netdev - [RFC PATCH 13/15] l2tp: add idr consistency check in session

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Message-Id: <1857469bef821f5ef31e2f7ef586d1e904b83037.1721733730.git.jchapman@katalix.com>
Date: Tue, 23 Jul 2024 14:51:41 +0100
From: James Chapman <jchapman@...alix.com>
To: netdev@...r.kernel.org
Cc: davem@...emloft.net,
	edumazet@...gle.com,
	kuba@...nel.org,
	pabeni@...hat.com,
	dsahern@...nel.org,
	tparkin@...alix.com
Subject: [RFC PATCH 13/15] l2tp: add idr consistency check in session_register

l2tp_session_register uses an idr_alloc then idr_replace pattern to
insert sessions into the session IDR. To catch invalid locking, add a
WARN_ON_ONCE if the IDR entry is modified by another thread between
alloc and replace steps.

Also add comments to make expectations clear.
---
 net/l2tp/l2tp_core.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/net/l2tp/l2tp_core.c b/net/l2tp/l2tp_core.c
index cd9b157e8b4d..fd03c17dd20c 100644
--- a/net/l2tp/l2tp_core.c
+++ b/net/l2tp/l2tp_core.c
@@ -474,6 +474,7 @@ int l2tp_session_register(struct l2tp_session *session,
 {
 	struct l2tp_net *pn = l2tp_pernet(tunnel->l2tp_net);
 	struct l2tp_session *other_session = NULL;
+	void *old = NULL;
 	u32 session_key;
 	int err;
 
@@ -517,13 +518,19 @@ int l2tp_session_register(struct l2tp_session *session,
 	WRITE_ONCE(session->tunnel, tunnel);
 	list_add_rcu(&session->list, &tunnel->session_list);
 
+	/* this makes session available to lockless getters */
 	if (tunnel->version == L2TP_HDR_VER_3) {
 		if (!other_session)
-			idr_replace(&pn->l2tp_v3_session_idr, session, session_key);
+			old = idr_replace(&pn->l2tp_v3_session_idr, session, session_key);
 	} else {
-		idr_replace(&pn->l2tp_v2_session_idr, session, session_key);
+		old = idr_replace(&pn->l2tp_v2_session_idr, session, session_key);
 	}
 
+	/* old should be NULL, unless something removed or modified
+	 * the IDR entry after our idr_alloc_32 above (which shouldn't
+	 * happen).
+	 */
+	WARN_ON_ONCE(old);
 out:
 	spin_unlock_bh(&pn->l2tp_session_idr_lock);
 	spin_unlock_bh(&tunnel->list_lock);
-- 
2.34.1