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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Sun, 22 Sep 2019 16:35:54 -0400

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 <[email-masked]> 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 <[email-masked]>
> >
> >
> > 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.

> > 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...

> >
> >
> > 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