selftests/bpf: Add test for l3 use of bpf_redirect_peer

Add a test case for using bpf_skb_change_head() in combination with
bpf_redirect_peer() to redirect a packet from a L3 device to veth and back.

The test uses a BPF program that adds L2 headers to the packet coming
from a L3 device and then calls bpf_redirect_peer() to redirect the packet
to a veth device. The test fails as skb->mac_len is not set properly and
thus the ethernet headers are not properly skb_pull'd in cls_bpf_classify(),
causing tcp_v4_rcv() to point the TCP header into middle of the IP header.
Signed-off-by: Jussi Maki <joamaki@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20210525102955.2811090-1-joamaki@gmail.com

tools/testing/selftests/bpf/progs/test_tc_peer.c

@@ -5,12 +5,17 @@
 #include <linux/bpf.h>
 #include <linux/stddef.h>
 #include <linux/pkt_cls.h>
+#include <linux/if_ether.h>
+#include <linux/ip.h>
 
 #include <bpf/bpf_helpers.h>
 
 volatile const __u32 IFINDEX_SRC;
 volatile const __u32 IFINDEX_DST;
 
+static const __u8 src_mac[] = {0x00, 0x11, 0x22, 0x33, 0x44, 0x55};
+static const __u8 dst_mac[] = {0x00, 0x22, 0x33, 0x44, 0x55, 0x66};
+
 SEC("classifier/chk_egress")
 int tc_chk(struct __sk_buff *skb)
 {
@@ -29,4 +34,30 @@ int tc_src(struct __sk_buff *skb)
 	return bpf_redirect_peer(IFINDEX_DST, 0);
 }
 
+SEC("classifier/dst_ingress_l3")
+int tc_dst_l3(struct __sk_buff *skb)
+{
+	return bpf_redirect(IFINDEX_SRC, 0);
+}
+
+SEC("classifier/src_ingress_l3")
+int tc_src_l3(struct __sk_buff *skb)
+{
+	__u16 proto = skb->protocol;
+
+	if (bpf_skb_change_head(skb, ETH_HLEN, 0) != 0)
+		return TC_ACT_SHOT;
+
+	if (bpf_skb_store_bytes(skb, 0, &src_mac, ETH_ALEN, 0) != 0)
+		return TC_ACT_SHOT;
+
+	if (bpf_skb_store_bytes(skb, ETH_ALEN, &dst_mac, ETH_ALEN, 0) != 0)
+		return TC_ACT_SHOT;
+
+	if (bpf_skb_store_bytes(skb, ETH_ALEN + ETH_ALEN, &proto, sizeof(__u16), 0) != 0)
+		return TC_ACT_SHOT;
+
+	return bpf_redirect_peer(IFINDEX_DST, 0);
+}
+
 char __license[] SEC("license") = "GPL";