linux-kernel - Re: [PATCH net] net: dsa: Do not leave DSA master with NULL netdev

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Message-ID: <CA+h21ho50twA=D=kZYxVuE=C6gf=8JeXmTEHhV30p_30oQZjjA@mail.gmail.com>
Date:   Mon, 4 May 2020 23:34:02 +0300
From:   Vladimir Oltean <olteanv@...il.com>
To:     Florian Fainelli <f.fainelli@...il.com>
Cc:     netdev <netdev@...r.kernel.org>, allen.pais@...cle.com,
        Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net] net: dsa: Do not leave DSA master with NULL netdev_ops

Hi Florian,

On Mon, 4 May 2020 at 23:19, Florian Fainelli <f.fainelli@...il.com> wrote:
>
> When ndo_get_phys_port_name() for the CPU port was added we introduced
> an early check for when the DSA master network device in
> dsa_master_ndo_setup() already implements ndo_get_phys_port_name(). When
> we perform the teardown operation in dsa_master_ndo_teardown() we would
> not be checking that cpu_dp->orig_ndo_ops was successfully allocated and
> non-NULL initialized.
>
> With network device drivers such as virtio_net, this leads to a NPD as
> soon as the DSA switch hanging off of it gets torn down because we are
> now assigning the virtio_net device's netdev_ops a NULL pointer.
>
> Fixes: da7b9e9b00d4 ("net: dsa: Add ndo_get_phys_port_name() for CPU port")
> Reported-by: Allen Pais <allen.pais@...cle.com>
> Signed-off-by: Florian Fainelli <f.fainelli@...il.com>
> ---

The fix makes complete sense.
But on another note, if we don't overlay an ndo_get_phys_port_name if
the master already has one, doesn't that render the entire mechanism
of having a reliable way for user space to determine the CPU port
number pointless?

>  net/dsa/master.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/net/dsa/master.c b/net/dsa/master.c
> index b5c535af63a3..a621367c6e8c 100644
> --- a/net/dsa/master.c
> +++ b/net/dsa/master.c
> @@ -289,7 +289,8 @@ static void dsa_master_ndo_teardown(struct net_device *dev)
>  {
>         struct dsa_port *cpu_dp = dev->dsa_ptr;
>
> -       dev->netdev_ops = cpu_dp->orig_ndo_ops;
> +       if (cpu_dp->orig_ndo_ops)
> +               dev->netdev_ops = cpu_dp->orig_ndo_ops;
>         cpu_dp->orig_ndo_ops = NULL;
>  }
>
> --
> 2.20.1
>

Regards,
-Vladimir