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/
Suggested-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Link: https://patch.msgid.link/20241202-netpoll_rcu_herbet_fix-v2-1-2b9d58edc76a@debian.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/net/core/netpoll.c b/net/core/netpoll.c
index 2e459b9d88eb52b7c5e4710bf3f7740045d9314b..99e5aa9cc992f429eecf20aeadd04dc293b8f22b 100644 (file)
--- 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);
        }