Messages in this thread | | | Subject | Re: [PATCH net-next] tuntap: Fallback to automq on TUNSETSTEERINGEBPF prog negative return | From | Jason Wang <> | Date | Mon, 23 Sep 2019 08:46:04 +0800 |
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On 2019/9/23 上午1:43, Matt Cover wrote: > On Sun, Sep 22, 2019 at 5:37 AM Michael S. Tsirkin <mst@redhat.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@stackpath.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.
> 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.
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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