Re: [PATCH v3 1/5] net: add header len parameter to tun_get_socket(), tap_get_socket()

On the receive side, where the tun device generates the virtio_net_hdr
but VIRITO_NET_F_MSG_RXBUF was negotiated and vhost-net needs to fill
in the 'num_buffers' field on top of the existing virtio_net_hdr, fix
that to use 'sock_hlen - 2' as the location, which means that it goes
in the right place regardless of whether the tun device is using an
additional tun_pi header or not. In this case, the user should have
configured the tun device with a vnet hdr size of 12, to make room.

Fixes: 8dd014adfea6f ("vhost-net: mergeable buffers support")
Signed-off-by: David Woodhouse <redacted>
---
  drivers/net/tap.c      |  5 ++++-
  drivers/net/tun.c      | 16 +++++++++++++++-
  drivers/vhost/net.c    | 31 +++++++++++++++----------------
  include/linux/if_tap.h |  4 ++--
  include/linux/if_tun.h |  4 ++--
  5 files changed, 38 insertions(+), 22 deletions(-)

diff --git a/drivers/net/tap.c b/drivers/net/tap.c
index 8e3a28ba6b28..2170a0d3d34c 100644
--- a/drivers/net/tap.c
+++ b/drivers/net/tap.c

@@ -1246,7 +1246,7 @@ static const struct proto_ops tap_socket_ops = {
   * attached to a device.  The returned object works like a packet socket, it
   * can be used for sock_sendmsg/sock_recvmsg.  The caller is responsible for
   * holding a reference to the file for as long as the socket is in use. */
-struct socket *tap_get_socket(struct file *file)
+struct socket *tap_get_socket(struct file *file, size_t *hlen)
  {
  	struct tap_queue *q;
  	if (file->f_op != &tap_fops)

@@ -1254,6 +1254,9 @@ struct socket *tap_get_socket(struct file *file)
  	q = file->private_data;
  	if (!q)
  		return ERR_PTR(-EBADFD);
+	if (hlen)
+		*hlen = (q->flags & IFF_VNET_HDR) ? q->vnet_hdr_sz : 0;
+
  	return &q->sock;
  }
  EXPORT_SYMBOL_GPL(tap_get_socket);

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index 4cf38be26dc9..67b406fa0881 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c

@@ -3649,7 +3649,7 @@ static void tun_cleanup(void)
   * attached to a device.  The returned object works like a packet socket, it
   * can be used for sock_sendmsg/sock_recvmsg.  The caller is responsible for
   * holding a reference to the file for as long as the socket is in use. */
-struct socket *tun_get_socket(struct file *file)
+struct socket *tun_get_socket(struct file *file, size_t *hlen)
  {
  	struct tun_file *tfile;
  	if (file->f_op != &tun_fops)

@@ -3657,6 +3657,20 @@ struct socket *tun_get_socket(struct file *file)
  	tfile = file->private_data;
  	if (!tfile)
  		return ERR_PTR(-EBADFD);
+
+	if (hlen) {
+		struct tun_struct *tun = tun_get(tfile);
+		size_t len = 0;
+
+		if (!tun)
+			return ERR_PTR(-ENOTCONN);
+		if (tun->flags & IFF_VNET_HDR)
+			len += READ_ONCE(tun->vnet_hdr_sz);
+		if (!(tun->flags & IFF_NO_PI))
+			len += sizeof(struct tun_pi);
+		tun_put(tun);
+		*hlen = len;
+	}
  	return &tfile->socket;
  }
  EXPORT_SYMBOL_GPL(tun_get_socket);

diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
index df82b124170e..b92a7144ed90 100644
--- a/drivers/vhost/net.c
+++ b/drivers/vhost/net.c

@@ -1143,7 +1143,8 @@ static void handle_rx(struct vhost_net *net)
  
  	vq_log = unlikely(vhost_has_feature(vq, VHOST_F_LOG_ALL)) ?
  		vq->log : NULL;
-	mergeable = vhost_has_feature(vq, VIRTIO_NET_F_MRG_RXBUF);
+	mergeable = vhost_has_feature(vq, VIRTIO_NET_F_MRG_RXBUF) &&
+		(vhost_hlen || sock_hlen >= sizeof(num_buffers));

  
  	do {
  		sock_len = vhost_net_rx_peek_head_len(net, sock->sk,

@@ -1213,9 +1214,10 @@ static void handle_rx(struct vhost_net *net)
  			}
  		} else {
  			/* Header came from socket; we'll need to patch
-			 * ->num_buffers over if VIRTIO_NET_F_MRG_RXBUF
+			 * ->num_buffers over the last two bytes if
+			 * VIRTIO_NET_F_MRG_RXBUF is enabled.
  			 */
-			iov_iter_advance(&fixup, sizeof(hdr));
+			iov_iter_advance(&fixup, sock_hlen - 2);