linux-kernel - [PATCH net-next v11 03/10] tun: Allow steering eBPF program to fall back

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Message-Id: <20250317-rss-v11-3-4cacca92f31f@daynix.com>
Date: Mon, 17 Mar 2025 19:57:53 +0900
From: Akihiko Odaki <akihiko.odaki@...nix.com>
To: Jonathan Corbet <corbet@....net>, 
 Willem de Bruijn <willemdebruijn.kernel@...il.com>, 
 Jason Wang <jasowang@...hat.com>, "David S. Miller" <davem@...emloft.net>, 
 Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, 
 Paolo Abeni <pabeni@...hat.com>, "Michael S. Tsirkin" <mst@...hat.com>, 
 Xuan Zhuo <xuanzhuo@...ux.alibaba.com>, Shuah Khan <shuah@...nel.org>, 
 linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org, 
 netdev@...r.kernel.org, kvm@...r.kernel.org, 
 virtualization@...ts.linux-foundation.org, linux-kselftest@...r.kernel.org, 
 Yuri Benditovich <yuri.benditovich@...nix.com>, 
 Andrew Melnychenko <andrew@...nix.com>, 
 Stephen Hemminger <stephen@...workplumber.org>, gur.stavi@...wei.com, 
 Lei Yang <leiyang@...hat.com>, Simon Horman <horms@...nel.org>, 
 Akihiko Odaki <akihiko.odaki@...nix.com>
Subject: [PATCH net-next v11 03/10] tun: Allow steering eBPF program to
 fall back

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

Signed-off-by: Akihiko Odaki <akihiko.odaki@...nix.com>
---
 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