Subject: Re: [CRAZY-RFF] debugfs: track open files and release on remove

From: Greg KH <[email-masked]>
Date: Fri, 9 Oct 2020 10:47:29 +0200

On Fri, Oct 09, 2020 at 10:19:02AM +0200, Johannes Berg wrote:
> On Fri, 2020-10-09 at 10:16 +0200, Greg KH wrote:
> > On Fri, Oct 09, 2020 at 10:06:14AM +0200, Johannes Berg wrote:
> > > We used to say the proxy_fops weren't needed and it wasn't an issue, and
> > > then still implemented it. Dunno. I'm not really too concerned about it
> > > myself, only root can hold the files open and remove modules ...
> > 
> > proxy_fops were needed because devices can be removed from the system at
> > any time, causing their debugfs files to want to also be removed.  It
> > wasn't because of unloading kernel code.
> 
> Indeed, that's true. Still, we lived with it for years.

Because no one wanted to fix the code, not because it was correct :)

> Anyway, like I said, I really just did this more to see that it _could_
> be done, not to suggest that it _should_ :-)

Agreed.

> I think adding the .owner everywhere would be good, and perhaps we can
> somehow put a check somewhere like
> 
> 	WARN_ON(is_module_address((unsigned long)fops) && !fops->owner);
> 
> to prevent the issue in the future?

That will fail for all of the debugfs_create_* operations, as there is
only one set of file operations for all of the different files created
with these calls.

Which, now that I remember it, is why we went down the proxy "solution"
in the first place :(

thanks,

greg k-h