From 00eaf20fae3d6a57862f1076810001b95cf16404 Mon Sep 17 00:00:00 2001
From: Jesper Dangaard Brouer <brouer@redhat.com>
Date: Tue, 13 Oct 2020 15:07:39 +0200
Subject: [PATCH] MTU-tests: Add tc_bpf program that increase packet size

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
---
 MTU-tests/Makefile              |  4 +-
 MTU-tests/tc_bpf_inc_pkt_size.c | 69 +++++++++++++++++++++++++++++++++
 2 files changed, 71 insertions(+), 2 deletions(-)
 create mode 100644 MTU-tests/tc_bpf_inc_pkt_size.c

diff --git a/MTU-tests/Makefile b/MTU-tests/Makefile
index 5b34c4c..7539848 100644
--- a/MTU-tests/Makefile
+++ b/MTU-tests/Makefile
@@ -1,8 +1,8 @@
 # SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
 
 USER_TARGETS :=
-BPF_TARGETS := tc_bpf_encap
-EXTRA_DEPS := encap.h
+BPF_TARGETS := tc_bpf_inc_pkt_size
+# EXTRA_DEPS := encap.h
 
 LIB_DIR = ../lib
 
diff --git a/MTU-tests/tc_bpf_inc_pkt_size.c b/MTU-tests/tc_bpf_inc_pkt_size.c
new file mode 100644
index 0000000..4c635cf
--- /dev/null
+++ b/MTU-tests/tc_bpf_inc_pkt_size.c
@@ -0,0 +1,69 @@
+#include <linux/bpf.h>
+#include <linux/in6.h>
+#include <bpf/bpf_helpers.h>
+#include <xdp/parsing_helpers.h>
+
+/* MTU is defined as L3 size (usually 1500 for Ethernet),
+ * but remember TC (and XDP) operate at L2.
+ */
+//#define PKT_SIZE_L3 1500
+#define PKT_SIZE_L3 1501
+//#define PKT_SIZE_L3 1505
+//#define PKT_SIZE_L3 1600
+//#define PKT_SIZE_L3 20000
+//#define PKT_SIZE_L3 65535
+
+#define OFFSET sizeof(struct iphdr)
+#define ENCAP_TYPE BPF_F_ADJ_ROOM_ENCAP_L3_IPV4
+
+SEC("classifier") int tc_inc_pkt_sz(struct __sk_buff *skb)
+{
+	volatile void *data, *data_end;
+	int ret = BPF_DROP;
+	struct ethhdr *eth;
+	struct iphdr *iph;
+	int extra_len;
+	int len;
+
+	/* Desired packet size at L2 */
+	int pkt_size_l2 = PKT_SIZE_L3 + sizeof(*eth) ;
+
+	data     = (void *)(long)skb->data;
+	data_end = (void *)(long)skb->data_end;
+	eth = (struct ethhdr *)data;
+
+	if (data + sizeof(*eth) > data_end)
+		return BPF_DROP;
+
+	/* Keep ARP resolution working */
+	if (eth->h_proto == bpf_htons(ETH_P_ARP)) {
+		ret = BPF_OK;
+		goto out;
+	}
+
+	len = (data_end - data);
+	extra_len = pkt_size_l2 - len;
+
+	if (bpf_skb_adjust_room(skb, extra_len, BPF_ADJ_ROOM_MAC, ENCAP_TYPE))
+		goto out;
+
+	// TODO: Handle if bpf_skb_adjust_room() cannot increase size,
+	// as it's only my patched kernel that drop the MTU check
+
+	/* Most re-load after bpf_skb_adjust_room() */
+	data     = (void *)(long)skb->data;
+	data_end = (void *)(long)skb->data_end;
+
+	eth = (void *)data;
+	iph = (void *)(eth +1);
+	if (iph +1 > data_end)
+		goto out;
+
+	eth->h_proto = bpf_htons(ETH_P_IP);
+
+	ret = BPF_OK;
+out:
+	return ret;
+}
+
+char _license[] SEC("license") = "GPL";