netdev - [PATCH RFC net-next 2/5] net/sched: act

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Message-ID: <20251006193103.2684156-3-edumazet@google.com>
Date: Mon,  6 Oct 2025 19:31:00 +0000
From: Eric Dumazet <edumazet@...gle.com>
To: "David S . Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, 
	Paolo Abeni <pabeni@...hat.com>
Cc: Simon Horman <horms@...nel.org>, Jamal Hadi Salim <jhs@...atatu.com>, 
	Cong Wang <xiyou.wangcong@...il.com>, Jiri Pirko <jiri@...nulli.us>, 
	Kuniyuki Iwashima <kuniyu@...gle.com>, Willem de Bruijn <willemb@...gle.com>, netdev@...r.kernel.org, 
	eric.dumazet@...il.com, Eric Dumazet <edumazet@...gle.com>
Subject: [PATCH RFC net-next 2/5] net/sched: act_mirred: add loop detection

We want to revert commit 0f022d32c3ec ("net/sched: Fix mirred deadlock
on device recursion") because it adds code in the fast path, even when
act_mirred is not used.

Use an additional device pointers array in struct netdev_xmit
and implement loop detection in tcf_mirred_is_act_redirect().

Signed-off-by: Eric Dumazet <edumazet@...gle.com>
---
 include/linux/netdevice_xmit.h |  9 ++++-
 net/sched/act_mirred.c         | 62 +++++++++++++---------------------
 2 files changed, 31 insertions(+), 40 deletions(-)

diff --git a/include/linux/netdevice_xmit.h b/include/linux/netdevice_xmit.h
index 813a19122ebbb2c6a04176330b1055b7c2b9c902..cc232508e695eefe95ea6e55a21978be11d5da83 100644
--- a/include/linux/netdevice_xmit.h
+++ b/include/linux/netdevice_xmit.h
@@ -2,6 +2,12 @@
 #ifndef _LINUX_NETDEVICE_XMIT_H
 #define _LINUX_NETDEVICE_XMIT_H
 
+#if IS_ENABLED(CONFIG_NET_ACT_MIRRED)
+#define MIRRED_NEST_LIMIT	4
+#endif
+
+struct net_device;
+
 struct netdev_xmit {
 	u16 recursion;
 	u8  more;
@@ -9,7 +15,8 @@ struct netdev_xmit {
 	u8  skip_txqueue;
 #endif
 #if IS_ENABLED(CONFIG_NET_ACT_MIRRED)
-	u8 sched_mirred_nest;
+	u8			sched_mirred_nest;
+	struct net_device	*sched_mirred_dev[MIRRED_NEST_LIMIT];
 #endif
 #if IS_ENABLED(CONFIG_NF_DUP_NETDEV)
 	u8 nf_dup_skb_recursion;
diff --git a/net/sched/act_mirred.c b/net/sched/act_mirred.c
index 5f01f567c934d3669d9a3058cff861a8fe5f88b6..f27b583def78e4afecc7112854b93d59c2520201 100644
--- a/net/sched/act_mirred.c
+++ b/net/sched/act_mirred.c
@@ -29,31 +29,6 @@
 static LIST_HEAD(mirred_list);
 static DEFINE_SPINLOCK(mirred_list_lock);
 
-#define MIRRED_NEST_LIMIT    4
-
-#ifndef CONFIG_PREEMPT_RT
-static u8 tcf_mirred_nest_level_inc_return(void)
-{
-	return __this_cpu_inc_return(softnet_data.xmit.sched_mirred_nest);
-}
-
-static void tcf_mirred_nest_level_dec(void)
-{
-	__this_cpu_dec(softnet_data.xmit.sched_mirred_nest);
-}
-
-#else
-static u8 tcf_mirred_nest_level_inc_return(void)
-{
-	return current->net_xmit.sched_mirred_nest++;
-}
-
-static void tcf_mirred_nest_level_dec(void)
-{
-	current->net_xmit.sched_mirred_nest--;
-}
-#endif
-
 static bool tcf_mirred_is_act_redirect(int action)
 {
 	return action == TCA_EGRESS_REDIR || action == TCA_INGRESS_REDIR;
@@ -439,44 +414,53 @@ TC_INDIRECT_SCOPE int tcf_mirred_act(struct sk_buff *skb,
 {
 	struct tcf_mirred *m = to_mirred(a);
 	int retval = READ_ONCE(m->tcf_action);
-	unsigned int nest_level;
+	struct netdev_xmit *xmit;
 	bool m_mac_header_xmit;
 	struct net_device *dev;
-	int m_eaction;
+	int i, m_eaction;
 	u32 blockid;
 
-	nest_level = tcf_mirred_nest_level_inc_return();
-	if (unlikely(nest_level > MIRRED_NEST_LIMIT)) {
+#ifdef CONFIG_PREEMPT_RT
+	xmit = &current->net_xmit;
+#else
+	xmit = this_cpu_ptr(&softnet_data.xmit);
+#endif
+	if (unlikely(xmit->sched_mirred_nest >= MIRRED_NEST_LIMIT)) {
 		net_warn_ratelimited("Packet exceeded mirred recursion limit on dev %s\n",
 				     netdev_name(skb->dev));
-		retval = TC_ACT_SHOT;
-		goto dec_nest_level;
+		return TC_ACT_SHOT;
 	}
 
 	tcf_lastuse_update(&m->tcf_tm);
 	tcf_action_update_bstats(&m->common, skb);
 
 	blockid = READ_ONCE(m->tcfm_blockid);
-	if (blockid) {
-		retval = tcf_blockcast(skb, m, blockid, res, retval);
-		goto dec_nest_level;
-	}
+	if (blockid)
+		return tcf_blockcast(skb, m, blockid, res, retval);
 
 	dev = rcu_dereference_bh(m->tcfm_dev);
 	if (unlikely(!dev)) {
 		pr_notice_once("tc mirred: target device is gone\n");
 		tcf_action_inc_overlimit_qstats(&m->common);
-		goto dec_nest_level;
+		return retval;
 	}
+	for (i = 0; i < xmit->sched_mirred_nest; i++) {
+		if (xmit->sched_mirred_dev[i] != dev)
+			continue;
+		pr_notice_once("tc mirred: loop on device %s\n",
+			       netdev_name(dev));
+		tcf_action_inc_overlimit_qstats(&m->common);
+		return retval;
+	}
+
+	xmit->sched_mirred_dev[xmit->sched_mirred_nest++] = dev;
 
 	m_mac_header_xmit = READ_ONCE(m->tcfm_mac_header_xmit);
 	m_eaction = READ_ONCE(m->tcfm_eaction);
 
 	retval = tcf_mirred_to_dev(skb, m, dev, m_mac_header_xmit, m_eaction,
 				   retval);
-
-dec_nest_level:
-	tcf_mirred_nest_level_dec();
+	xmit->sched_mirred_nest--;
 
 	return retval;
 }
-- 
2.51.0.618.g983fd99d29-goog