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

Matthew Cover
2019-09-20T18:59:15Z

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>
---
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