netdev - Re: [PATCH ipsec] xfrm: set dst dev to blackhole_netdev instead of loopback

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Date:   Fri, 13 May 2022 14:56:20 -0400
From:   Xin Long <lucien.xin@...il.com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     network dev <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Steffen Klassert <steffen.klassert@...unet.com>
Subject: Re: [PATCH ipsec] xfrm: set dst dev to blackhole_netdev instead of
 loopback_dev in ifdown

On Fri, May 13, 2022 at 12:22 PM Eric Dumazet <edumazet@...gle.com> wrote:
>
> On Fri, May 13, 2022 at 8:47 AM Xin Long <lucien.xin@...il.com> wrote:
> >
> > The global blackhole_netdev has replaced pernet loopback_dev to become the
> > one given to the object that holds an netdev when ifdown in many places of
> > ipv4 and ipv6 since commit 8d7017fd621d ("blackhole_netdev: use
> > blackhole_netdev to invalidate dst entries").
> >
> > Especially after commit faab39f63c1f ("net: allow out-of-order netdev
> > unregistration"), it's no longer safe to use loopback_dev that may be
> > freed before other netdev.
>
> Maybe add it formally in Fixes: tag.
>
Sure. :)

Fixes: faab39f63c1f ("net: allow out-of-order netdev unregistration")

> >
> > This patch is to set dst dev to blackhole_netdev instead of loopback_dev
> > in ifdown.
> >
> > Signed-off-by: Xin Long <lucien.xin@...il.com>
> > ---
> >  net/xfrm/xfrm_policy.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/net/xfrm/xfrm_policy.c b/net/xfrm/xfrm_policy.c
> > index 00bd0ecff5a1..f1876ea61fdc 100644
> > --- a/net/xfrm/xfrm_policy.c
> > +++ b/net/xfrm/xfrm_policy.c
> > @@ -3744,7 +3744,7 @@ static int stale_bundle(struct dst_entry *dst)
> >  void xfrm_dst_ifdown(struct dst_entry *dst, struct net_device *dev)
> >  {
> >         while ((dst = xfrm_dst_child(dst)) && dst->xfrm && dst->dev == dev) {
> > -               dst->dev = dev_net(dev)->loopback_dev;
> > +               dst->dev = blackhole_netdev;
>
> I assume the XFRM layer is ready to deal with dst->dev set to blackhole ?
>
> No initial setup needed ?
I don't see why it's not ready, since it's been using loopback_dev.
In early time, commit 8d7017fd621d replaced loopback_dev quite straightforward
for ipv4/6.

BTW, there's still another one left in dn_dst_ifdown(), I will fix it
in another patch.

Thanks.

>
> Thanks
>
> >                 dev_hold(dst->dev);
> >                 dev_put(dev);
> >         }
> > --
> > 2.31.1
> >