Re: [PATCH 1/3] kernfs: take kernfs_rename_lock for same-parent renames too

[Greg Kroah-Hartman]

On Wed, Sep 02, 2026 at 09:02:51PM -0700, Shakeel Butt wrote:
> kernfs_rename_ns() only takes kernfs_rename_lock when the rename moves
> the node to a new parent.  A rename that keeps the same parent, like
> renaming a network interface, changes kernfs_node::name with only
> kernfs_rwsem held.  So the lock protects ->__parent but not ->name, and
> a reader that wants a stable name has to take kernfs_rwsem, the same
> lock every path lookup needs.
> 
> That also makes for a small but real bug.  kernfs_path_from_node()
> takes kernfs_rename_lock for reading, and kernfs_path_from_node_locked()
> then reads the name of each ancestor.  It reads each one once, so a
> single same-parent rename only moves the answer from the old path to the
> new one, but two of them landing inside one walk build a path that never
> existed:
> 
>   CPU0                                   CPU1
>   kernfs_path_from_node() on /a/b/c
>     reads the name of a, gets "a"
>                                          renames a to a2
>                                          renames b to b2
>     reads the name of b, gets "b2"
>     returns "/a/b2/c"
> 
> This hits roots without KERNFS_ROOT_INVARIANT_PARENT: sysfs, where the
> bad path can reach sysfs_warn_dup() and pr_cont_kernfs_path(), and
> resctrl, which renames a mon group inside its mon_groups directory.
> cgroup sets the flag, so it skips the lock and reads names under RCU
> alone; that case needs something else and is not addressed here.
> 
> So take the lock in both cases, and let kernfs_rcu_name() accept it the
> way kernfs_parent() already does for ->__parent.  Same-parent renames
> are rare, the lock is per filesystem, and the locked section is at most
> three stores.  It also gives a future rename sequence counter one place
> to sit that covers every rename.
> 
> Fixes: 741c10b096bc ("kernfs: Use RCU to access kernfs_node::name.")
> Signed-off-by: Shakeel Butt <shakeel.butt@xxxxxxxxx>
> ---
>  fs/kernfs/dir.c             | 28 +++++++++++++++-------------
>  fs/kernfs/kernfs-internal.h |  9 ++++++++-
>  2 files changed, 23 insertions(+), 14 deletions(-)

How was this found and tested?  Did you forget an Assisted-by: tag?

thanks,

greg k-h