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

[Shakeel Butt <shakeel.butt@linux.dev>]

On Thu, Sep 03, 2026 at 06:31:58AM +0200, Greg Kroah-Hartman wrote:
> 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@linux.dev>
> > ---
> >  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?

I am working on a series to improve kernfs_rwsem and going through
review-prompt with AI to review my series and these were existing 
issues AI found. I have created reproducers with AI for these and
tested that these patches those.