ns: add asserts for initial namespace reference counts

They always remain fixed at one. Notice when that assumptions is broken.

Link: https://patch.msgid.link/20251110-work-namespace-nstree-fixes-v1-13-e8a9264e0fb9@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: brauner

include/linux/ns_common.h

@@ -60,10 +60,17 @@ static __always_inline __must_check int __ns_ref_active_read(const struct ns_com
 	return atomic_read(&ns->__ns_ref_active);
 }
 
+static __always_inline __must_check int __ns_ref_read(const struct ns_common *ns)
+{
+	return refcount_read(&ns->__ns_ref);
+}
+
 static __always_inline __must_check bool __ns_ref_put(struct ns_common *ns)
 {
-	if (is_ns_init_id(ns))
+	if (is_ns_init_id(ns)) {
+		VFS_WARN_ON_ONCE(__ns_ref_read(ns) != 1);
 		return false;
+	}
 	if (refcount_dec_and_test(&ns->__ns_ref)) {
 		VFS_WARN_ON_ONCE(__ns_ref_active_read(ns));
 		return true;
@@ -73,31 +80,32 @@ static __always_inline __must_check bool __ns_ref_put(struct ns_common *ns)
 
 static __always_inline __must_check bool __ns_ref_get(struct ns_common *ns)
 {
-	if (is_ns_init_id(ns))
+	if (is_ns_init_id(ns)) {
+		VFS_WARN_ON_ONCE(__ns_ref_read(ns) != 1);
 		return true;
+	}
 	if (refcount_inc_not_zero(&ns->__ns_ref))
 		return true;
 	VFS_WARN_ON_ONCE(__ns_ref_active_read(ns));
 	return false;
 }
 
-static __always_inline __must_check int __ns_ref_read(const struct ns_common *ns)
-{
-	return refcount_read(&ns->__ns_ref);
-}
-
 static __always_inline void __ns_ref_inc(struct ns_common *ns)
 {
-	if (is_ns_init_id(ns))
+	if (is_ns_init_id(ns)) {
+		VFS_WARN_ON_ONCE(__ns_ref_read(ns) != 1);
 		return;
+	}
 	refcount_inc(&ns->__ns_ref);
 }
 
 static __always_inline __must_check bool __ns_ref_dec_and_lock(struct ns_common *ns,
 							       spinlock_t *ns_lock)
 {
-	if (is_ns_init_id(ns))
+	if (is_ns_init_id(ns)) {
+		VFS_WARN_ON_ONCE(__ns_ref_read(ns) != 1);
 		return false;
+	}
 	return refcount_dec_and_lock(&ns->__ns_ref, ns_lock);
 }