linux-kernel - [PATCH 6/8] ns: add asserts for active refcount underflow

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Message-Id: <20251109-namespace-6-19-fixes-v1-6-ae8a4ad5a3b3@kernel.org>
Date: Sun, 09 Nov 2025 22:11:27 +0100
From: Christian Brauner <brauner@...nel.org>
To: linux-fsdevel@...r.kernel.org, Josef Bacik <josef@...icpanda.com>, 
 Jeff Layton <jlayton@...nel.org>
Cc: Jann Horn <jannh@...gle.com>, Mike Yuan <me@...dnzj.com>, 
 Zbigniew Jędrzejewski-Szmek <zbyszek@...waw.pl>, 
 Lennart Poettering <mzxreary@...inter.de>, 
 Daan De Meyer <daan.j.demeyer@...il.com>, Aleksa Sarai <cyphar@...har.com>, 
 Amir Goldstein <amir73il@...il.com>, Tejun Heo <tj@...nel.org>, 
 Johannes Weiner <hannes@...xchg.org>, Thomas Gleixner <tglx@...utronix.de>, 
 Alexander Viro <viro@...iv.linux.org.uk>, Jan Kara <jack@...e.cz>, 
 linux-kernel@...r.kernel.org, cgroups@...r.kernel.org, bpf@...r.kernel.org, 
 Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, 
 netdev@...r.kernel.org, Arnd Bergmann <arnd@...db.de>, 
 Christian Brauner <brauner@...nel.org>
Subject: [PATCH 6/8] ns: add asserts for active refcount underflow

Add a few more assert to detect active reference count underflows.

Signed-off-by: Christian Brauner <brauner@...nel.org>
---
 include/linux/ns_common.h |  1 -
 kernel/nscommon.c         | 18 ++++++++++++++----
 2 files changed, 14 insertions(+), 5 deletions(-)

diff --git a/include/linux/ns_common.h b/include/linux/ns_common.h
index 3aaba2ca31d7..66ea09b48377 100644
--- a/include/linux/ns_common.h
+++ b/include/linux/ns_common.h
@@ -294,7 +294,6 @@ void __ns_ref_active_put(struct ns_common *ns);
 
 static __always_inline struct ns_common *__must_check ns_get_unless_inactive(struct ns_common *ns)
 {
-	VFS_WARN_ON_ONCE(__ns_ref_active_read(ns) && !__ns_ref_read(ns));
 	if (!__ns_ref_active_read(ns)) {
 		VFS_WARN_ON_ONCE(is_ns_init_id(ns));
 		return NULL;
diff --git a/kernel/nscommon.c b/kernel/nscommon.c
index bfd2d6805776..c910b979e433 100644
--- a/kernel/nscommon.c
+++ b/kernel/nscommon.c
@@ -170,8 +170,10 @@ void __ns_ref_active_put(struct ns_common *ns)
 	if (is_ns_init_id(ns))
 		return;
 
-	if (!atomic_dec_and_test(&ns->__ns_ref_active))
+	if (!atomic_dec_and_test(&ns->__ns_ref_active)) {
+		VFS_WARN_ON_ONCE(__ns_ref_active_read(ns) < 0);
 		return;
+	}
 
 	VFS_WARN_ON_ONCE(is_ns_init_id(ns));
 	VFS_WARN_ON_ONCE(!__ns_ref_read(ns));
@@ -181,8 +183,10 @@ void __ns_ref_active_put(struct ns_common *ns)
 		if (!ns)
 			return;
 		VFS_WARN_ON_ONCE(is_ns_init_id(ns));
-		if (!atomic_dec_and_test(&ns->__ns_ref_active))
+		if (!atomic_dec_and_test(&ns->__ns_ref_active)) {
+			VFS_WARN_ON_ONCE(__ns_ref_active_read(ns) < 0);
 			return;
+		}
 	}
 }
 
@@ -280,12 +284,16 @@ void __ns_ref_active_put(struct ns_common *ns)
  */
 void __ns_ref_active_get(struct ns_common *ns)
 {
+	int prev;
+
 	/* Initial namespaces are always active. */
 	if (is_ns_init_id(ns))
 		return;
 
 	/* If we didn't resurrect the namespace we're done. */
-	if (atomic_fetch_add(1, &ns->__ns_ref_active))
+	prev = atomic_fetch_add(1, &ns->__ns_ref_active);
+	VFS_WARN_ON_ONCE(prev < 0);
+	if (likely(prev))
 		return;
 
 	/*
@@ -298,7 +306,9 @@ void __ns_ref_active_get(struct ns_common *ns)
 			return;
 
 		VFS_WARN_ON_ONCE(is_ns_init_id(ns));
-		if (atomic_fetch_add(1, &ns->__ns_ref_active))
+		prev = atomic_fetch_add(1, &ns->__ns_ref_active);
+		VFS_WARN_ON_ONCE(prev < 0);
+		if (likely(prev))
 			return;
 	}
 }

-- 
2.47.3