Date:   Mon, 23 Sep 2019 10:32:31 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Matt Cover <werekraken@...il.com>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>, davem@...emloft.net,
        ast@...nel.org, daniel@...earbox.net, kafai@...com,
        songliubraving@...com, yhs@...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 2019/9/23 上午9:20, Matt Cover wrote:
> On Sun, Sep 22, 2019 at 5:46 PM Jason Wang <jasowang@...hat.com> wrote:
>>
>> On 2019/9/23 上午1:43, 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)
>>
>> Well, using ebpf means it need to take care of all the cases. E.g you
>> can easily implement the fallback through eBPF as well.
>>
> I really think there is value in being
> able to implement a scoped special
> case while leaving the rest of the
> packets in the kernel's hands.


This is only work when some fucntion could not be done by eBPF itself 
and then we can provide the function through eBPF helpers. But this is 
not the case here.


>
> Having to reimplement automq makes
> this hookpoint less accessible to
> beginners and experienced alike.


Note that automq itself is kind of complicated, it's best effort that is 
hard to be documented accurately. It has several limitations (e.g flow 
caches etc.) that may not work well in some conditions.

It's not hard to implement a user programmable steering policy through 
maps which could have much deterministic behavior than automq. The goal 
of steering ebpf is to get rid of automq completely not partially rely 
on it.

And I don't see how relying on automq can simplify anything.

Thanks


>
>>>    2. determine there isn't sufficient information
>>>       to do proper queue selection; return
>>>       negative and use the default automq logic
>>>       for the unknown
>>
>> Same as above.
>>
>>
>>>    3. implement a noop prog (e.g. do
>>>       bpf_trace_printk() then return negative and
>>>       use the default automq logic for everything)
>>
>> ditto.
>>
>>
>>>> 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.
>>
>> Yes, eBPF can return probably wrong value, but what kernel did is just
>> to make sure it doesn't harm anything.
>>
>> I would rather just drop the packet in this case.
>>
> In addition to TUN_SSE_ABORT, we can
> add TUN_SSE_DROP. That could be made the
> default for any undefined negative
> return as well.
>
>> Thanks
>>
>>
>>>> 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?
>>>
>>>> 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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