Re: [PATCH net] ipv4: fib: treat an unbuildable encapsulation as a nexthop mismatch

[Ido Schimmel <idosch@nvidia.com>]

On Fri, Sep 18, 2026 at 05:28:33PM +0200, Gabriel Goller wrote:
> fib_encap_match() builds the requested lwtunnel state and compares it
> against the nexthop of a candidate route. When lwtunnel_build_state()
> failed it left result at 0, which is interpreted as "the nexthop
> matches", so fib_nh_match() continues to compare only oif and gateway.
> 
> So if there comes along a RTM_DELROUTE which carries an encapsulation
> the kernel rejects, it could delete a different route with a different
> encapsulation.

Did you hit this in practice? Can you share details?

Note that the modern alternative is to install nexthop objects with
encapsulation and then associate these objects with routes.

> 
> Fixes: 571e722676fe ("ipv4: support for fib route lwtunnel encap attributes")
> Signed-off-by: Gabriel Goller <g.goller@proxmox.com>
> ---
>  net/ipv4/fib_semantics.c | 14 +++++++-------
>  1 file changed, 7 insertions(+), 7 deletions(-)
> 
> diff --git a/net/ipv4/fib_semantics.c b/net/ipv4/fib_semantics.c
> index 50e96f86ca59..951e48ae87da 100644
> --- a/net/ipv4/fib_semantics.c
> +++ b/net/ipv4/fib_semantics.c
> @@ -911,17 +911,17 @@ static int fib_encap_match(struct net *net, u16 encap_type,
>  			   struct netlink_ext_ack *extack)
>  {
>  	struct lwtunnel_state *lwtstate;
> -	int ret, result = 0;
> +	int result;
>  
>  	if (encap_type == LWTUNNEL_ENCAP_NONE)
>  		return 0;

This is also "unbuildable", but will match.

lwtunnel_build_state() returns an error in this case, so you can just
remove this check.

>  
> -	ret = lwtunnel_build_state(net, encap_type, encap, AF_INET,
> -				   cfg, &lwtstate, extack);
> -	if (!ret) {
> -		result = lwtunnel_cmp_encap(lwtstate, nh->fib_nh_lws);
> -		lwtstate_free(lwtstate);
> -	}
> +	if (lwtunnel_build_state(net, encap_type, encap, AF_INET, cfg,
> +				 &lwtstate, extack))
> +		return 1;
> +
> +	result = lwtunnel_cmp_encap(lwtstate, nh->fib_nh_lws);
> +	lwtstate_free(lwtstate);
>  
>  	return result;
>  }
> -- 
> 2.47.3
> 
>