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

On 20.09.2026 12:08, Ido Schimmel wrote:

On Fri, Sep 18, 2026 at 05:28:33PM +0200, Gabriel Goller wrote:

quoted

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?

Nope, found this in the code while writing
https://lore.kernel.org/netdev/arD-5t6LiBiPXAAa@luna.proxmox.com/ (local)

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

Yep.

quoted

Fixes: 571e722676fe ("ipv4: support for fib route lwtunnel encap attributes")
Signed-off-by: Gabriel Goller <redacted>
---
 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.

Makes sense, will send a v2 soon.

Thanks for the review!
Gabriel

quoted

-	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;
 }