nstree: assign fixed ids to the initial namespaces

The initial set of namespace comes with fixed inode numbers making it
easy for userspace to identify them solely based on that information.
This has long preceeded anything here.

Similarly, let's assign fixed namespace ids for the initial namespaces.

Kill the cookie and use a sequentially increasing number. This has the
nice side-effect that the owning user namespace will always have a
namespace id that is smaller than any of it's descendant namespaces.

Link: https://patch.msgid.link/20251029-work-namespace-nstree-listns-v4-15-2e6f823ebdc0@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>

3 files changed, 37 insertions, 5 deletions

diff --git a/include/linux/ns_common.h b/include/linux/ns_common.h
index 7a3c71b3a76f..009a6dea724f 100644
--- a/include/linux/ns_common.h
+++ b/include/linux/ns_common.h
@@ -173,6 +173,17 @@ static __always_inline bool is_initial_namespace(struct ns_common *ns)
 		struct user_namespace *:   &init_user_ns,   \
 		struct uts_namespace *:    &init_uts_ns)
 
+#define ns_init_id(__ns)						\
+	_Generic((__ns),						\
+		struct cgroup_namespace *:	CGROUP_NS_INIT_ID,	\
+		struct ipc_namespace *:		IPC_NS_INIT_ID,		\
+		struct mnt_namespace *:		MNT_NS_INIT_ID,		\
+		struct net *:			NET_NS_INIT_ID,		\
+		struct pid_namespace *:		PID_NS_INIT_ID,		\
+		struct time_namespace *:	TIME_NS_INIT_ID,	\
+		struct user_namespace *:	USER_NS_INIT_ID,	\
+		struct uts_namespace *:		UTS_NS_INIT_ID)
+
 #define to_ns_operations(__ns)                                                                         \
 	_Generic((__ns),                                                                               \
 		struct cgroup_namespace *: (IS_ENABLED(CONFIG_CGROUPS) ? &cgroupns_operations : NULL), \
@@ -198,7 +209,7 @@ static __always_inline bool is_initial_namespace(struct ns_common *ns)
 #define NS_COMMON_INIT(nsname, refs)							\
 {											\
 	.ns_type		= ns_common_type(&nsname),				\
-	.ns_id			= 0,							\
+	.ns_id			= ns_init_id(&nsname),					\
 	.inum			= ns_init_inum(&nsname),				\
 	.ops			= to_ns_operations(&nsname),				\
 	.stashed		= NULL,							\
diff --git a/include/linux/nstree.h b/include/linux/nstree.h
index 43aa262c0ea1..38674c6fa4f7 100644
--- a/include/linux/nstree.h
+++ b/include/linux/nstree.h
@@ -9,6 +9,7 @@
 #include <linux/seqlock.h>
 #include <linux/rculist.h>
 #include <linux/cookie.h>
+#include <uapi/linux/nsfs.h>
 
 extern struct ns_tree cgroup_ns_tree;
 extern struct ns_tree ipc_ns_tree;
@@ -30,7 +31,11 @@ extern struct ns_tree uts_ns_tree;
 		struct user_namespace *:   &(user_ns_tree),	\
 		struct uts_namespace *:    &(uts_ns_tree))
 
-u64 ns_tree_gen_id(struct ns_common *ns);
+#define ns_tree_gen_id(__ns)                 \
+	__ns_tree_gen_id(to_ns_common(__ns), \
+			 (((__ns) == ns_init_ns(__ns)) ? ns_init_id(__ns) : 0))
+
+u64 __ns_tree_gen_id(struct ns_common *ns, u64 id);
 void __ns_tree_add_raw(struct ns_common *ns, struct ns_tree *ns_tree);
 void __ns_tree_remove(struct ns_common *ns, struct ns_tree *ns_tree);
 struct ns_common *ns_tree_lookup_rcu(u64 ns_id, int ns_type);
@@ -38,9 +43,9 @@ struct ns_common *__ns_tree_adjoined_rcu(struct ns_common *ns,
 					 struct ns_tree *ns_tree,
 					 bool previous);
 
-static inline void __ns_tree_add(struct ns_common *ns, struct ns_tree *ns_tree)
+static inline void __ns_tree_add(struct ns_common *ns, struct ns_tree *ns_tree, u64 id)
 {
-	ns_tree_gen_id(ns);
+	__ns_tree_gen_id(ns, id);
 	__ns_tree_add_raw(ns, ns_tree);
 }
 
@@ -60,7 +65,9 @@ static inline void __ns_tree_add(struct ns_common *ns, struct ns_tree *ns_tree)
  * This function assigns a new id to the namespace and adds it to the
  * appropriate namespace tree and list.
  */
-#define ns_tree_add(__ns) __ns_tree_add(to_ns_common(__ns), to_ns_tree(__ns))
+#define ns_tree_add(__ns)                                   \
+	__ns_tree_add(to_ns_common(__ns), to_ns_tree(__ns), \
+		      (((__ns) == ns_init_ns(__ns)) ? ns_init_id(__ns) : 0))
 
 /**
  * ns_tree_remove - Remove a namespace from a namespace tree
diff --git a/include/uapi/linux/nsfs.h b/include/uapi/linux/nsfs.h
index e098759ec917..f8bc2aad74d6 100644
--- a/include/uapi/linux/nsfs.h
+++ b/include/uapi/linux/nsfs.h
@@ -67,4 +67,18 @@ struct nsfs_file_handle {
 #define NSFS_FILE_HANDLE_SIZE_VER0 16 /* sizeof first published struct */
 #define NSFS_FILE_HANDLE_SIZE_LATEST sizeof(struct nsfs_file_handle) /* sizeof latest published struct */
 
+enum init_ns_id {
+	IPC_NS_INIT_ID		= 1ULL,
+	UTS_NS_INIT_ID		= 2ULL,
+	USER_NS_INIT_ID		= 3ULL,
+	PID_NS_INIT_ID		= 4ULL,
+	CGROUP_NS_INIT_ID	= 5ULL,
+	TIME_NS_INIT_ID		= 6ULL,
+	NET_NS_INIT_ID		= 7ULL,
+	MNT_NS_INIT_ID		= 8ULL,
+#ifdef __KERNEL__
+	NS_LAST_INIT_ID		= MNT_NS_INIT_ID,
+#endif
+};
+
 #endif /* __LINUX_NSFS_H */