netdev - [PATCH net-next v2] netpoll: Use rtnl_dereference() for npinfo pointer access

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Message-Id: <20241122-netpoll_rcu_herbet_fix-v2-1-93a41fdbb19a@debian.org>
Date: Fri, 22 Nov 2024 05:17:57 -0800
From: Breno Leitao <leitao@...ian.org>
To: "David S. Miller" <davem@...emloft.net>, 
 Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, 
 Paolo Abeni <pabeni@...hat.com>, Simon Horman <horms@...nel.org>, 
 Michal Kubiak <michal.kubiak@...el.com>
Cc: netdev@...r.kernel.org, linux-kernel@...r.kernel.org, 
 kernel-team@...a.com, Herbert Xu <herbert@...dor.apana.org.au>, 
 Breno Leitao <leitao@...ian.org>, Jacob Keller <jacob.e.keller@...el.com>
Subject: [PATCH net-next v2] netpoll: Use rtnl_dereference() for npinfo
 pointer access

In the __netpoll_setup() function, when accessing the device's npinfo
pointer, replace rcu_access_pointer() with rtnl_dereference(). This
change is more appropriate, as suggested by Herbert Xu[1].

The function is called with the RTNL mutex held, and the pointer is
being dereferenced later, so, dereference earlier and just reuse the
pointer for the if/else.

The replacement ensures correct pointer access while maintaining
the existing locking and RCU semantics of the netpoll subsystem.

Link: https://lore.kernel.org/lkml/Zz1cKZYt1e7elibV@gondor.apana.org.au/ [1]
Suggested-by: Herbert Xu <herbert@...dor.apana.org.au>
Signed-off-by: Breno Leitao <leitao@...ian.org>
Reviewed-by: Jacob Keller <jacob.e.keller@...el.com>
Acked-by: Herbert Xu <herbert@...dor.apana.org.au>
---
Changes in v2:
- Targeting net-next instead of net
- Added the Acked-by and Reviewed-by
- Added the link in the commit summary
- Link to v1: https://lore.kernel.org/r/20241121-netpoll_rcu_herbet_fix-v1-1-457c5fee9dfd@debian.org
---
 net/core/netpoll.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/net/core/netpoll.c b/net/core/netpoll.c
index 45fb60bc4803958eb07d4038028269fc0c19622e..30152811e0903a369ccca30500e11e696be158fd 100644
--- a/net/core/netpoll.c
+++ b/net/core/netpoll.c
@@ -626,7 +626,8 @@ int __netpoll_setup(struct netpoll *np, struct net_device *ndev)
 		goto out;
 	}

-	if (!rcu_access_pointer(ndev->npinfo)) {
+	npinfo = rtnl_dereference(ndev->npinfo);
+	if (!npinfo) {
 		npinfo = kmalloc(sizeof(*npinfo), GFP_KERNEL);
 		if (!npinfo) {
 			err = -ENOMEM;
@@ -646,7 +647,6 @@ int __netpoll_setup(struct netpoll *np, struct net_device *ndev)
 				goto free_npinfo;
 		}
 	} else {
-		npinfo = rtnl_dereference(ndev->npinfo);
 		refcount_inc(&npinfo->refcnt);
 	}

---
base-commit: 66418447d27b7f4c027587582a133dd0bc0a663b
change-id: 20241121-netpoll_rcu_herbet_fix-3f0a433b7860

Best regards,
-- 
Breno Leitao <leitao@...ian.org>