Date:   Sun, 22 Sep 2019 15:30:29 -0700
From:   Matt Cover <werekraken@...il.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     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>,
        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 Sun, Sep 22, 2019 at 1:36 PM Michael S. Tsirkin <mst@...hat.com> wrote:
>
> On Sun, Sep 22, 2019 at 10:43:19AM -0700, Matt Cover wrote:
> > On Sun, Sep 22, 2019 at 5:37 AM Michael S. Tsirkin <mst@...hat.com> wrote:
> > >
> > > On Fri, Sep 20, 2019 at 11:58:43AM -0700, Matthew Cover 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>
> > >
> > >
> > > Could you add a bit more motivation data here?
> >
> > Thank you for these questions Michael.
> >
> > I'll plan on adding the below information to the
> > commit message and submitting a v2 of this patch
> > when net-next reopens. In the meantime, it would
> > be very helpful to know if these answers address
> > some of your concerns.
> >
> > > 1. why is this a good idea
> >
> > This change allows TUNSETSTEERINGEBPF progs to
> > do any of the following.
> >  1. implement queue selection for a subset of
> >     traffic (e.g. special queue selection logic
> >     for ipv4, but return negative and use the
> >     default automq logic for ipv6)
> >  2. determine there isn't sufficient information
> >     to do proper queue selection; return
> >     negative and use the default automq logic
> >     for the unknown
> >  3. implement a noop prog (e.g. do
> >     bpf_trace_printk() then return negative and
> >     use the default automq logic for everything)
> >
> > > 2. how do we know existing userspace does not rely on existing behaviour
> >
> > Prior to this change a negative return from a
> > TUNSETSTEERINGEBPF prog would have been cast
> > into a u16 and traversed netdev_cap_txqueue().
> >
> > In most cases netdev_cap_txqueue() would have
> > found this value to exceed real_num_tx_queues
> > and queue_index would be updated to 0.
> >
> > It is possible that a TUNSETSTEERINGEBPF prog
> > return a negative value which when cast into a
> > u16 results in a positive queue_index less than
> > real_num_tx_queues. For example, on x86_64, a
> > return value of -65535 results in a queue_index
> > of 1; which is a valid queue for any multiqueue
> > device.
> >
> > It seems unlikely, however as stated above is
> > unfortunately possible, that existing
> > TUNSETSTEERINGEBPF programs would choose to
> > return a negative value rather than return the
> > positive value which holds the same meaning.
> >
> > It seems more likely that future
> > TUNSETSTEERINGEBPF programs would leverage a
> > negative return and potentially be loaded into
> > a kernel with the old behavior.
>
> OK if we are returning a special
> value, shouldn't we limit it? How about a special
> value with this meaning?
> If we are changing an ABI let's at least make it
> extensible.
>

A special value with this meaning sounds
good to me. I'll plan on adding a define
set to -1 to cause the fallback to automq.

The way I was initially viewing the old
behavior was that returning negative was
undefined; it happened to have the
outcomes I walked through, but not
necessarily by design.

In order to keep the new behavior
extensible, how should we state that a
negative return other than -1 is
undefined and therefore subject to
change. Is something like this
sufficient?

  Documentation/networking/tc-actions-env-rules.txt

Additionally, what should the new
behavior implement when a negative other
than -1 is returned? I would like to have
it do the same thing as -1 for now, but
with the understanding that this behavior
is undefined. Does this sound reasonable?

> > > 3. why doesn't userspace need a way to figure out whether it runs on a kernel with and
> > >    without this patch
> >
> > There may be some value in exposing this fact
> > to the ebpf prog loader. What is the standard
> > practice here, a define?
>
>
> We'll need something at runtime - people move binaries between kernels
> without rebuilding then. An ioctl is one option.
> A sysfs attribute is another, an ethtool flag yet another.
> A combination of these is possible.
>
> And if we are doing this anyway, maybe let userspace select
> the new behaviour? This way we can stay compatible with old
> userspace...
>

Understood. I'll look into adding an
ioctl to activate the new behavior. And
perhaps a method of checking which is
behavior is currently active (in case we
ever want to change the default, say
after some suitably long transition
period).

> > >
> > >
> > > thanks,
> > > MST
> > >
> > > > ---
> > > >  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

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