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Date:   Wed, 6 May 2020 12:08:35 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Michal Kubecek <mkubecek@...e.cz>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        syzbot <syzbot+e73ceacfd8560cc8a3ca@...kaller.appspotmail.com>,
        syzbot+c2fb6f9ddcea95ba49b5@...kaller.appspotmail.com,
        Jarod Wilson <jarod@...hat.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Jay Vosburgh <j.vosburgh@...il.com>,
        Jann Horn <jannh@...gle.com>
Subject: Re: [Patch net] net: fix a potential recursive NETDEV_FEAT_CHANGE

On Tue, May 5, 2020 at 10:26 PM Michal Kubecek <mkubecek@...e.cz> wrote:
>
> On Tue, May 05, 2020 at 03:35:27PM -0700, Cong Wang wrote:
> > On Tue, May 5, 2020 at 3:27 PM Michal Kubecek <mkubecek@...e.cz> wrote:
> > > On Tue, May 05, 2020 at 02:58:19PM -0700, Cong Wang wrote:
> > > > diff --git a/net/core/dev.c b/net/core/dev.c
> > > > index 522288177bbd..ece50ae346c3 100644
> > > > --- a/net/core/dev.c
> > > > +++ b/net/core/dev.c
> > > > @@ -8907,7 +8907,7 @@ static void netdev_sync_lower_features(struct net_device *upper,
> > > >                       netdev_dbg(upper, "Disabling feature %pNF on lower dev %s.\n",
> > > >                                  &feature, lower->name);
> > > >                       lower->wanted_features &= ~feature;
> > > > -                     netdev_update_features(lower);
> > > > +                     __netdev_update_features(lower);
> > > >
> > > >                       if (unlikely(lower->features & feature))
> > > >                               netdev_WARN(upper, "failed to disable %pNF on %s!\n",
> > >
> > > Wouldn't this mean that when we disable LRO on a bond manually with
> > > "ethtool -K", LRO will be also disabled on its slaves but no netlink
> > > notification for them would be sent to userspace?
> >
> > What netlink notification are you talking about?
>
> There is ethtool notification sent by ethnl_netdev_event() and rtnetlink
> notification sent by rtnetlink_event(). Both are triggered by
> NETDEV_FEAT_CHANGE notifier so unless I missed something, when you
> suppress the notifier, there will be no netlink notifications to
> userspace.

Oh, interesting, why ethtool_notify() can't be called directly, for example,
in netdev_update_features()? To me, using a NETDEV_FEAT_CHANGE
event handler seems unnecessary for ethtool netlink.

BTW, as pointed out by Jay, actually we only need to skip
NETDEV_FEAT_CHANGE for failure case, so I will update my patch.

Thanks.

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