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Date:   Tue, 13 Sep 2022 17:55:52 +0100
From:   "Russell King (Oracle)" <linux@armlinux.org.uk>
To:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc:     mw@semihalf.com, linux-kernel@vger.kernel.org,
        "David S. Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>, netdev@vger.kernel.org,
        stable <stable@kernel.org>
Subject: Re: [PATCH net] net: mvpp2: debugfs: fix memory leak when using
 debugfs_lookup()
Message-ID: <YyC2GGbzEuCuZzMk@shell.armlinux.org.uk>
References: <20220902134111.280657-1-gregkh@linuxfoundation.org>
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In-Reply-To: <20220902134111.280657-1-gregkh@linuxfoundation.org>
Sender: Russell King (Oracle) <linux@armlinux.org.uk>
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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.

-- 
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