Re: [PATCH] net: lpc_eth: Fix a possible memory leak in lpc_mii_probe()

[Ma Ke <make24@iscas.ac.cn>]

>Hello Ma Ke.
>
>On 4/1/26 16:18, Ma Ke wrote:
>> On 3/30/26 13:04, Vladimir Zapolskiy wrote:
>>> On 3/30/26 11:16, Ma Ke wrote:
>>>> lpc_mii_probe() calls of_phy_find_device() to obtain a phy_device
>>>> pointer. of_phy_find_device() increments the refcount of the device.
>>>> The current implementation does not decrement the refcount after using
>>>> the pointer, which leads to a memory leak.
>>>
>>> this is correct, there is an actual detected bug.
>>>
>>>>
>>>> Add phy_device_free() to balance the refcount.
>>>
>>> But this does not sound right, you shoud use of_node_put(pldat->phy_node).
>>>
>>>>
>>>> Found by code review.
>>>>
>>>> Signed-off-by: Ma Ke <make24@iscas.ac.cn>
>>>> Cc: stable@vger.kernel.org
>>>> Fixes: 3503bf024b3e ("net: lpc_eth: parse phy nodes from device tree")
>>>> ---
>>>>    drivers/net/ethernet/nxp/lpc_eth.c | 11 ++++++-----
>>>>    1 file changed, 6 insertions(+), 5 deletions(-)
>>>>
>>>> diff --git a/drivers/net/ethernet/nxp/lpc_eth.c b/drivers/net/ethernet/nxp/lpc_eth.c
>>>> index 8b9a3e3bba30..8ce7c9bb6dd6 100644
>>>> --- a/drivers/net/ethernet/nxp/lpc_eth.c
>>>> +++ b/drivers/net/ethernet/nxp/lpc_eth.c
>>>> @@ -751,7 +751,7 @@ static void lpc_handle_link_change(struct net_device *ndev)
>>>>    static int lpc_mii_probe(struct net_device *ndev)
>>>>    {
>>>>    	struct netdata_local *pldat = netdev_priv(ndev);
>>>> -	struct phy_device *phydev;
>>>> +	struct phy_device *phydev, *phydev_tmp;
>>>>    
>>>>    	/* Attach to the PHY */
>>>>    	if (lpc_phy_interface_mode(&pldat->pdev->dev) == PHY_INTERFACE_MODE_MII)
>>>> @@ -760,17 +760,18 @@ static int lpc_mii_probe(struct net_device *ndev)
>>>>    		netdev_info(ndev, "using RMII interface\n");
>>>>    
>>>>    	if (pldat->phy_node)
>>>> -		phydev =  of_phy_find_device(pldat->phy_node);
>>>> +		phydev_tmp =  of_phy_find_device(pldat->phy_node);
>>>>    	else
>>>> -		phydev = phy_find_first(pldat->mii_bus);
>>>> -	if (!phydev) {
>>>> +		phydev_tmp = phy_find_first(pldat->mii_bus);
>>>> +	if (!phydev_tmp) {
>>>
>>> I didn't get it, why the new phydev_tmp is needed above, please
>>> restore the original code above.
>>>
>>>>    		netdev_err(ndev, "no PHY found\n");
>>>>    		return -ENODEV;
>>>>    	}
>>>>    
>>>> -	phydev = phy_connect(ndev, phydev_name(phydev),
>>>> +	phydev = phy_connect(ndev, phydev_name(phydev_tmp),
>>>>    			     &lpc_handle_link_change,
>>>>    			     lpc_phy_interface_mode(&pldat->pdev->dev));
>>>> +	phy_device_free(phydev_tmp);
>>>
>>> This is plainly wrong and has to be dropped or changed to
>>>
>>> 	if (pldat->phy_node)
>>> 		of_node_put(pldat->phy_node);
>>>
>>>>    	if (IS_ERR(phydev)) {
>>>>    		netdev_err(ndev, "Could not attach to PHY\n");
>>>>    		return PTR_ERR(phydev);
>>>
>>> Is it AI generated fix or what?.. The change looks bad, it introduces
>>> more severe issues than it fixes.
>>>
>>> If you think you cannot create a proper change, let me know.
>>>
>> Thank you very much for your detailed review and guidance.
>> 
>> Now I think your point probably is: you are saying that the real leak
>> is not from of_phy_find_device(), but from the device node
>
>I was pretty indelicate in my comment, let's split the change into parts.
>
>1) I still do not understand, why phydev_tmp is introduced, please explain
>or remove this part of the change;
>
>2) phydev = of_phy_find_device() requires phy_device_free(phydev), but
>I do not see why phy_find_first() requires it, while it was added in your
>change.
>
>Let's start from resolving these two points.
>
>> pldat->phy_node which was obtained earlier (probably by
>> of_parse_phandle()) and never freed by of_node_put(). And you suggest
>> to add of_node_put(pldat->phy_node) instead of my wrong
>> phy_device_free().
>> 
>> However, I am still a little confused. In lpc_mii_probe(),
>> of_phy_find_device() is called. From my understanding, this function
>> increases the reference count of the device. To balance it, I thought
>> phy_device_free() (which calls put_device()) should be used.
>> 
>> Could you please kindly advise the correct patch? I will follow your
>> guidance and submit a proper fix.
>> 
>> I apologize again for my previous wrong patch. Thank you very much for
>> your help.
>
> -- 
> Best wishes,
> Vladimir
Hello Vladimir,

Thank you for the detailed explanation and for pointing out my mistakes.

> 1) I still do not understand, why phydev_tmp is introduced, please explain
> or remove this part of the change;

I added phydev_tmp because I thought I needed to keep the original 
phy_device pointer for releasing after phy_connect(). But as you 
implied, it's perhaps unnecessary and only makes the code less 
readable. I will drop this change completely in the next version.

> 2) phydev = of_phy_find_device() requires phy_device_free(phydev), but
> I do not see why phy_find_first() requires it, while it was added in your
> change.

You are absolutely right. I mistakenly assumed that both functions 
return a reference-counted pointer. phy_find_first() does not 
increment the refcount, so calling phy_device_free() on it is wrong 
and dangerous. My patch introduced a new bug there.

Now I understand that only the of_phy_find_device() branch needs a 
corresponding put_device(). I will prepare a corrected patch that only
releases the reference in that specific path (including on the error 
path after phy_connect() failure). I will also look at the phy_node 
reference leak you hinted at.

Thank you again for your guidance. I will send a v2 after fixing it 
properly.

Best regards,
Ma Ke