Re: [PATCH net] net: mvpp2: debugfs: fix memory leak when using debugfs_lookup()

["Russell King (Oracle)" <linux@armlinux.org.uk>]

On Wed, Sep 14, 2022 at 08:03:08PM +0200, Greg Kroah-Hartman wrote:
> On Tue, Sep 13, 2022 at 05:55:52PM +0100, Russell King (Oracle) wrote:
> > On Fri, Sep 02, 2022 at 03:41:11PM +0200, Greg Kroah-Hartman wrote:
> > > When calling debugfs_lookup() the result must have dput() called on it,
> > > otherwise the memory will leak over time.  Fix this up to be much
> > > simpler logic and only create the root debugfs directory once when the
> > > driver is first accessed.  That resolves the memory leak and makes
> > > things more obvious as to what the intent is.
> > 
> > To clarify a bit more on the original patch rather than one of the
> > backported stable patches of this.
> > 
> > This patch introduces a bug, whereby if the driver is a module, and
> > is inserted, binds to a device, then is removed and re-inserted,
> > mvpp2_root will be NULL on the first call to mvpp2_dbgfs_init(),
> > so we will attempt to call debugfs_create_dir(). However, the
> > directory was already previously created, so this will fail, and
> > mvpp2_root will be the EEXIST error pointer.
> > 
> > Since we never clean up this directory, the original code does NOT
> > result in a memory leak - since the increase in refcount caused by
> > debugfs_lookup() has absolutely no effect - because we never remove
> > this directory once it's been created.
> > 
> > If the driver /did/ remove the directory when the module is removed,
> > then yes, maybe there's an argument for this fix. However, as things
> > currently stand, this is in no way a fix, but actually introduces a
> > debugfs regression.
> > 
> > Please can the change be reverted in mainline and all stable trees.
> 
> I never considered the 'rmmod the driver and then load it again' as a
> valid thing to worry about.  And I doubt that many others would either :)
> 
> Given that the current code does NOT clean up when it is removed, I
> assumed that no one cared abou this, but yes, it is crazy but the
> current code does work, but it leaks a dentry.  I'll send a follow-on
> patch to do this "correctly" when I return from the Plumbers conference
> next week.
> 
> But for now, this patch is correct, and does not leak memory anymore
> like the code without this change currently does, so I think it should
> stay.

Please can you explain which memory isn't leaked as a result of the
patch?

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