netdev - Re: [PATCH net] net: mvpp2: debugfs: fix memory leak when using debugfs

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Date:   Tue, 13 Sep 2022 17:55:52 +0100
From:   "Russell King (Oracle)" <linux@...linux.org.uk>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     mw@...ihalf.com, linux-kernel@...r.kernel.org,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org,
        stable <stable@...nel.org>
Subject: Re: [PATCH net] net: mvpp2: debugfs: fix memory leak when using
 debugfs_lookup()

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.

Thanks.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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