On Tue, Nov 03, 2009 at 01:12:33PM +0100, Arnd Bergmann wrote:

On Monday 02 November 2009, Michael S. Tsirkin wrote:

quoted

Tun device looks similar to a packet socket
in that both pass complete frames from/to userspace.

This patch fills in enough fields in the socket underlying tun driver
to support sendmsg/recvmsg operations, and message flags
MSG_TRUNC and MSG_DONTWAIT, and exports access to this socket
to modules.  Regular read/write behaviour is unchanged.

This way, code using raw sockets to inject packets
into a physical device, can support injecting
packets into host network stack almost without modification.

First user of this interface will be vhost virtualization
accelerator.

You mentioned before that you wanted to export the socket
using some ioctl function returning an open file descriptor,
which seemed to be a cleaner approach than this one.

Note that a similar feature can be implemented on top of tun_get_socket,
as seen from patch below.

What was your reason for changing?

It turns out socket structure is really bound to specific a file, so we
can not have 2 files referencing the same socket.  Instead, as I say
above, it's possible to make sendmsg/recvmsg work on tap file directly.

For vhost, the advantage of such a feature over using tun_get_socket
directly would be that vhost module won't depend on tun module then.  I
have implemented this (patch below), but decided to go with the simple
thing first.  Since no userspace-visible changes are involved, let's do
this by small steps: it will be easier to figure out when vhost
is upstream.


---

Note: patch below aplies on top of patch tun: export underlying socket.
It is not intended for merge yet.

net: convert tun device to socket

Add callback to file_ops to retrieve socket from
file structure. Use this to make tun character device
accept sendmsg/recvmsg calls.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index b58095a..53e1806 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c

@@ -1405,7 +1405,8 @@ static const struct file_operations tun_fops = {
 	.unlocked_ioctl = tun_chr_ioctl,
 	.open	= tun_chr_open,
 	.release = tun_chr_close,
-	.fasync = tun_chr_fasync
+	.fasync = tun_chr_fasync,
+	.get_socket = tun_get_socket,
 };
 
 static struct miscdevice tun_miscdev = {

diff --git a/include/linux/fs.h b/include/linux/fs.h
index 2620a8c..f2b381f 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h

@@ -1506,6 +1506,9 @@ struct file_operations {
 	ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, loff_t *, size_t, unsigned int);
 	ssize_t (*splice_read)(struct file *, loff_t *, struct pipe_inode_info *, size_t, unsigned int);
 	int (*setlease)(struct file *, long, struct file_lock **);
+#ifdef CONFIG_NET
+	struct socket *(*get_socket)(struct file *file);
+#endif
 };
 
 struct inode_operations {

diff --git a/net/socket.c b/net/socket.c
index 9dff31c..700efcb 100644
--- a/net/socket.c
+++ b/net/socket.c

@@ -119,6 +119,11 @@ static ssize_t sock_splice_read(struct file *file, loff_t *ppos,
 			        struct pipe_inode_info *pipe, size_t len,
 				unsigned int flags);
 
+static struct socket *sock_get_socket(struct file *file)
+{
+	return file->private_data;	/* set in sock_map_fd */
+}
+
 /*
  *	Socket files have a set of 'special' operations as well as the generic file ones. These don't appear
  *	in the operation structures but are done directly via the socketcall() multiplexor.

@@ -141,6 +146,7 @@ static const struct file_operations socket_file_ops = {
 	.sendpage =	sock_sendpage,
 	.splice_write = generic_splice_sendpage,
 	.splice_read =	sock_splice_read,
+	.get_socket =   sock_get_socket,
 };
 
 /*

@@ -416,8 +422,8 @@ int sock_map_fd(struct socket *sock, int flags)
 
 static struct socket *sock_from_file(struct file *file, int *err)
 {
-	if (file->f_op == &socket_file_ops)
-		return file->private_data;	/* set in sock_map_fd */
+	if (file->f_op->get_socket)
+		return file->f_op->get_socket(file);
 
 	*err = -ENOTSOCK;
 	return NULL;

-- 
MST

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