nstree: introduce a unified tree

This will allow userspace to lookup and stat a namespace simply by its
identifier without having to know what type of namespace it is.

Link: https://patch.msgid.link/20251029-work-namespace-nstree-listns-v4-13-2e6f823ebdc0@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>

include/linux/ns_common.h

@@ -109,6 +109,9 @@ struct ns_common {
 	union {
 		struct {
 			u64 ns_id;
+			struct /* global namespace rbtree and list */ {
+				struct rb_node ns_unified_tree_node;
+			};
 			struct /* per type rbtree and list */ {
 				struct rb_node ns_tree_node;
 				struct list_head ns_list_node;

kernel/nscommon.c

@@ -62,6 +62,7 @@ int __ns_common_init(struct ns_common *ns, u32 ns_type, const struct proc_ns_ope
 	ns->ns_id = 0;
 	ns->ns_type = ns_type;
 	RB_CLEAR_NODE(&ns->ns_tree_node);
+	RB_CLEAR_NODE(&ns->ns_unified_tree_node);
 	INIT_LIST_HEAD(&ns->ns_list_node);
 
 #ifdef CONFIG_DEBUG_VFS

kernel/nstree.c

@@ -5,31 +5,30 @@
 #include <linux/proc_ns.h>
 #include <linux/vfsdebug.h>
 
+static __cacheline_aligned_in_smp DEFINE_SEQLOCK(ns_tree_lock);
+static struct rb_root ns_unified_tree = RB_ROOT; /* protected by ns_tree_lock */
+
 /**
  * struct ns_tree - Namespace tree
  * @ns_tree: Rbtree of namespaces of a particular type
  * @ns_list: Sequentially walkable list of all namespaces of this type
- * @ns_tree_lock: Seqlock to protect the tree and list
  * @type: type of namespaces in this tree
  */
 struct ns_tree {
-       struct rb_root ns_tree;
-       struct list_head ns_list;
-       seqlock_t ns_tree_lock;
-       int type;
+	struct rb_root ns_tree;
+	struct list_head ns_list;
+	int type;
 };
 
 struct ns_tree mnt_ns_tree = {
 	.ns_tree = RB_ROOT,
 	.ns_list = LIST_HEAD_INIT(mnt_ns_tree.ns_list),
-	.ns_tree_lock = __SEQLOCK_UNLOCKED(mnt_ns_tree.ns_tree_lock),
 	.type = CLONE_NEWNS,
 };
 
 struct ns_tree net_ns_tree = {
 	.ns_tree = RB_ROOT,
 	.ns_list = LIST_HEAD_INIT(net_ns_tree.ns_list),
-	.ns_tree_lock = __SEQLOCK_UNLOCKED(net_ns_tree.ns_tree_lock),
 	.type = CLONE_NEWNET,
 };
 EXPORT_SYMBOL_GPL(net_ns_tree);
@@ -37,42 +36,36 @@ EXPORT_SYMBOL_GPL(net_ns_tree);
 struct ns_tree uts_ns_tree = {
 	.ns_tree = RB_ROOT,
 	.ns_list = LIST_HEAD_INIT(uts_ns_tree.ns_list),
-	.ns_tree_lock = __SEQLOCK_UNLOCKED(uts_ns_tree.ns_tree_lock),
 	.type = CLONE_NEWUTS,
 };
 
 struct ns_tree user_ns_tree = {
 	.ns_tree = RB_ROOT,
 	.ns_list = LIST_HEAD_INIT(user_ns_tree.ns_list),
-	.ns_tree_lock = __SEQLOCK_UNLOCKED(user_ns_tree.ns_tree_lock),
 	.type = CLONE_NEWUSER,
 };
 
 struct ns_tree ipc_ns_tree = {
 	.ns_tree = RB_ROOT,
 	.ns_list = LIST_HEAD_INIT(ipc_ns_tree.ns_list),
-	.ns_tree_lock = __SEQLOCK_UNLOCKED(ipc_ns_tree.ns_tree_lock),
 	.type = CLONE_NEWIPC,
 };
 
 struct ns_tree pid_ns_tree = {
 	.ns_tree = RB_ROOT,
 	.ns_list = LIST_HEAD_INIT(pid_ns_tree.ns_list),
-	.ns_tree_lock = __SEQLOCK_UNLOCKED(pid_ns_tree.ns_tree_lock),
 	.type = CLONE_NEWPID,
 };
 
 struct ns_tree cgroup_ns_tree = {
 	.ns_tree = RB_ROOT,
 	.ns_list = LIST_HEAD_INIT(cgroup_ns_tree.ns_list),
-	.ns_tree_lock = __SEQLOCK_UNLOCKED(cgroup_ns_tree.ns_tree_lock),
 	.type = CLONE_NEWCGROUP,
 };
 
 struct ns_tree time_ns_tree = {
 	.ns_tree = RB_ROOT,
 	.ns_list = LIST_HEAD_INIT(time_ns_tree.ns_list),
-	.ns_tree_lock = __SEQLOCK_UNLOCKED(time_ns_tree.ns_tree_lock),
 	.type = CLONE_NEWTIME,
 };
 
@@ -85,6 +78,13 @@ static inline struct ns_common *node_to_ns(const struct rb_node *node)
 	return rb_entry(node, struct ns_common, ns_tree_node);
 }
 
+static inline struct ns_common *node_to_ns_unified(const struct rb_node *node)
+{
+	if (!node)
+		return NULL;
+	return rb_entry(node, struct ns_common, ns_unified_tree_node);
+}
+
 static inline int ns_cmp(struct rb_node *a, const struct rb_node *b)
 {
 	struct ns_common *ns_a = node_to_ns(a);
@@ -99,15 +99,27 @@ static inline int ns_cmp(struct rb_node *a, const struct rb_node *b)
 	return 0;
 }
 
+static inline int ns_cmp_unified(struct rb_node *a, const struct rb_node *b)
+{
+	struct ns_common *ns_a = node_to_ns_unified(a);
+	struct ns_common *ns_b = node_to_ns_unified(b);
+	u64 ns_id_a = ns_a->ns_id;
+	u64 ns_id_b = ns_b->ns_id;
+
+	if (ns_id_a < ns_id_b)
+		return -1;
+	if (ns_id_a > ns_id_b)
+		return 1;
+	return 0;
+}
+
 void __ns_tree_add_raw(struct ns_common *ns, struct ns_tree *ns_tree)
 {
 	struct rb_node *node, *prev;
 
 	VFS_WARN_ON_ONCE(!ns->ns_id);
 
-	write_seqlock(&ns_tree->ns_tree_lock);
-
-	VFS_WARN_ON_ONCE(ns->ns_type != ns_tree->type);
+	write_seqlock(&ns_tree_lock);
 
 	node = rb_find_add_rcu(&ns->ns_tree_node, &ns_tree->ns_tree, ns_cmp);
 	/*
@@ -120,7 +132,8 @@ void __ns_tree_add_raw(struct ns_common *ns, struct ns_tree *ns_tree)
 	else
 		list_add_rcu(&ns->ns_list_node, &node_to_ns(prev)->ns_list_node);
 
-	write_sequnlock(&ns_tree->ns_tree_lock);
+	rb_find_add_rcu(&ns->ns_unified_tree_node, &ns_unified_tree, ns_cmp_unified);
+	write_sequnlock(&ns_tree_lock);
 
 	VFS_WARN_ON_ONCE(node);
 
@@ -139,11 +152,12 @@ void __ns_tree_remove(struct ns_common *ns, struct ns_tree *ns_tree)
 	VFS_WARN_ON_ONCE(list_empty(&ns->ns_list_node));
 	VFS_WARN_ON_ONCE(ns->ns_type != ns_tree->type);
 
-	write_seqlock(&ns_tree->ns_tree_lock);
+	write_seqlock(&ns_tree_lock);
 	rb_erase(&ns->ns_tree_node, &ns_tree->ns_tree);
+	rb_erase(&ns->ns_unified_tree_node, &ns_unified_tree);
 	list_bidir_del_rcu(&ns->ns_list_node);
 	RB_CLEAR_NODE(&ns->ns_tree_node);
-	write_sequnlock(&ns_tree->ns_tree_lock);
+	write_sequnlock(&ns_tree_lock);
 }
 EXPORT_SYMBOL_GPL(__ns_tree_remove);
 
@@ -159,6 +173,17 @@ static int ns_find(const void *key, const struct rb_node *node)
 	return 0;
 }
 
+static int ns_find_unified(const void *key, const struct rb_node *node)
+{
+	const u64 ns_id = *(u64 *)key;
+	const struct ns_common *ns = node_to_ns_unified(node);
+
+	if (ns_id < ns->ns_id)
+		return -1;
+	if (ns_id > ns->ns_id)
+		return 1;
+	return 0;
+}
 
 static struct ns_tree *ns_tree_from_type(int ns_type)
 {
@@ -184,28 +209,51 @@ static struct ns_tree *ns_tree_from_type(int ns_type)
 	return NULL;
 }
 
-struct ns_common *ns_tree_lookup_rcu(u64 ns_id, int ns_type)
+static struct ns_common *__ns_unified_tree_lookup_rcu(u64 ns_id)
+{
+	struct rb_node *node;
+	unsigned int seq;
+
+	do {
+		seq = read_seqbegin(&ns_tree_lock);
+		node = rb_find_rcu(&ns_id, &ns_unified_tree, ns_find_unified);
+		if (node)
+			break;
+	} while (read_seqretry(&ns_tree_lock, seq));
+
+	return node_to_ns_unified(node);
+}
+
+static struct ns_common *__ns_tree_lookup_rcu(u64 ns_id, int ns_type)
 {
 	struct ns_tree *ns_tree;
 	struct rb_node *node;
 	unsigned int seq;
 
-	RCU_LOCKDEP_WARN(!rcu_read_lock_held(), "suspicious ns_tree_lookup_rcu() usage");
-
 	ns_tree = ns_tree_from_type(ns_type);
 	if (!ns_tree)
 		return NULL;
 
 	do {
-		seq = read_seqbegin(&ns_tree->ns_tree_lock);
+		seq = read_seqbegin(&ns_tree_lock);
 		node = rb_find_rcu(&ns_id, &ns_tree->ns_tree, ns_find);
 		if (node)
 			break;
-	} while (read_seqretry(&ns_tree->ns_tree_lock, seq));
+	} while (read_seqretry(&ns_tree_lock, seq));
 
 	return node_to_ns(node);
 }
 
+struct ns_common *ns_tree_lookup_rcu(u64 ns_id, int ns_type)
+{
+	RCU_LOCKDEP_WARN(!rcu_read_lock_held(), "suspicious ns_tree_lookup_rcu() usage");
+
+	if (ns_type)
+		return __ns_tree_lookup_rcu(ns_id, ns_type);
+
+	return __ns_unified_tree_lookup_rcu(ns_id);
+}
+
 /**
  * ns_tree_adjoined_rcu - find the next/previous namespace in the same
  * tree