Re: [PATCH] block: fix "Directory XXXXX with parent 'block' already present!"

[Ming Lei <ming.lei@redhat.com>]

On Thu, Apr 21, 2022 at 09:09:01AM -0700, Christoph Hellwig wrote:
> On Thu, Apr 21, 2022 at 04:34:31PM +0800, Ming Lei wrote:
> > q->debugfs_dir is used by blk-mq debugfs and blktrace. The dentry is
> > created when adding disk, and removed when releasing request queue.
> > 
> > There is small window between releasing disk and releasing request
> > queue, and during the period, one disk with same name may be created
> > and added, so debugfs_create_dir() may complain with "Directory XXXXX
> > with parent 'block' already present!"
> > 
> > Fixes the issue by moving debugfs_create_dir() into blk_alloc_queue(),
> > and the dir name is named with q->id from beginning, and switched to
> > disk name when adding disk, and finally changed to q->id in disk_release().
> 
> Is there any good reason to not just debugfs_remove_recursive in
> blk_unregister_queue and do away with all the renaming?

Please see the following reasons:

1) disk_release_mq() calls elevator_exit()/rq_qos_exit(), and the two
may trigger UAF if q->debugfs_dir is removed in blk_unregister_queue().

2) after deleting disk, blktrace still should/can work for tracing
passthrough request.

3) "debugfs directory deleted with blktrace active" in block/002 could
be triggered



thanks,
Ming