netdev - Re: [RFC PATCH net 1/2] nexthop: Do not flush blackhole nexthops when loopback goes down

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Date:   Sun, 28 Feb 2021 16:40:13 -0700
From:   David Ahern <dsahern@...il.com>
To:     Ido Schimmel <idosch@...sch.org>, netdev@...r.kernel.org
Cc:     davem@...emloft.net, kuba@...nel.org, roopa@...dia.com,
        sharpd@...dia.com, mlxsw@...dia.com,
        Ido Schimmel <idosch@...dia.com>
Subject: Re: [RFC PATCH net 1/2] nexthop: Do not flush blackhole nexthops when
 loopback goes down

On 2/28/21 7:26 AM, Ido Schimmel wrote:
> From: Ido Schimmel <idosch@...dia.com>
> 
> As far as user space is concerned, blackhole nexthops do not have a
> nexthop device and therefore should not be affected by the
> administrative or carrier state of any netdev.
> 
> However, when the loopback netdev goes down all the blackhole nexthops
> are flushed. This happens because internally the kernel associates
> blackhole nexthops with the loopback netdev.

That was not intended, so definitely a bug.

> 
> This behavior is both confusing to those not familiar with kernel
> internals and also diverges from the legacy API where blackhole IPv4
> routes are not flushed when the loopback netdev goes down:
> 
>  # ip route add blackhole 198.51.100.0/24
>  # ip link set dev lo down
>  # ip route show 198.51.100.0/24
>  blackhole 198.51.100.0/24
> 
> Blackhole IPv6 routes are flushed, but at least user space knows that
> they are associated with the loopback netdev:
> 
>  # ip -6 route show 2001:db8:1::/64
>  blackhole 2001:db8:1::/64 dev lo metric 1024 pref medium
> 
> Fix this by only flushing blackhole nexthops when the loopback netdev is
> unregistered.
> 
> Fixes: ab84be7e54fc ("net: Initial nexthop code")
> Signed-off-by: Ido Schimmel <idosch@...dia.com>
> Reported-by: Donald Sharp <sharpd@...dia.com>
> ---
>  net/ipv4/nexthop.c | 10 +++++++---
>  1 file changed, 7 insertions(+), 3 deletions(-)
> 
> diff --git a/net/ipv4/nexthop.c b/net/ipv4/nexthop.c
> index f1c6cbdb9e43..743777bce179 100644
> --- a/net/ipv4/nexthop.c
> +++ b/net/ipv4/nexthop.c
> @@ -1399,7 +1399,7 @@ static int insert_nexthop(struct net *net, struct nexthop *new_nh,
>  
>  /* rtnl */
>  /* remove all nexthops tied to a device being deleted */
> -static void nexthop_flush_dev(struct net_device *dev)
> +static void nexthop_flush_dev(struct net_device *dev, unsigned long event)
>  {
>  	unsigned int hash = nh_dev_hashfn(dev->ifindex);
>  	struct net *net = dev_net(dev);
> @@ -1411,6 +1411,10 @@ static void nexthop_flush_dev(struct net_device *dev)
>  		if (nhi->fib_nhc.nhc_dev != dev)
>  			continue;
>  
> +		if (nhi->reject_nh &&
> +		    (event == NETDEV_DOWN || event == NETDEV_CHANGE))
> +			continue;
> +
>  		remove_nexthop(net, nhi->nh_parent, NULL);
>  	}
>  }
> @@ -2189,11 +2193,11 @@ static int nh_netdev_event(struct notifier_block *this,
>  	switch (event) {
>  	case NETDEV_DOWN:
>  	case NETDEV_UNREGISTER:
> -		nexthop_flush_dev(dev);
> +		nexthop_flush_dev(dev, event);
>  		break;
>  	case NETDEV_CHANGE:
>  		if (!(dev_get_flags(dev) & (IFF_RUNNING | IFF_LOWER_UP)))
> -			nexthop_flush_dev(dev);
> +			nexthop_flush_dev(dev, event);
>  		break;
>  	case NETDEV_CHANGEMTU:
>  		info_ext = ptr;
> 

LGTM. I suggest submitting without the RFC.

Reviewed-by: David Ahern <dsahern@...il.com>