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")
_xdp_mark(struct xdp_md * ctx)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")
_tc_mark(struct __sk_buff * ctx)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