linux-kernel - Re: [PATCH RFC/RFT net-next v2 1/5] net: dsa: deny bridge VLAN with existing 8021q upper on any port

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Message-ID: <CAOiHx=mog+8Grm1QTnqU_F3=BnWmJqTj+ko-nZiRMAb4-hvSqw@mail.gmail.com>
Date: Mon, 1 Dec 2025 20:52:34 +0100
From: Jonas Gorski <jonas.gorski@...il.com>
To: Simon Horman <horms@...nel.org>
Cc: Andrew Lunn <andrew@...n.ch>, Vladimir Oltean <olteanv@...il.com>, 
	"David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, 
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, Shuah Khan <shuah@...nel.org>, 
	Florian Fainelli <f.fainelli@...il.com>, Vladimir Oltean <vladimir.oltean@....com>, netdev@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH RFC/RFT net-next v2 1/5] net: dsa: deny bridge VLAN with
 existing 8021q upper on any port

Hi,

On Mon, Dec 1, 2025 at 3:48 PM Simon Horman <horms@...nel.org> wrote:
>
> On Mon, Dec 01, 2025 at 11:28:13AM +0100, Jonas Gorski wrote:
>
> ...
>
> > diff --git a/net/dsa/user.c b/net/dsa/user.c
> > index f59d66f0975d..fa1fe0f1493a 100644
> > --- a/net/dsa/user.c
> > +++ b/net/dsa/user.c
> > @@ -653,21 +653,30 @@ static int dsa_user_port_attr_set(struct net_device *dev, const void *ctx,
> >
> >  /* Must be called under rcu_read_lock() */
> >  static int
> > -dsa_user_vlan_check_for_8021q_uppers(struct net_device *user,
> > +dsa_user_vlan_check_for_8021q_uppers(struct dsa_port *dp,
> >                                    const struct switchdev_obj_port_vlan *vlan)
> >  {
> > -     struct net_device *upper_dev;
> > -     struct list_head *iter;
> > +     struct dsa_switch *ds = dp->ds;
> > +     struct dsa_port *other_dp;
> >
> > -     netdev_for_each_upper_dev_rcu(user, upper_dev, iter) {
> > -             u16 vid;
> > +     dsa_switch_for_each_user_port(other_dp, ds) {
> > +             struct net_device *user = other_dp->user;
>
> Hi Jonas,
>
> The AI robot is concerned that user may be NULL here.
> And I can't convince myself that cannot be the case.
>
> Could you take a look?
>
> https://netdev-ai.bots.linux.dev/ai-review.html?id=3d47057e-e740-4b66-9d60-9ec2a7ee92a1#patch-0

At this point it can be NULL. But it being NULL is not an issue, as ...
>
> > +             struct net_device *upper_dev;
> > +             struct list_head *iter;
> >
> > -             if (!is_vlan_dev(upper_dev))
> > +             if (!dsa_port_bridge_same(dp, other_dp))
> >                       continue;

... this condition will filter all cases where it is NULL. For
dsa_port_bridge_same() to return true both ports need to be attached
to a bridge (and to the same bridge), and to be attached to a bridge a
net_device is required, so other_dp->user cannot be NULL. And we only
access user after here.

Best regards,
Jonas