kernfs: Use RCU to access kernfs_node::name.

Using RCU lifetime rules to access kernfs_node::name can avoid the
trouble with kernfs_rename_lock in kernfs_name() and kernfs_path_from_node()
if the fs was created with KERNFS_ROOT_INVARIANT_PARENT. This is usefull
as it allows to implement kernfs_path_from_node() only with RCU
protection and avoiding kernfs_rename_lock. The lock is only required if
the __parent node can be changed and the function requires an unchanged
hierarchy while it iterates from the node to its parent.
The change is needed to allow the lookup of the node's path
(kernfs_path_from_node()) from context which runs always with disabled
preemption and or interrutps even on PREEMPT_RT. The problem is that
kernfs_rename_lock becomes a sleeping lock on PREEMPT_RT.

I went through all ::name users and added the required access for the lookup
with a few extensions:
- rdtgroup_pseudo_lock_create() drops all locks and then uses the name
  later on. resctrl supports rename with different parents. Here I made
  a temporal copy of the name while it is used outside of the lock.

- kernfs_rename_ns() accepts NULL as new_parent. This simplifies
  sysfs_move_dir_ns() where it can set NULL in order to reuse the current
  name.

- kernfs_rename_ns() is only using kernfs_rename_lock if the parents are
  different. All users use either kernfs_rwsem (for stable path view) or
  just RCU for the lookup. The ::name uses always RCU free.

Use RCU lifetime guarantees to access kernfs_node::name.

Suggested-by: Tejun Heo <tj@kernel.org>
Acked-by: Tejun Heo <tj@kernel.org>
Reported-by: syzbot+6ea37e2e6ffccf41a7e6@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/lkml/67251dc6.050a0220.529b6.015e.GAE@google.com/
Reported-by: Hillf Danton <hdanton@sina.com>
Closes: https://lore.kernel.org/20241102001224.2789-1-hdanton@sina.com
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Link: https://lore.kernel.org/r/20250213145023.2820193-7-bigeasy@linutronix.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 5 insertions, 0 deletions

diff --git a/fs/kernfs/kernfs-internal.h b/fs/kernfs/kernfs-internal.h
index c43bee18b79f..40a2a9cd819d 100644
--- a/fs/kernfs/kernfs-internal.h
+++ b/fs/kernfs/kernfs-internal.h
@@ -107,6 +107,11 @@ static inline bool kernfs_root_is_locked(const struct kernfs_node *kn)
 	return lockdep_is_held(&kernfs_root(kn)->kernfs_rwsem);
 }
 
+static inline const char *kernfs_rcu_name(const struct kernfs_node *kn)
+{
+	return rcu_dereference_check(kn->name, kernfs_root_is_locked(kn));
+}
+
 static inline struct kernfs_node *kernfs_parent(const struct kernfs_node *kn)
 {
 	/*