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

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Date:   Fri, 13 May 2022 09:22:02 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     Xin Long <lucien.xin@...il.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 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.

>
> 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 ?

Thanks

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