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

[Matt Cover]

On Sun, Sep 22, 2019 at 5:51 PM Jason Wang <jasowang@xxxxxxxxxx> wrote:
>
>
> On 2019/9/23 äå6:30, Matt Cover wrote:
> > On Sun, Sep 22, 2019 at 1:36 PM Michael S. Tsirkin <mst@xxxxxxxxxx> 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@xxxxxxxxxx> 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@xxxxxxxxxxxxx>
> >>>>
> >>>> 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.
>
>
> Can it really return -1?
>
> I see:
>
> static inline u32 bpf_prog_run_clear_cb(const struct bpf_prog *prog,
>                                          struct sk_buff *skb)
> ...
>
>
> >
> > 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.
>
>
> Having such fallback may bring extra troubles, it requires the eBPF
> program know the existence of the behavior which is not a part of kernel
> ABI actually. And then some eBPF program may start to rely on that which
> is pretty dangerous. Note, one important consideration is to have
> macvtap support where does not have any stuffs like automq.
>
> Thanks
>

How about we call this TUN_SSE_ABORT
instead of TUN_SSE_DO_AUTOMQ?

TUN_SSE_ABORT could be documented as
falling back to the default queue
selection method in either space
(presumably macvtap has some queue
selection method when there is no prog).

>
> >
> > 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