linux-kernel - Re: [PATCH] backing-dev: no need to check return value of debugfs

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Message-ID: <20190122171908.c7geuvluezkjp3s7@linutronix.de>
Date:   Tue, 22 Jan 2019 18:19:08 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Anders Roxell <anders.roxell@...aro.org>,
        Arnd Bergmann <arnd@...db.de>, Michal Hocko <mhocko@...e.com>,
        linux-mm@...ck.org
Subject: Re: [PATCH] backing-dev: no need to check return value of
 debugfs_create functions

On 2019-01-22 17:25:03 [+0100], Greg Kroah-Hartman wrote:
> > >  }
> > >  
> > >  static void bdi_debug_unregister(struct backing_dev_info *bdi)
> > >  {
> > > -	debugfs_remove(bdi->debug_stats);
> > > -	debugfs_remove(bdi->debug_dir);
> > > +	debugfs_remove_recursive(bdi->debug_dir);
> > 
> > this won't remove it.
> 
> Which is fine, you don't care.

but if you cat the stats file then it will dereference the bdi struct
which has been free(), right?

> But step back, how could that original call be NULL?  That only happens
> if you pass it a bad parent dentry (which you didn't), or the system is
> totally out of memory (in which case you don't care as everything else
> is on fire).

debugfs_get_inode() could do -ENOMEM and then the directory creation
fails with NULL.

> > If you return for "debug_dir == NULL" then it is a nice cleanup.
> 
> No, that's not a valid thing to check for, you should not care as it
> will not happen.  And if it does happen, it's ok, it's only debugfs, no
> one can rely on it, it is only for debugging.

It might happen with ENOMEM as of now. It could happen for other reasons
in future if the code changes.

> thanks,
> 
> greg k-h

Sebastian