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

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Message-ID: <CACGkMEvyaY0rdNTj_jP_4HF5TrCCNeTKZZ8U2LsMikLqyctkzA@mail.gmail.com>
Date: Thu, 5 Jun 2025 09:55:29 +0800
From: Jason Wang <jasowang@...hat.com>
To: Akihiko Odaki <akihiko.odaki@...nix.com>
Cc: Jonathan Corbet <corbet@....net>, Willem de Bruijn <willemdebruijn.kernel@...il.com>, 
	"David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, 
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, 
	"Michael S. Tsirkin" <mst@...hat.com>, Xuan Zhuo <xuanzhuo@...ux.alibaba.com>, 
	Shuah Khan <shuah@...nel.org>, linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org, 
	netdev@...r.kernel.org, kvm@...r.kernel.org, 
	virtualization@...ts.linux-foundation.org, linux-kselftest@...r.kernel.org, 
	Yuri Benditovich <yuri.benditovich@...nix.com>, Andrew Melnychenko <andrew@...nix.com>, 
	Stephen Hemminger <stephen@...workplumber.org>, gur.stavi@...wei.com, 
	Lei Yang <leiyang@...hat.com>, Simon Horman <horms@...nel.org>
Subject: Re: [PATCH net-next v12 03/10] tun: Allow steering eBPF program to
 fall back

On Wed, Jun 4, 2025 at 3:24 PM Akihiko Odaki <akihiko.odaki@...nix.com> wrote:
>
> On 2025/06/04 10:27, Jason Wang wrote:
> > On Fri, May 30, 2025 at 12:50 PM Akihiko Odaki <akihiko.odaki@...nix.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@...nix.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).

Yes, that's my original point. Let's start from something that is
simpler and safer.

Thanks