Re: [PATCH 2/5] blktrace: fix debugfs use after free

[Luis Chamberlain <mcgrof@kernel.org>]

On Thu, Apr 16, 2020 at 12:29 AM Ming Lei <ming.lei@redhat.com> wrote:
>
> On Thu, Apr 16, 2020 at 06:20:54AM +0000, Luis Chamberlain wrote:
> > On Thu, Apr 16, 2020 at 01:47:50PM +0800, Ming Lei wrote:
> > > On Thu, Apr 16, 2020 at 05:25:24AM +0000, Luis Chamberlain wrote:
> > > > On Thu, Apr 16, 2020 at 10:10:36AM +0800, Ming Lei wrote:
> > > > > In theory, multiple partitions can be traced concurrently, but looks
> > > > > it never works, so it won't cause trouble for multiple partition trace.
> > > > >
> > > > > One userspace visible change is that blktrace debugfs dir name is switched
> > > > > to disk name from partition name in case of partition trace, will it
> > > > > break some utilities?
> > > >
> > > > How is this possible, its not clear to me, we go from:
> > > >
> > > > - q->debugfs_dir = debugfs_create_dir(kobject_name(q->kobj.parent),
> > > > -                                     blk_debugfs_root);
> > > >
> > > > To this:
> > > >
> > > > + q->debugfs_dir = debugfs_create_dir(kobject_name(q->kobj.parent),
> > > > +                                     blk_debugfs_root);
> > > >
> > > >
> > > > Maybe I am overlooking something.
> > >
> > > Your patch removes the blktrace debugfs dir:
> > >
> > > do_blk_trace_setup()
> > >
> > > -       dir = debugfs_lookup(buts->name, blk_debugfs_root);
> > > -       if (!dir)
> > > -               bt->dir = dir = debugfs_create_dir(buts->name, blk_debugfs_root);
> > > -
> > >
> > > Then create blktrace attributes under the dir of q->debugfs_dir.
> > >
> > > However, buts->name could be one partition device name, but
> >
> > I can see how buts->name is set to bdevname() which expands to
> > disk_name(bdev->bd_disk, bdev->bd_part->partno, buf).
> >
> > > q->debugfs_dir has to be disk name.
> >
> > I can't see this, can you point me to where it is clear the
> > request_queue kobject's parent is sure to be the disk name?
>
> blk_register_queue():
>         ...
>         ret = kobject_add(&q->kobj, kobject_get(&dev->kobj), "%s", "queue");
>         ...

Alright, I have a fix for this now, and I do have also a further
explanation as to *why* the debugfs_lookup() doesn't help us here.
I'll follow up with more patches.

  Luis