netdev - [PATCH net-next 05/10] net: dst: annotate data-races around dst->output

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Message-ID: <20250627112526.3615031-6-edumazet@google.com>
Date: Fri, 27 Jun 2025 11:25:21 +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>, Kuniyuki Iwashima <kuniyu@...gle.com>, 
	David Ahern <dsahern@...nel.org>, netdev@...r.kernel.org, eric.dumazet@...il.com, 
	Eric Dumazet <edumazet@...gle.com>
Subject: [PATCH net-next 05/10] net: dst: annotate data-races around dst->output

dst_dev_put() can overwrite dst->output while other
cpus might read this field (for instance from dst_output())

Add READ_ONCE()/WRITE_ONCE() annotations to suppress
potential issues.

We will likely need RCU protection in the future.

Fixes: 4a6ce2b6f2ec ("net: introduce a new function dst_dev_put()")
Signed-off-by: Eric Dumazet <edumazet@...gle.com>
---
 include/net/dst.h      | 2 +-
 include/net/lwtunnel.h | 4 ++--
 net/core/dst.c         | 2 +-
 net/ipv4/route.c       | 2 +-
 4 files changed, 5 insertions(+), 5 deletions(-)

diff --git a/include/net/dst.h b/include/net/dst.h
index c0f8b6d8e70746fe09a68037f14d6e5bf1d1c57e..b6acfde7d587c40489aaf869f715479478f548ca 100644
--- a/include/net/dst.h
+++ b/include/net/dst.h
@@ -458,7 +458,7 @@ INDIRECT_CALLABLE_DECLARE(int ip_output(struct net *, struct sock *,
 /* Output packet to network from transport.  */
 static inline int dst_output(struct net *net, struct sock *sk, struct sk_buff *skb)
 {
-	return INDIRECT_CALL_INET(skb_dst(skb)->output,
+	return INDIRECT_CALL_INET(READ_ONCE(skb_dst(skb)->output),
 				  ip6_output, ip_output,
 				  net, sk, skb);
 }
diff --git a/include/net/lwtunnel.h b/include/net/lwtunnel.h
index eaac07d505959e263e479e0fe288424371945f5d..26232f603e33c9c7a1499a0dd2f69c0ba10cc381 100644
--- a/include/net/lwtunnel.h
+++ b/include/net/lwtunnel.h
@@ -138,8 +138,8 @@ int bpf_lwt_push_ip_encap(struct sk_buff *skb, void *hdr, u32 len,
 static inline void lwtunnel_set_redirect(struct dst_entry *dst)
 {
 	if (lwtunnel_output_redirect(dst->lwtstate)) {
-		dst->lwtstate->orig_output = dst->output;
-		dst->output = lwtunnel_output;
+		dst->lwtstate->orig_output = READ_ONCE(dst->output);
+		WRITE_ONCE(dst->output, lwtunnel_output);
 	}
 	if (lwtunnel_input_redirect(dst->lwtstate)) {
 		dst->lwtstate->orig_input = READ_ONCE(dst->input);
diff --git a/net/core/dst.c b/net/core/dst.c
index 13c629dc7123da1eaeb07e4546ae6c3f38265af1..52e824e57c1755a39008fede0d97c7ed7be56855 100644
--- a/net/core/dst.c
+++ b/net/core/dst.c
@@ -149,7 +149,7 @@ void dst_dev_put(struct dst_entry *dst)
 	if (dst->ops->ifdown)
 		dst->ops->ifdown(dst, dev);
 	WRITE_ONCE(dst->input, dst_discard);
-	dst->output = dst_discard_out;
+	WRITE_ONCE(dst->output, dst_discard_out);
 	dst->dev = blackhole_netdev;
 	netdev_ref_replace(dev, blackhole_netdev, &dst->dev_tracker,
 			   GFP_ATOMIC);
diff --git a/net/ipv4/route.c b/net/ipv4/route.c
index 75a1f9eabd6b6350b1ebc9d7dc8166b3d9364a03..ce6aba4f01ff25a8f9238271a7ae2295f7c7bb93 100644
--- a/net/ipv4/route.c
+++ b/net/ipv4/route.c
@@ -1688,7 +1688,7 @@ struct rtable *rt_dst_clone(struct net_device *dev, struct rtable *rt)
 			new_rt->rt_gw6 = rt->rt_gw6;
 
 		new_rt->dst.input = READ_ONCE(rt->dst.input);
-		new_rt->dst.output = rt->dst.output;
+		new_rt->dst.output = READ_ONCE(rt->dst.output);
 		new_rt->dst.error = rt->dst.error;
 		new_rt->dst.lastuse = jiffies;
 		new_rt->dst.lwtstate = lwtstate_get(rt->dst.lwtstate);
-- 
2.50.0.727.gbf7dc18ff4-goog