Re: [PATCH net-next v2 6/8] netdev: depend on netdev->lock for xdp features

[Stanislav Fomichev <stfomichev@gmail.com>]

On 04/10, Jakub Kicinski wrote:
> On Thu, 10 Apr 2025 19:10:28 -0700 Jakub Kicinski wrote:
> > On Thu, 10 Apr 2025 10:10:01 -0700 Kuniyuki Iwashima wrote:
> > > syzkaller reported splats in register_netdevice() and
> > > unregister_netdevice_many_notify().
> > > 
> > > In register_netdevice(), some devices cannot use
> > > netdev_assert_locked().
> > > 
> > > In unregister_netdevice_many_notify(), maybe we need to
> > > hold ops lock in UNREGISTER as you initially suggested.
> > > Now do_setlink() deadlock does not happen.  
> > 
> > Ah...  Thank you.
> > 
> > Do you have a reference to use as Reported-by, or its from a
> > non-public instance ?
> > 
> > I'll test this shortly:
> > 
> > diff --git a/net/core/netdev-genl.c b/net/core/netdev-genl.c
> > index b64c614a00c4..891e2f60922f 100644
> > --- a/net/core/netdev-genl.c
> > +++ b/net/core/netdev-genl.c
> > @@ -38,7 +38,8 @@ netdev_nl_dev_fill(struct net_device *netdev, struct sk_buff *rsp,
> >         u64 xdp_rx_meta = 0;
> >         void *hdr;
> >  
> > -       netdev_assert_locked(netdev); /* note: rtnl_lock may not be held! */
> > +       /* note: rtnl_lock may or may not be held! */
> > +       netdev_assert_locked_or_invisible(netdev);
> >  
> >         hdr = genlmsg_iput(rsp, info);
> >         if (!hdr)
> > @@ -966,7 +967,9 @@ static int netdev_genl_netdevice_event(struct notifier_block *nb,
> >                 netdev_genl_dev_notify(netdev, NETDEV_CMD_DEV_ADD_NTF);
> >                 break;
> >         case NETDEV_UNREGISTER:
> > +               netdev_lock(netdev);
> >                 netdev_genl_dev_notify(netdev, NETDEV_CMD_DEV_DEL_NTF);
> > +               netdev_unlock(netdev);
> >                 break;
> >         case NETDEV_XDP_FEAT_CHANGE:
> >                 netdev_genl_dev_notify(netdev, NETDEV_CMD_DEV_CHANGE_NTF);
> 
> Ugh, REGISTER is ops locked we'd need conditional locking here.
> 
> Stanislav, I can make the REGISTERED notifier fully locked, right?
> I suspect any new object we add that's protected by the instance
> lock will want to lock the dev.

Are you suggesting to do s/netdev_lock_ops/netdev_lock/ around
call_netdevice_notifiers in register_netdevice? We can try, the biggest
concern, as usual, are the stacking devices (with an extra lock), but
casually grepping for NETDEV_REGISTER doesn't bring up anything
suspicious.

But if you're gonna do conditional locking for NETDEV_UNREGISTER, any
reason not to play it safe and add conditional locking to NETDEV_REGISTER
in netdev_genl_netdevice_event?