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

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Message-Id: <20241202-netpoll_rcu_herbet_fix-v2-1-2b9d58edc76a@debian.org>
Date: Mon, 02 Dec 2024 16:22:05 -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, 
 Herbert Xu <herbert@...dor.apana.org.au>, Breno Leitao <leitao@...ian.org>, 
 Jacob Keller <jacob.e.keller@...el.com>
Subject: [PATCH RESEND 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>
---
v2 RESEND:
- Resending the same v2, since I sent it when the net-next was closed.  :-P
- Link to v2: https://lore.kernel.org/r/20241122-netpoll_rcu_herbet_fix-v2-1-93a41fdbb19a@debian.org

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 2e459b9d88eb52b7c5e4710bf3f7740045d9314b..99e5aa9cc992f429eecf20aeadd04dc293b8f22b 100644
--- a/net/core/netpoll.c
+++ b/net/core/netpoll.c
@@ -634,7 +634,8 @@ 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;
@@ -654,7 +655,6 @@ 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: 65ae975e97d5aab3ee9dc5ec701b12090572ed43
change-id: 20241121-netpoll_rcu_herbet_fix-3f0a433b7860

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