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

[Matthew Cover <werekraken@gmail.com>]

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