netdev - Re: [RFC PATCH net-next 5/9] net: phylink: Add phylink_create

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Message-ID: <20190523223511.hjk5m3mxadkra26z@shell.armlinux.org.uk>
Date:   Thu, 23 May 2019 23:35:11 +0100
From:   Russell King - ARM Linux admin <linux@...linux.org.uk>
To:     Vladimir Oltean <olteanv@...il.com>
Cc:     Ioana Ciornei <ioana.ciornei@....com>,
        "f.fainelli@...il.com" <f.fainelli@...il.com>,
        "andrew@...n.ch" <andrew@...n.ch>,
        "hkallweit1@...il.com" <hkallweit1@...il.com>,
        "maxime.chevallier@...tlin.com" <maxime.chevallier@...tlin.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "davem@...emloft.net" <davem@...emloft.net>
Subject: Re: [RFC PATCH net-next 5/9] net: phylink: Add phylink_create_raw

On Fri, May 24, 2019 at 01:04:01AM +0300, Vladimir Oltean wrote:
> On Fri, 24 May 2019 at 00:55, Russell King - ARM Linux admin
> <linux@...linux.org.uk> wrote:
> >
> > On Thu, May 23, 2019 at 01:20:40AM +0000, Ioana Ciornei wrote:
> > > +     if (pl->ops) {
> > > +             pl->ops->mac_link_up(ndev, pl->link_an_mode,
> > >                            pl->phy_state.interface,
> > >                            pl->phydev);
> > >
> > > +             netif_carrier_on(ndev);
> > >
> > > +             netdev_info(ndev,
> > > +                         "Link is Up - %s/%s - flow control %s\n",
> > > +                         phy_speed_to_str(link_state.speed),
> > > +                         phy_duplex_to_str(link_state.duplex),
> > > +                         phylink_pause_to_str(link_state.pause));
> > > +     } else {
> > > +             blocking_notifier_call_chain(&pl->notifier_chain,
> > > +                                          PHYLINK_MAC_LINK_UP, &info);
> > > +             phydev_info(pl->phydev,
> > > +                         "Link is Up - %s/%s - flow control %s\n",
> > > +                         phy_speed_to_str(link_state.speed),
> > > +                         phy_duplex_to_str(link_state.duplex),
> > > +                         phylink_pause_to_str(link_state.pause));
> > > +     }
> >
> > So if we don't have pl->ops, what happens when we call phydev_info()
> > with a NULL phydev, which is a very real possibility: one of phylink's
> > whole points is to support dynamic presence of a PHY.
> >
> > What will happen in that case is this will oops, due to dereferencing
> > an offset NULL pointer via:
> >
> > #define phydev_info(_phydev, format, args...)   \
> >         dev_info(&_phydev->mdio.dev, format, ##args)
> >
> > You can't just decide that if there's no netdev, we will be guaranteed
> > a phy.
> >
> > --
> > RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
> > FTTC broadband for 0.8mile line in suburbia: sync at 12.1Mbps down 622kbps up
> > According to speedtest.net: 11.9Mbps down 500kbps up
> 
> True, however it does not crash:
> 
> [    2.539949] (NULL device *): Link is Up - 1Gbps/Full - flow control off
> 
> I agree that a better printing system has to be established though.

The only reason that happens is because struct mdio_device is at the
start of struct phy_device, and struct device is at the start of
struct mdio_device.

Should either of these move, that breaks and we get an oops.  Sorry,
that's way too fragile.

Plus, of course, do we think that printing "(NULL device *):" is
really acceptable?  We completely lose any information about _what_
link came up or went down.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
FTTC broadband for 0.8mile line in suburbia: sync at 12.1Mbps down 622kbps up
According to speedtest.net: 11.9Mbps down 500kbps up