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From: Arnd Bergmann on 3 Nov 2009 07:20
On Monday 02 November 2009, Michael S. Tsirkin wrote:
> 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.

What was your reason for changing?

> index 3f5fd52..404abe0 100644
> --- a/include/linux/if_tun.h
> +++ b/include/linux/if_tun.h
> @@ -86,4 +86,18 @@ struct tun_filter {
> __u8 addr[0][ETH_ALEN];
> };
>
> +#ifdef __KERNEL__
> +#if defined(CONFIG_TUN) || defined(CONFIG_TUN_MODULE)
> +struct socket *tun_get_socket(struct file *);
> +#else
> +#include <linux/err.h>
> +#include <linux/errno.h>
> +struct file;
> +struct socket;
> +static inline struct socket *tun_get_socket(struct file *f)
> +{
> + return ERR_PTR(-EINVAL);
> +}
> +#endif /* CONFIG_TUN */
> +#endif /* __KERNEL__ */
> #endif /* __IF_TUN_H */

Is this a leftover from testing? Exporting the function for !__KERNEL__
seems pointless.

Arnd <><

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From: Michael S. Tsirkin on 3 Nov 2009 07:40
On Tue, Nov 03, 2009 at 01:12:33PM +0100, Arnd Bergmann wrote:
> On Monday 02 November 2009, Michael S. Tsirkin wrote:
> > 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(a)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;


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From: Arnd Bergmann on 3 Nov 2009 07:50
On Tuesday 03 November 2009, Michael S. Tsirkin wrote:
> > 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.

Ah, I see.

> 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.

This may even make it easier for me to do the same with macvtap
if I resume work on that.

> @@ -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;

Or maybe do both (socket_file_ops and get_socket), to avoid an indirect
function call.

Arnd <><
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From: David Miller on 4 Nov 2009 08:40
From: "Michael S. Tsirkin" <mst(a)redhat.com>
Date: Tue, 3 Nov 2009 19:24:00 +0200

> Assuming it's okay with davem, I think it makes sense to merge this
> patch through Rusty's tree because vhost is the first user of the new
> interface. Posted here for completeness.

I'm fine with that, please add my:

Acked-by: David S. Miller <davem(a)davemloft.net>
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From: Michael S. Tsirkin on 5 Nov 2009 06:40
On Wed, Nov 04, 2009 at 07:09:05PM +0100, Arnd Bergmann wrote:
> On Tuesday 03 November 2009, Arnd Bergmann wrote:
> > > index 3f5fd52..404abe0 100644
> > > --- a/include/linux/if_tun.h
> > > +++ b/include/linux/if_tun.h
> > > @@ -86,4 +86,18 @@ struct tun_filter {
> > > __u8 addr[0][ETH_ALEN];
> > > };
> > >
> > > +#ifdef __KERNEL__
> > > +#if defined(CONFIG_TUN) || defined(CONFIG_TUN_MODULE)
> > > +struct socket *tun_get_socket(struct file *);
> > > +#else
> > > +#include <linux/err.h>
> > > +#include <linux/errno.h>
> > > +struct file;
> > > +struct socket;
> > > +static inline struct socket *tun_get_socket(struct file *f)
> > > +{
> > > + return ERR_PTR(-EINVAL);
> > > +}
> > > +#endif /* CONFIG_TUN */
> > > +#endif /* __KERNEL__ */
> > > #endif /* __IF_TUN_H */
> >
> > Is this a leftover from testing? Exporting the function for !__KERNEL__
> > seems pointless.
> >
>
> Michael, you didn't reply on this comment and the code is still there in v8.
> Do you actually need this? What for?
>
> Arnd <><

Sorry, missed the question. If you look closely it is not exported for
!__KERNEL__ at all. The stub is for when CONFIG_TUN is undefined.
Maybe I'll add a comment near #else, even though this is a bit strange
since the #if is just 2 lines above it.

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