[PATCH net-next 4/4] vxlan: report a circular route as SKB_DROP_REASON_RECURSION_LIMIT

[Anton Danilov <littlesmilingcloud@gmail.com>]

udp_tunnel_dst_lookup() and udp_tunnel6_dst_lookup() return -ELOOP
when the route to the remote end goes out of the vxlan device itself,
and vxlan_xmit_one() counts that in collisions, apart from the
-ENETUNREACH of a failed lookup. The drop reason does not tell them
apart: both are reported as SKB_DROP_REASON_IP_OUTNOROUTES, which
describes a failed route lookup.

Such a route would send the encapsulated packet back into the device
it leaves, which is the dead loop on a virtual device that
SKB_DROP_REASON_RECURSION_LIMIT describes. Report the -ELOOP case with
that reason, and add the tunnel case to the description of the reason
as an example. ip_tunnel and ip6_tunnel make the same check,
rt->dst.dev == dev in ip_tunnel_xmit() and ip_md_tunnel_xmit() and
tdev == dev in ip6_tnl_xmit(), and can report it with the same reason.

Assisted-by: LLM
Signed-off-by: Anton Danilov <littlesmilingcloud@gmail.com>
---
 drivers/net/vxlan/vxlan_core.c | 10 ++++++++--
 include/net/dropreason-core.h  |  6 +++++-
 2 files changed, 13 insertions(+), 3 deletions(-)

diff --git a/drivers/net/vxlan/vxlan_core.c b/drivers/net/vxlan/vxlan_core.c
index 0d98e29e2d72..7c94a630d184 100644
--- a/drivers/net/vxlan/vxlan_core.c
+++ b/drivers/net/vxlan/vxlan_core.c
@@ -2547,7 +2547,10 @@ void vxlan_xmit_one(struct sk_buff *skb, struct net_device *dev,
 					   tos, use_cache ? dst_cache : NULL);
 		if (IS_ERR(rt)) {
 			err = PTR_ERR(rt);
-			reason = SKB_DROP_REASON_IP_OUTNOROUTES;
+			if (err == -ELOOP)
+				reason = SKB_DROP_REASON_RECURSION_LIMIT;
+			else
+				reason = SKB_DROP_REASON_IP_OUTNOROUTES;
 			goto tx_error;
 		}
 
@@ -2633,7 +2636,10 @@ void vxlan_xmit_one(struct sk_buff *skb, struct net_device *dev,
 		if (IS_ERR(ndst)) {
 			err = PTR_ERR(ndst);
 			ndst = NULL;
-			reason = SKB_DROP_REASON_IP_OUTNOROUTES;
+			if (err == -ELOOP)
+				reason = SKB_DROP_REASON_RECURSION_LIMIT;
+			else
+				reason = SKB_DROP_REASON_IP_OUTNOROUTES;
 			goto tx_error;
 		}
 
diff --git a/include/net/dropreason-core.h b/include/net/dropreason-core.h
index 40a27d8887af..664249924219 100644
--- a/include/net/dropreason-core.h
+++ b/include/net/dropreason-core.h
@@ -613,7 +613,11 @@ enum skb_drop_reason {
 	SKB_DROP_REASON_PSP_INPUT,
 	/** @SKB_DROP_REASON_PSP_OUTPUT: PSP output checks failed */
 	SKB_DROP_REASON_PSP_OUTPUT,
-	/** @SKB_DROP_REASON_RECURSION_LIMIT: Dead loop on virtual device. */
+	/**
+	 * @SKB_DROP_REASON_RECURSION_LIMIT: Dead loop on virtual device, e.g. a
+	 * tunnel whose route to its remote end goes out of the tunnel device
+	 * itself.
+	 */
 	SKB_DROP_REASON_RECURSION_LIMIT,
 	/**
 	 * @SKB_DROP_REASON_MAX: the maximum of core drop reasons, which
-- 
2.47.3