[PATCH v2 2/2] NFS: Fix RCU dereference of cl_xprt in nfs_compare_super_address

[Sean Chang <seanwascoding@gmail.com>]

The cl_xprt pointer in struct rpc_clnt is marked as __rcu. Accessing
it directly in nfs_compare_super_address() is unsafe and triggers
Sparse warnings.

Fix this by wrapping the access with rcu_read_lock() and using
rcu_dereference() to safely retrieve the transport pointer. This
ensures the xprt structure remains memory-safe during the comparison
of network namespaces and addresses.

Additionally, add a check for the XPRT_CONNECTED state bit. While RCU
guarantees the memory remains valid, checking XPRT_CONNECTED ensures
the transport is still logically active, preventing operations on a
transport that is already undergoing teardown.

Fixes: 7e3fcf61abde ("nfs: don't share mounts between network namespaces")
Signed-off-by: Sean Chang <seanwascoding@gmail.com>
---
 fs/nfs/super.c | 21 ++++++++++++++++++---
 1 file changed, 18 insertions(+), 3 deletions(-)

diff --git a/fs/nfs/super.c b/fs/nfs/super.c
index 7a318581f85b..c9044d9d64cc 100644
--- a/fs/nfs/super.c
+++ b/fs/nfs/super.c
@@ -1166,12 +1166,23 @@ static int nfs_set_super(struct super_block *s, struct fs_context *fc)
 static int nfs_compare_super_address(struct nfs_server *server1,
 				     struct nfs_server *server2)
 {
+	struct rpc_xprt *xprt1, *xprt2;
 	struct sockaddr *sap1, *sap2;
-	struct rpc_xprt *xprt1 = server1->client->cl_xprt;
-	struct rpc_xprt *xprt2 = server2->client->cl_xprt;
+
+	rcu_read_lock();
+
+	xprt1 = rcu_dereference(server1->client->cl_xprt);
+	xprt2 = rcu_dereference(server2->client->cl_xprt);
+
+	if (!xprt1 || !xprt2 ||
+	    !test_bit(XPRT_CONNECTED, &xprt1->state) ||
+	    !test_bit(XPRT_CONNECTED, &xprt2->state))
+		goto out_unlock;
 
 	if (!net_eq(xprt1->xprt_net, xprt2->xprt_net))
-		return 0;
+		goto out_unlock;
+
+	rcu_read_unlock();
 
 	sap1 = (struct sockaddr *)&server1->nfs_client->cl_addr;
 	sap2 = (struct sockaddr *)&server2->nfs_client->cl_addr;
@@ -1203,6 +1214,10 @@ static int nfs_compare_super_address(struct nfs_server *server1,
 	}
 
 	return 1;
+
+out_unlock:
+	rcu_read_unlock();
+	return 0;
 }
 
 static int nfs_compare_userns(const struct nfs_server *old,
-- 
2.43.0