[PATCH net-next v10 03/10] tun: Allow steering eBPF program to fall back

From: Akihiko Odaki
Date: Thu Mar 13 2025 - 03:05:57 EST


This clarifies a steering eBPF program takes precedence over the other
steering algorithms.

Signed-off-by: Akihiko Odaki <akihiko.odaki@xxxxxxxxxx>
---
Documentation/networking/tuntap.rst | 7 +++++++
drivers/net/tun.c | 28 +++++++++++++++++-----------
include/uapi/linux/if_tun.h | 9 +++++++++
3 files changed, 33 insertions(+), 11 deletions(-)

diff --git a/Documentation/networking/tuntap.rst b/Documentation/networking/tuntap.rst
index 4d7087f727be..86b4ae8caa8a 100644
--- a/Documentation/networking/tuntap.rst
+++ b/Documentation/networking/tuntap.rst
@@ -206,6 +206,13 @@ enable is true we enable it, otherwise we disable it::
return ioctl(fd, TUNSETQUEUE, (void *)&ifr);
}

+3.4 Reference
+-------------
+
+``linux/if_tun.h`` defines the interface described below:
+
+.. kernel-doc:: include/uapi/linux/if_tun.h
+
Universal TUN/TAP device driver Frequently Asked Question
=========================================================

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index d8f4d3e996a7..9133ab9ed3f5 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -476,21 +476,29 @@ 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 bool tun_ebpf_select_queue(struct tun_struct *tun, struct sk_buff *skb,
+ u16 *ret)
{
struct tun_prog *prog;
u32 numqueues;
- u16 ret = 0;
+ u32 prog_ret;
+
+ prog = rcu_dereference(tun->steering_prog);
+ if (!prog)
+ return false;

numqueues = READ_ONCE(tun->numqueues);
- if (!numqueues)
- return 0;
+ if (!numqueues) {
+ *ret = 0;
+ return true;
+ }

- prog = rcu_dereference(tun->steering_prog);
- if (prog)
- ret = bpf_prog_run_clear_cb(prog->prog, skb);
+ prog_ret = bpf_prog_run_clear_cb(prog->prog, skb);
+ if (prog_ret == TUN_STEERINGEBPF_FALLBACK)
+ return false;

- return ret % numqueues;
+ *ret = (u16)prog_ret % numqueues;
+ return true;
}

static u16 tun_select_queue(struct net_device *dev, struct sk_buff *skb,
@@ -500,9 +508,7 @@ static u16 tun_select_queue(struct net_device *dev, struct sk_buff *skb,
u16 ret;

rcu_read_lock();
- if (rcu_dereference(tun->steering_prog))
- ret = tun_ebpf_select_queue(tun, skb);
- else
+ if (!tun_ebpf_select_queue(tun, skb, &ret))
ret = tun_automq_select_queue(tun, skb);
rcu_read_unlock();

diff --git a/include/uapi/linux/if_tun.h b/include/uapi/linux/if_tun.h
index 287cdc81c939..980de74724fc 100644
--- a/include/uapi/linux/if_tun.h
+++ b/include/uapi/linux/if_tun.h
@@ -115,4 +115,13 @@ struct tun_filter {
__u8 addr[][ETH_ALEN];
};

+/**
+ * define TUN_STEERINGEBPF_FALLBACK - A steering eBPF return value to fall back
+ *
+ * A steering eBPF program may return this value to fall back to the steering
+ * algorithm that should have been used if the program was not set. This allows
+ * selectively overriding the steering decision.
+ */
+#define TUN_STEERINGEBPF_FALLBACK -1
+
#endif /* _UAPI__IF_TUN_H */

--
2.48.1