xdp2skb_meta_kern.c (3095B)
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 */ 27struct meta_info { 28 __u32 mark; 29} __attribute__((aligned(4))); 30 31SEC("xdp_mark") 32int _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 62SEC("tc_mark") 63int _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: 84export DEV=ixgbe2 85export FILE=xdp2skb_meta_kern.o 86 87# via TC command 88tc qdisc del dev $DEV clsact 2> /dev/null 89tc qdisc add dev $DEV clsact 90tc filter add dev $DEV ingress prio 1 handle 1 bpf da obj $FILE sec tc_mark 91tc filter show dev $DEV ingress 92 93# XDP via IP command: 94ip link set dev $DEV xdp off 95ip link set dev $DEV xdp obj $FILE sec xdp_mark 96 97# Use iptable to "see" if SKBs are marked 98iptables -I INPUT -p icmp -m mark --mark 41 # == 0x29 99iptables -I INPUT -p icmp -m mark --mark 42 # == 0x2a 100 101# Hint: catch XDP_ABORTED errors via 102perf record -e xdp:* 103perf script 104 105*/