linux-kernel - Re: [PATCH net-next] tuntap: Fallback to automq on TUNSETSTEERINGEBPF prog negative return

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Date:   Fri, 20 Sep 2019 12:45:56 -0700
From:   Matt Cover <werekraken@...il.com>
To:     davem@...emloft.net, ast@...nel.org, daniel@...earbox.net,
        kafai@...com, songliubraving@...com, yhs@...com,
        Jason Wang <jasowang@...hat.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Stanislav Fomichev <sdf@...gle.com>, mst@...hat.com,
        Matthew Cover <matthew.cover@...ckpath.com>,
        mail@...urcelik.de, pabeni@...hat.com,
        Nicolas Dichtel <nicolas.dichtel@...nd.com>,
        wangli39@...du.com, lifei.shirley@...edance.com,
        tglx@...utronix.de, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, bpf@...r.kernel.org
Subject: Re: [PATCH net-next] tuntap: Fallback to automq on TUNSETSTEERINGEBPF
 prog negative return

On Fri, Sep 20, 2019 at 11:59 AM Matthew Cover <werekraken@...il.com> wrote:
>
> Treat a negative return from a TUNSETSTEERINGEBPF bpf prog as a signal
> to fallback to tun_automq_select_queue() for tx queue selection.
>
> Compilation of this exact patch was tested.
>
> For functional testing 3 additional printk()s were added.
>
> Functional testing results (on 2 txq tap device):
>
>   [Fri Sep 20 18:33:27 2019] ========== tun no prog ==========
>   [Fri Sep 20 18:33:27 2019] tuntap: tun_ebpf_select_queue() returned '-1'
>   [Fri Sep 20 18:33:27 2019] tuntap: tun_automq_select_queue() ran
>   [Fri Sep 20 18:33:27 2019] ========== tun prog -1 ==========
>   [Fri Sep 20 18:33:27 2019] tuntap: bpf_prog_run_clear_cb() returned '-1'
>   [Fri Sep 20 18:33:27 2019] tuntap: tun_ebpf_select_queue() returned '-1'
>   [Fri Sep 20 18:33:27 2019] tuntap: tun_automq_select_queue() ran
>   [Fri Sep 20 18:33:27 2019] ========== tun prog 0 ==========
>   [Fri Sep 20 18:33:27 2019] tuntap: bpf_prog_run_clear_cb() returned '0'
>   [Fri Sep 20 18:33:27 2019] tuntap: tun_ebpf_select_queue() returned '0'
>   [Fri Sep 20 18:33:27 2019] ========== tun prog 1 ==========
>   [Fri Sep 20 18:33:27 2019] tuntap: bpf_prog_run_clear_cb() returned '1'
>   [Fri Sep 20 18:33:27 2019] tuntap: tun_ebpf_select_queue() returned '1'
>   [Fri Sep 20 18:33:27 2019] ========== tun prog 2 ==========
>   [Fri Sep 20 18:33:27 2019] tuntap: bpf_prog_run_clear_cb() returned '2'
>   [Fri Sep 20 18:33:27 2019] tuntap: tun_ebpf_select_queue() returned '0'
>
> Signed-off-by: Matthew Cover <matthew.cover@...ckpath.com>
> ---
>  drivers/net/tun.c | 20 +++++++++++---------
>  1 file changed, 11 insertions(+), 9 deletions(-)
>
> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> index aab0be4..173d159 100644
> --- a/drivers/net/tun.c
> +++ b/drivers/net/tun.c
> @@ -583,35 +583,37 @@ 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 int tun_ebpf_select_queue(struct tun_struct *tun, struct sk_buff *skb)
>  {
>         struct tun_prog *prog;
>         u32 numqueues;
> -       u16 ret = 0;
> +       int ret = -1;
>
>         numqueues = READ_ONCE(tun->numqueues);
>         if (!numqueues)
>                 return 0;
>
> +       rcu_read_lock();
>         prog = rcu_dereference(tun->steering_prog);
>         if (prog)
>                 ret = bpf_prog_run_clear_cb(prog->prog, skb);
> +       rcu_read_unlock();
>
> -       return ret % numqueues;
> +       if (ret >= 0)
> +               ret %= numqueues;
> +
> +       return ret;
>  }
>
>  static u16 tun_select_queue(struct net_device *dev, struct sk_buff *skb,
>                             struct net_device *sb_dev)
>  {
>         struct tun_struct *tun = netdev_priv(dev);
> -       u16 ret;
> +       int ret;
>
> -       rcu_read_lock();
> -       if (rcu_dereference(tun->steering_prog))
> -               ret = tun_ebpf_select_queue(tun, skb);
> -       else
> +       ret = tun_ebpf_select_queue(tun, skb);
> +       if (ret < 0)
>                 ret = tun_automq_select_queue(tun, skb);
> -       rcu_read_unlock();
>
>         return ret;
>  }
> --
> 1.8.3.1
>

Sorry for sending this while net-next is closed... I
should have been more careful.

Please let me know if I should resubmit once net-next
is open again.