On 2025/06/04 10:27, Jason Wang wrote:

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

I assumed it is fine to repurpose one of the 32-bit integer values since 32-bit integer is too big to specify the queue number, but it may not be fine. I don't have a concrete use case either.

Perhaps it is safer to note that TUNSETSTEERINGEBPF takes precedence over TUNSETVNETRSS to allow such an extension in the future (but without implementing one now).

...

•   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"?

That sounds indeed better to me.

Regards, Akihiko Odaki