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

[Chris Mason <clm@meta.com>]

On Mon, 1 Dec 2025 20:52:34 +0100 Jonas Gorski <jonas.gorski@gmail.com> wrote:

> Hi,
> 
> On Mon, Dec 1, 2025 at 3:48 PM Simon Horman <horms@kernel.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=47057e-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.

I reproduced this false positive here, thanks for the explanation.  This is an
example of a class of review mistakes I've wanted to fix, so I used it to
improve the prompts around NULL pointers that are protected via other checks.

I'll test this on some more commits and push it out.

-chris