Re: [PATCH net-next v12 03/10] tun: Allow steering eBPF program to fall back

[Jason Wang <jasowang@redhat.com>]

On Fri, May 30, 2025 at 12:50 PM Akihiko Odaki <akihiko.odaki@daynix.com> wrote:
>
> This clarifies a steering eBPF program takes precedence over the other
> steering algorithms.

Let's give an example on the use case for this.

>
> Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
> ---
>  Documentation/networking/tuntap.rst |  7 +++++++
>  drivers/net/tun.c                   | 28 +++++++++++++++++-----------
>  include/uapi/linux/if_tun.h         |  9 +++++++++
>  3 files changed, 33 insertions(+), 11 deletions(-)
>
> diff --git a/Documentation/networking/tuntap.rst b/Documentation/networking/tuntap.rst
> index 4d7087f727be..86b4ae8caa8a 100644
> --- a/Documentation/networking/tuntap.rst
> +++ b/Documentation/networking/tuntap.rst
> @@ -206,6 +206,13 @@ enable is true we enable it, otherwise we disable it::
>        return ioctl(fd, TUNSETQUEUE, (void *)&ifr);
>    }
>
> +3.4 Reference
> +-------------
> +
> +``linux/if_tun.h`` defines the interface described below:
> +
> +.. kernel-doc:: include/uapi/linux/if_tun.h
> +
>  Universal TUN/TAP device driver Frequently Asked Question
>  =========================================================
>
> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> index d8f4d3e996a7..9133ab9ed3f5 100644
> --- a/drivers/net/tun.c
> +++ b/drivers/net/tun.c
> @@ -476,21 +476,29 @@ static u16 tun_automq_select_queue(struct tun_struct *tun, struct sk_buff *skb)
>         return txq;
>  }
>
> -static u16 tun_ebpf_select_queue(struct tun_struct *tun, struct sk_buff *skb)
> +static bool tun_ebpf_select_queue(struct tun_struct *tun, struct sk_buff *skb,
> +                                 u16 *ret)
>  {
>         struct tun_prog *prog;
>         u32 numqueues;
> -       u16 ret = 0;
> +       u32 prog_ret;
> +
> +       prog = rcu_dereference(tun->steering_prog);
> +       if (!prog)
> +               return false;
>
>         numqueues = READ_ONCE(tun->numqueues);
> -       if (!numqueues)
> -               return 0;
> +       if (!numqueues) {
> +               *ret = 0;
> +               return true;
> +       }
>
> -       prog = rcu_dereference(tun->steering_prog);
> -       if (prog)
> -               ret = bpf_prog_run_clear_cb(prog->prog, skb);
> +       prog_ret = bpf_prog_run_clear_cb(prog->prog, skb);
> +       if (prog_ret == TUN_STEERINGEBPF_FALLBACK)
> +               return false;

This seems to break the uAPI. So I think we need a new ioctl to enable
the behaviour

>
> -       return ret % numqueues;
> +       *ret = (u16)prog_ret % numqueues;
> +       return true;
>  }
>
>  static u16 tun_select_queue(struct net_device *dev, struct sk_buff *skb,
> @@ -500,9 +508,7 @@ static u16 tun_select_queue(struct net_device *dev, struct sk_buff *skb,
>         u16 ret;
>
>         rcu_read_lock();
> -       if (rcu_dereference(tun->steering_prog))
> -               ret = tun_ebpf_select_queue(tun, skb);
> -       else
> +       if (!tun_ebpf_select_queue(tun, skb, &ret))
>                 ret = tun_automq_select_queue(tun, skb);
>         rcu_read_unlock();
>
> diff --git a/include/uapi/linux/if_tun.h b/include/uapi/linux/if_tun.h
> index 287cdc81c939..980de74724fc 100644
> --- a/include/uapi/linux/if_tun.h
> +++ b/include/uapi/linux/if_tun.h
> @@ -115,4 +115,13 @@ struct tun_filter {
>         __u8   addr[][ETH_ALEN];
>  };
>
> +/**
> + * define TUN_STEERINGEBPF_FALLBACK - A steering eBPF return value to fall back
> + *
> + * A steering eBPF program may return this value to fall back to the steering
> + * algorithm that should have been used if the program was not set. This allows
> + * selectively overriding the steering decision.
> + */
> +#define TUN_STEERINGEBPF_FALLBACK -1

Not a native speaker, consider it works more like XDP_PASS, would it
be better to use "TUN_STERRING_PASS"?

Thanks

> +
>  #endif /* _UAPI__IF_TUN_H */
>
> --
> 2.49.0
>