSUPP;
> > @@ -1173,19 +1190,34 @@ static int vsock_dgram_sendmsg(struct socket *sock, struct msghdr *msg,
> >  
> >  	lock_sock(sk);
> >  
> > -	transport = vsk->transport;
> > -
> >  	err = vsock_auto_bind(vsk);
> >  	if (err)
> >  		goto out;
> >  
> > -
> >  	/* If the provided message contains an address, use that.  Otherwise
> >  	 * fall back on the socket's remote handle (if it has been connected).
> >  	 */
> >  	if (msg->msg_name &&
> >  	    vsock_addr_cast(msg->msg_name, msg->msg_namelen,
> >  			    &remote_addr) == 0) {
> > +		transport = vsock_dgram_lookup_transport(remote_addr->svm_cid,
> > +							 remote_addr->svm_flags);
> > +		if (!transport) {
> > +			err = -EINVAL;
> > +			goto out;
> > +		}
> > +
> > +		if (!try_module_get(transport->module)) {
> > +			err = -ENODEV;
> > +			goto out;
> > +		}
> > +
> > +		/* When looking up a transport dynamically and acquiring a
> > +		 * reference on the module, we need to remember to release the
> > +		 * reference later.
> > +		 */
> > +		module_got = true;
> > +
> >  		/* Ensure this address is of the right type and is a valid
> >  		 * destination.
> >  		 */
> > @@ -1200,6 +1232,7 @@ static int vsock_dgram_sendmsg(struct socket *sock, struct msghdr *msg,
> >  	} else if (sock->state == SS_CONNECTED) {
> >  		remote_addr = &vsk->remote_addr;
> >  
> > +		transport = vsk->transport;
> >  		if (remote_addr->svm_cid == VMADDR_CID_ANY)
> >  			remote_addr->svm_cid = transport->get_local_cid();
> >  
> > @@ -1224,6 +1257,8 @@ static int vsock_dgram_sendmsg(struct socket *sock, struct msghdr *msg,
> >  	err = transport->dgram_enqueue(vsk, remote_addr, msg, len);
> >  
> >  out:
> > +	if (module_got)
> > +		module_put(transport->module);
> >  	release_sock(sk);
> >  	return err;
> >  }
> > @@ -1256,13 +1291,18 @@ static int vsock_dgram_connect(struct socket *sock,
> >  	if (err)
> >  		goto out;
> >  
> > +	memcpy(&vsk->remote_addr, remote_addr, sizeof(vsk->remote_addr));
> > +
> > +	err = vsock_assign_transport(vsk, NULL);
> > +	if (err)
> > +		goto out;
> > +
> >  	if (!vsk->transport->dgram_allow(remote_addr->svm_cid,
> >  					 remote_addr->svm_port)) {
> >  		err = -EINVAL;
> >  		goto out;
> >  	}
> >  
> > -	memcpy(&vsk->remote_addr, remote_addr, sizeof(vsk->remote_addr));
> >  	sock->state = SS_CONNECTED;
> >  
> >  	/* sock map disallows redirection of non-TCP sockets with sk_state !=
> > @@ -2487,7 +2527,7 @@ int vsock_core_register(const struct vsock_transport *t, int features)
> >  
> >  	t_h2g = transport_h2g;
> >  	t_g2h = transport_g2h;
> > -	t_dgram = transport_dgram;
> > +	t_dgram = transport_dgram_fallback;
> >  	t_local = transport_local;
> >  
> >  	if (features & VSOCK_TRANSPORT_F_H2G) {
> > @@ -2506,7 +2546,7 @@ int vsock_core_register(const struct vsock_transport *t, int features)
> >  		t_g2h = t;
> >  	}
> >  
> > -	if (features & VSOCK_TRANSPORT_F_DGRAM) {
> > +	if (features & VSOCK_TRANSPORT_F_DGRAM_FALLBACK) {
> >  		if (t_dgram) {
> >  			err = -EBUSY;
> >  			goto err_busy;
> > @@ -2524,7 +2564,7 @@ int vsock_core_register(const struct vsock_transport *t, int features)
> >  
> >  	transport_h2g = t_h2g;
> >  	transport_g2h = t_g2h;
> > -	transport_dgram = t_dgram;
> > +	transport_dgram_fallback = t_dgram;
> >  	transport_local = t_local;
> >  
> >  err_busy:
> > @@ -2543,8 +2583,8 @@ void vsock_core_unregister(const struct vsock_transport *t)
> >  	if (transport_g2h == t)
> >  		transport_g2h = NULL;
> >  
> > -	if (transport_dgram == t)
> > -		transport_dgram = NULL;
> > +	if (transport_dgram_fallback == t)
> > +		transport_dgram_fallback = NULL;
> >  
> >  	if (transport_local == t)
> >  		transport_local = NULL;
> > diff --git a/net/vmw_vsock/hyperv_transport.c b/net/vmw_vsock/hyperv_transport.c
> > index 7f1ea434656d..c29000f2612a 100644
> > --- a/net/vmw_vsock/hyperv_transport.c
> > +++ b/net/vmw_vsock/hyperv_transport.c
> > @@ -551,11 +551,6 @@ static void hvs_destruct(struct vsock_sock *vsk)
> >  	kfree(hvs);
> >  }
> >  
> > -static int hvs_dgram_bind(struct vsock_sock *vsk, struct sockaddr_vm *addr)
> > -{
> > -	return -EOPNOTSUPP;
> > -}
> > -
> >  static int hvs_dgram_enqueue(struct vsock_sock *vsk,
> >  			     struct sockaddr_vm *remote, struct msghdr *msg,
> >  			     size_t dgram_len)
> > @@ -826,7 +821,6 @@ static struct vsock_transport hvs_transport = {
> >  	.connect                  = hvs_connect,
> >  	.shutdown                 = hvs_shutdown,
> >  
> > -	.dgram_bind               = hvs_dgram_bind,
> >  	.dgram_enqueue            = hvs_dgram_enqueue,
> >  	.dgram_allow              = hvs_dgram_allow,
> >  
> > diff --git a/net/vmw_vsock/virtio_transport.c b/net/vmw_vsock/virtio_transport.c
> > index 66edffdbf303..ac2126c7dac5 100644
> > --- a/net/vmw_vsock/virtio_transport.c
> > +++ b/net/vmw_vsock/virtio_transport.c
> > @@ -428,7 +428,6 @@ static struct virtio_transport virtio_transport = {
> >  		.shutdown                 = virtio_transport_shutdown,
> >  		.cancel_pkt               = virtio_transport_cancel_pkt,
> >  
> > -		.dgram_bind               = virtio_transport_dgram_bind,
> >  		.dgram_enqueue            = virtio_transport_dgram_enqueue,
> >  		.dgram_allow              = virtio_transport_dgram_allow,
> >  
> > diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
> > index 01ea1402ad40..ffcbdd77feaa 100644
> > --- a/net/vmw_vsock/virtio_transport_common.c
> > +++ b/net/vmw_vsock/virtio_transport_common.c
> > @@ -781,13 +781,6 @@ bool virtio_transport_stream_allow(u32 cid, u32 port)
> >  }
> >  EXPORT_SYMBOL_GPL(virtio_transport_stream_allow);
> >  
> > -int virtio_transport_dgram_bind(struct vsock_sock *vsk,
> > -				struct sockaddr_vm *addr)
> > -{
> > -	return -EOPNOTSUPP;
> > -}
> > -EXPORT_SYMBOL_GPL(virtio_transport_dgram_bind);
> > -
> >  bool virtio_transport_dgram_allow(u32 cid, u32 port)
> >  {
> >  	return false;
> > diff --git a/net/vmw_vsock/vmci_transport.c b/net/vmw_vsock/vmci_transport.c
> > index 0bbbdb222245..857b0461f856 100644
> > --- a/net/vmw_vsock/vmci_transport.c
> > +++ b/net/vmw_vsock/vmci_transport.c
> > @@ -2072,7 +2072,7 @@ static int __init vmci_transport_init(void)
> >  	/* Register only with dgram feature, other features (H2G, G2H) will be
> >  	 * registered when the first host or guest becomes active.
> >  	 */
> > -	err = vsock_core_register(&vmci_transport, VSOCK_TRANSPORT_F_DGRAM);
> > +	err = vsock_core_register(&vmci_transport, VSOCK_TRANSPORT_F_DGRAM_FALLBACK);
> >  	if (err < 0)
> >  		goto err_unsubscribe;
> >  
> > diff --git a/net/vmw_vsock/vsock_loopback.c b/net/vmw_vsock/vsock_loopback.c
> > index 2a59dd177c74..278235ea06c4 100644
> > --- a/net/vmw_vsock/vsock_loopback.c
> > +++ b/net/vmw_vsock/vsock_loopback.c
> > @@ -61,7 +61,6 @@ static struct virtio_transport loopback_transport = {
> >  		.shutdown                 = virtio_transport_shutdown,
> >  		.cancel_pkt               = vsock_loopback_cancel_pkt,
> >  
> > -		.dgram_bind               = virtio_transport_dgram_bind,
> >  		.dgram_enqueue            = virtio_transport_dgram_enqueue,
> >  		.dgram_allow              = virtio_transport_dgram_allow,
> >  
> >