netdev - Re: [PATCH net] phy: Don't increment MDIO bus refcount unless it's a different owner

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Message-ID: <6492ed34-69ae-84df-3fad-c8c4c0510af7@gmail.com>
Date:   Thu, 8 Dec 2016 09:54:08 -0800
From:   Florian Fainelli <f.fainelli@...il.com>
To:     Johan Hovold <johan@...nel.org>
Cc:     netdev@...r.kernel.org, rmk+kernel@....linux.org.uk, andrew@...n.ch
Subject: Re: [PATCH net] phy: Don't increment MDIO bus refcount unless it's a
 different owner

On 12/08/2016 09:01 AM, Johan Hovold wrote:
> On Thu, Dec 08, 2016 at 08:47:54AM -0800, Florian Fainelli wrote:
>> On 12/08/2016 08:27 AM, Johan Hovold wrote:
>>> On Tue, Dec 06, 2016 at 08:54:43PM -0800, Florian Fainelli wrote:
>>>> Commit 3e3aaf649416 ("phy: fix mdiobus module safety") fixed the way we
>>>> dealt with MDIO bus module reference count, but sort of introduced a
>>>> regression in that, if an Ethernet driver registers its own MDIO bus
>>>> driver, as is common, we will end up with the Ethernet driver's
>>>> module->refnct set to 1, thus preventing this driver from any removal.
>>>>
>>>> Fix this by comparing the network device's device driver owner against
>>>> the MDIO bus driver owner, and only if they are different, increment the
>>>> MDIO bus module refcount.
>>>>
>>>> Fixes: 3e3aaf649416 ("phy: fix mdiobus module safety")
>>>> Signed-off-by: Florian Fainelli <f.fainelli@...il.com>
>>>> ---
>>>> Russell,
>>>>
>>>> I verified this against the ethoc driver primarily (on a TS7300 board)
>>>> and bcmgenet.
>>>>
>>>> Thanks!
>>>>
>>>>  drivers/net/phy/phy_device.c | 16 +++++++++++++---
>>>>  1 file changed, 13 insertions(+), 3 deletions(-)
>>>>
>>>> diff --git a/drivers/net/phy/phy_device.c b/drivers/net/phy/phy_device.c
>>>> index 1a4bf8acad78..c4ceb082e970 100644
>>>> --- a/drivers/net/phy/phy_device.c
>>>> +++ b/drivers/net/phy/phy_device.c
>>>> @@ -857,11 +857,17 @@ EXPORT_SYMBOL(phy_attached_print);
>>>>  int phy_attach_direct(struct net_device *dev, struct phy_device *phydev,
>>>>  		      u32 flags, phy_interface_t interface)
>>>>  {
>>>> +	struct module *ndev_owner = dev->dev.parent->driver->owner;
>>>
>>> Is this really safe? A driver does not need to set a parent device, and
>>> in that case you get a NULL-deref here (I tried using cpsw).
>>
>> Humm, cpsw does call SET_NETDEV_DEV() which should take care of that, is
>> the call made too late? Do you have an example oops?
> 
> Sorry if I was being unclear, cpsw does set a parent device, but there
> are network driver that do not. Perhaps such drivers will never hit this
> code path, but I can't say for sure and everything appear to work for
> cpsw if you comment out that SET_NETDEV_DEV (well, at least before this
> patch).

You were clear, I did not understand that you exercised this with cpsw
to see whether this was safe in all conditions.

> 
>> I don't mind safeguarding this with a check against dev->dev.parent, but
>> I would like to fix the drivers where relevant too, since
>> SET_NETDEV_DEV() should really be called, otherwise a number of things
>> just don't work
> 
> I grepped for for register_netdev and think I saw a number of drivers
> which do not call SET_NETDEV_DEV.
> 
> Again, perhaps they will never hit this path, but thought I should ask.

You are absolutely right, this is a potential problem, so far I found
two legitimate drivers that do not call SET_NETDEV_DEV (lantiq_etop.c
and cpmac.c, both fixed), and Freescale's FMAN driver, which I have a
hard time understanding what it does with mac_dev->net_dev...

Thanks!
-- 
Florian