1 /* SPDX-License-Identifier: GPL-2.0
2  * Copyright (c) 2018 Jesper Dangaard Brouer, Red Hat Inc.
3  *
4  * Example howto transfer info from XDP to SKB, e.g. skb->mark
5  * -----------------------------------------------------------
6  * This uses the XDP data_meta infrastructure, and is a cooperation
7  * between two bpf-programs (1) XDP and (2) clsact at TC-ingress hook.
8  *
9  * Notice: This example does not use the BPF C-loader,
10  * but instead rely on the iproute2 TC tool for loading BPF-objects.
11  */
12 #include <uapi/linux/bpf.h>
13 #include <uapi/linux/pkt_cls.h>
14 
15 #include <bpf/bpf_helpers.h>
16 
17 /*
18  * This struct is stored in the XDP 'data_meta' area, which is located
19  * just in-front-of the raw packet payload data.  The meaning is
20  * specific to these two BPF programs that use it as a communication
21  * channel.  XDP adjust/increase the area via a bpf-helper, and TC use
22  * boundary checks to see if data have been provided.
23  *
24  * The struct must be 4 byte aligned, which here is enforced by the
25  * struct __attribute__((aligned(4))).
26  */
27 struct meta_info {
28 	__u32 mark;
29 } __attribute__((aligned(4)));
30 
31 SEC("xdp_mark")
32 int _xdp_mark(struct xdp_md *ctx)
33 {
34 	struct meta_info *meta;
35 	void *data, *data_end;
36 	int ret;
37 
38 	/* Reserve space in-front of data pointer for our meta info.
39 	 * (Notice drivers not supporting data_meta will fail here!)
40 	 */
41 	ret = bpf_xdp_adjust_meta(ctx, -(int)sizeof(*meta));
42 	if (ret < 0)
43 		return XDP_ABORTED;
44 
45 	/* Notice: Kernel-side verifier requires that loading of
46 	 * ctx->data MUST happen _after_ helper bpf_xdp_adjust_meta(),
47 	 * as pkt-data pointers are invalidated.  Helpers that require
48 	 * this are determined/marked by bpf_helper_changes_pkt_data()
49 	 */
50 	data = (void *)(unsigned long)ctx->data;
51 
52 	/* Check data_meta have room for meta_info struct */
53 	meta = (void *)(unsigned long)ctx->data_meta;
54 	if (meta + 1 > data)
55 		return XDP_ABORTED;
56 
57 	meta->mark = 42;
58 
59 	return XDP_PASS;
60 }
61 
62 SEC("tc_mark")
63 int _tc_mark(struct __sk_buff *ctx)
64 {
65 	void *data      = (void *)(unsigned long)ctx->data;
66 	void *data_end  = (void *)(unsigned long)ctx->data_end;
67 	void *data_meta = (void *)(unsigned long)ctx->data_meta;
68 	struct meta_info *meta = data_meta;
69 
70 	/* Check XDP gave us some data_meta */
71 	if (meta + 1 > data) {
72 		ctx->mark = 41;
73 		 /* Skip "accept" if no data_meta is avail */
74 		return TC_ACT_OK;
75 	}
76 
77 	/* Hint: See func tc_cls_act_is_valid_access() for BPF_WRITE access */
78 	ctx->mark = meta->mark; /* Transfer XDP-mark to SKB-mark */
79 
80 	return TC_ACT_OK;
81 }
82 
83 /* Manually attaching these programs:
84 export DEV=ixgbe2
85 export FILE=xdp2skb_meta_kern.o
86 
87 # via TC command
88 tc qdisc del dev $DEV clsact 2> /dev/null
89 tc qdisc add dev $DEV clsact
90 tc filter  add dev $DEV ingress prio 1 handle 1 bpf da obj $FILE sec tc_mark
91 tc filter show dev $DEV ingress
92 
93 # XDP via IP command:
94 ip link set dev $DEV xdp off
95 ip link set dev $DEV xdp obj $FILE sec xdp_mark
96 
97 # Use iptable to "see" if SKBs are marked
98 iptables -I INPUT -p icmp -m mark --mark 41  # == 0x29
99 iptables -I INPUT -p icmp -m mark --mark 42  # == 0x2a
100 
101 # Hint: catch XDP_ABORTED errors via
102 perf record -e xdp:*
103 perf script
104 
105 */
106