[net-next] can-raw: add msg_flags to distinguish local traffic

CAN has no addressing scheme. It is currently impossible
for userspace to tell is a received CAN frame comes from
another process on the local host, or from a remote CAN
device.
This patch add support for userspace applications to distinguish
between 'own', 'local' and 'remote' CAN traffic.
Distinction is made by returning some flags in msg->msg_flags
in the call to recvmsg.
MSG_CONFIRM flag means 'own', as in 'transmission confirmation'
MSG_DONTROUTE flag means 'local', not routed.
Obviously, msgs with MSG_CONFIRM will have MSG_DONTROUTE set too.

Please note that on SocketCAN mailing list, different opinions
exist on the exact meaning of MSG_DONTROUTE. Better (=more
intuitive) alternatives are appreciated.

Signed-off-by: Kurt Van Dijck <kurt.van.dijck@skynet.be>
---
 net/can/raw.c |   33 ++++++++++++++++++++++++++++++---
 1 files changed, 30 insertions(+), 3 deletions(-)

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On Fri, Oct 15, 2010 at 10:37 AM, Kurt Van Dijck <kurt.van.dijck@eia.be> wrote:
> CAN has no addressing scheme. It is currently impossible
> for userspace to tell is a received CAN frame comes from
> another process on the local host, or from a remote CAN
> device.
> This patch add support for userspace applications to distinguish
> between 'own', 'local' and 'remote' CAN traffic.
> Distinction is made by returning some flags in msg->msg_flags
> in the call to recvmsg.
> MSG_CONFIRM flag means 'own', as in 'transmission confirmation'
> MSG_DONTROUTE flag means 'local', not routed.
> Obviously, msgs with MSG_CONFIRM will have MSG_DONTROUTE set too.
>
> Please note that on SocketCAN mailing list, different opinions
> exist on the exact meaning of MSG_DONTROUTE. Better (=more
> intuitive) alternatives are appreciated.
>
> Signed-off-by: Kurt Van Dijck <kurt.van.dijck@skynet.be>
> ---
>  net/can/raw.c |   33 ++++++++++++++++++++++++++++++---
>  1 files changed, 30 insertions(+), 3 deletions(-)
>
> diff --git a/net/can/raw.c b/net/can/raw.c
> index 7d77e67..f98709e 100644
> --- a/net/can/raw.c
> +++ b/net/can/raw.c
> @@ -90,23 +90,39 @@ struct raw_sock {
>        can_err_mask_t err_mask;
>  };
>
> +/*
> + * return some space to store extra msg flags in.
> + * We use 1 int beyond the 'struct sockaddr_can' in skb->cb
> + * to store those.
> + * These flags will be use in raw_recvmsg()
> + */
> +static inline int *raw_flags(struct sk_buff *skb)
> +{
> +       BUILD_BUG_ON(sizeof(skb->cb)
> +                       <= (sizeof(struct sockaddr_can) + sizeof(int)));
> +       /* return pointer after struct sockaddr_can */
> +       return (int *)(&((struct sockaddr_can *)skb->cb)[1]);

Since msg_flags is unsigned, it would be nice if this function returns unsigned.

> +}
> +
>  static inline struct raw_sock *raw_sk(const struct sock *sk)
>  {
>        return (struct raw_sock *)sk;
>  }
>
> -static void raw_rcv(struct sk_buff *skb, void *data)
> +static void raw_rcv(struct sk_buff *oskb, void *data)
>  {
>        struct sock *sk = (struct sock *)data;
>        struct raw_sock *ro = raw_sk(sk);
>        struct sockaddr_can *addr;
> +       struct sk_buff *skb;
> +       int *pflags;
>
>        /* check the received tx sock reference */
> -       if (!ro->recv_own_msgs && skb->sk == sk)
> +       if (!ro->recv_own_msgs && oskb->sk == sk)
>                return;
>
>        /* clone the given skb to be able to enqueue it into the rcv queue */
> -       skb = skb_clone(skb, GFP_ATOMIC);
> +       skb = skb_clone(oskb, GFP_ATOMIC);
>        if (!skb)
>                return;
>
> @@ -123,6 +139,14 @@ static void raw_rcv(struct sk_buff *skb, void *data)
>        addr->can_family  = AF_CAN;
>        addr->can_ifindex = skb->dev->ifindex;
>
> +       /* prepare the flags for raw_recvmsg() */
> +       pflags = raw_flags(skb);
> +       *pflags = 0;
> +       if (oskb->sk)
> +               *pflags |= MSG_DONTROUTE;
> +       if (oskb->sk == sk)
> +               *pflags |= MSG_CONFIRM;
> +
>        if (sock_queue_rcv_skb(sk, skb) < 0)
>                kfree_skb(skb);
>  }
> @@ -707,6 +731,9 @@ static int raw_recvmsg(struct kiocb *iocb, struct socket *sock,
>                memcpy(msg->msg_name, skb->cb, msg->msg_namelen);
>        }
>
> +       /* assign the flags that have been recorded in in raw_rcv() */
small typo: double in
> +       msg->msg_flags |= *(raw_flags(skb));
> +
>        skb_free_datagram(sk, skb);
>
>        return size;

thanks,
Daniel.
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