Re: [PATCH net-next V2 2/2] tun: allow to attach ebpf socket filter

[Willem de Bruijn]

On Fri, Jan 5, 2018 at 4:54 AM, Jason Wang <jasowang@xxxxxxxxxx> wrote:
> This patch allows userspace to attach eBPF filter to tun. This will
> allow to implement VM dataplane filtering in a more efficient way
> compared to cBPF filter by allowing either qemu or libvirt to
> attach eBPF filter to tun.
>
> Signed-off-by: Jason Wang <jasowang@xxxxxxxxxx>
> ---
>  drivers/net/tun.c           | 39 +++++++++++++++++++++++++++++++++++----
>  include/uapi/linux/if_tun.h |  1 +
>  2 files changed, 36 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> index 0853829..9fc8b70 100644
> --- a/drivers/net/tun.c
> +++ b/drivers/net/tun.c
> @@ -238,6 +238,12 @@ struct tun_struct {
>         struct tun_pcpu_stats __percpu *pcpu_stats;
>         struct bpf_prog __rcu *xdp_prog;
>         struct tun_prog __rcu *steering_prog;
> +       struct tun_prog __rcu *filter_prog;
> +};
> +
> +struct veth {
> +       __be16 h_vlan_proto;
> +       __be16 h_vlan_TCI;
>  };
>
>  static int tun_napi_receive(struct napi_struct *napi, int budget)
> @@ -984,12 +990,25 @@ static void tun_automq_xmit(struct tun_struct *tun, struct sk_buff *skb)
>  #endif
>  }
>
> +static unsigned int run_ebpf_filter(struct tun_struct *tun,
> +                                   struct sk_buff *skb,
> +                                   int len)
> +{
> +       struct tun_prog *prog = rcu_dereference(tun->filter_prog);
> +
> +       if (prog)
> +               len = bpf_prog_run_clear_cb(prog->prog, skb);
> +
> +       return len;
> +}
> +
>  /* Net device start xmit */
>  static netdev_tx_t tun_net_xmit(struct sk_buff *skb, struct net_device *dev)
>  {
>         struct tun_struct *tun = netdev_priv(dev);
>         int txq = skb->queue_mapping;
>         struct tun_file *tfile;
> +       int len = skb->len;
>
>         rcu_read_lock();
>         tfile = rcu_dereference(tun->tfiles[txq]);
> @@ -1015,6 +1034,16 @@ static netdev_tx_t tun_net_xmit(struct sk_buff *skb, struct net_device *dev)
>             sk_filter(tfile->socket.sk, skb))
>                 goto drop;
>
> +       len = run_ebpf_filter(tun, skb, len);
> +
> +       /* Trim extra bytes since we may inster vlan proto & TCI

inster -> insert

> +        * in tun_put_user().
> +        */
> +       if (skb_vlan_tag_present(skb))
> +               len -= skb_vlan_tag_present(skb) ? sizeof(struct veth) : 0;

no need for testing skb_vlan_tag_present twice.

more importantly, why trim these bytes unconditionally?

only if the filter trims a packet to a length shorter than the the minimum
could this cause problems. sk_filter_trim_cap with a lower bound avoids
that: skb_vlan_tag_present(skb) ? sizeof(struct vlan_ethhdr) : 0;