Re: [PATCH net-next] tuntap: Fallback to automq on TUNSETSTEERINGEBPF prog negative return
From: Jason Wang
Date: Mon Sep 23 2019 - 01:08:56 EST
On 2019/9/23 äå11:00, Matt Cover wrote:
On Sun, Sep 22, 2019 at 7:32 PM Jason Wang <jasowang@xxxxxxxxxx> wrote:
On 2019/9/23 äå9:20, Matt Cover wrote:
On Sun, Sep 22, 2019 at 5:46 PM Jason Wang <jasowang@xxxxxxxxxx> wrote:
On 2019/9/23 äå1:43, 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)
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
I'm not suggesting that we document automq.
I'm suggesting that we add a return value
which is documented as signaling to the
kernel to implement whatever queue
selection method is used when there is no
ebpf prog attached.
Again, this only work when there's something that could not be done
through eBPF. And then we can provide eBPF helper there.
That behavior today is
automq.
Automq is not good, e.g tun_ebpf_select_queue() has already provided a
fallback, anything that automq can do better than that?
There is nothing about this return value
which would harder to change the default
queue selection later. The default already
exists today when there is no program
loaded.
The patch depends on incorrect behavior of tuntap (updating flow caches
when steering prog is set). I think it's wrong to update flow caches
even when steering program is set which leads extra overhead. Will
probably submit a patch to disable that behavior.
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