[PATCH bpf-next V8 7/8] selftests/bpf: use bpf_check_mtu in selftest test_cls_redirect

[Jesper Dangaard Brouer <brouer@redhat.com>]

This demonstrate how bpf_check_mtu() helper can easily be used together
with bpf_skb_adjust_room() helper, prior to doing size adjustment, as
delta argument is already setup.

Hint: This specific test can be selected like this:
 ./test_progs -t cls_redirect

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
---
 .../selftests/bpf/progs/test_cls_redirect.c        |    7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/tools/testing/selftests/bpf/progs/test_cls_redirect.c b/tools/testing/selftests/bpf/progs/test_cls_redirect.c
index c9f8464996ea..3c1e042962e6 100644
--- a/tools/testing/selftests/bpf/progs/test_cls_redirect.c
+++ b/tools/testing/selftests/bpf/progs/test_cls_redirect.c
@@ -70,6 +70,7 @@ typedef struct {
 	uint64_t errors_total_encap_adjust_failed;
 	uint64_t errors_total_encap_buffer_too_small;
 	uint64_t errors_total_redirect_loop;
+	uint64_t errors_total_encap_mtu_violate;
 } metrics_t;
 
 typedef enum {
@@ -407,6 +408,7 @@ static INLINING ret_t forward_with_gre(struct __sk_buff *skb, encap_headers_t *e
 		payload_off - sizeof(struct ethhdr) - sizeof(struct iphdr);
 	int32_t delta = sizeof(struct gre_base_hdr) - encap_overhead;
 	uint16_t proto = ETH_P_IP;
+	uint32_t mtu_len = 0;
 
 	/* Loop protection: the inner packet's TTL is decremented as a safeguard
 	 * against any forwarding loop. As the only interesting field is the TTL
@@ -479,6 +481,11 @@ static INLINING ret_t forward_with_gre(struct __sk_buff *skb, encap_headers_t *e
 		}
 	}
 
+	if (bpf_check_mtu(skb, skb->ifindex, &mtu_len, delta, 0)) {
+		metrics->errors_total_encap_mtu_violate++;
+		return TC_ACT_SHOT;
+	}
+
 	if (bpf_skb_adjust_room(skb, delta, BPF_ADJ_ROOM_NET,
 				BPF_F_ADJ_ROOM_FIXED_GSO |
 				BPF_F_ADJ_ROOM_NO_CSUM_RESET) ||