xdp2skb_meta_kern.c
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/* SPDX-License-Identifier: GPL-2.0
* Copyright (c) 2018 Jesper Dangaard Brouer, Red Hat Inc.
*
* Example howto transfer info from XDP to SKB, e.g. skb->mark
* -----------------------------------------------------------
* This uses the XDP data_meta infrastructure, and is a cooperation
* between two bpf-programs (1) XDP and (2) clsact at TC-ingress hook.
*
* Notice: This example does not use the BPF C-loader (bpf_load.c),
* but instead rely on the iproute2 TC tool for loading BPF-objects.
*/
#include <uapi/linux/bpf.h>
#include <uapi/linux/pkt_cls.h>
#include "bpf_helpers.h"
/*
* This struct is stored in the XDP 'data_meta' area, which is located
* just in-front-of the raw packet payload data. The meaning is
* specific to these two BPF programs that use it as a communication
* channel. XDP adjust/increase the area via a bpf-helper, and TC use
* boundary checks to see if data have been provided.
*
* The struct must be 4 byte aligned, which here is enforced by the
* struct __attribute__((aligned(4))).
*/
struct meta_info {
__u32 mark;
} __attribute__((aligned(4)));
SEC("xdp_mark")
int _xdp_mark(struct xdp_md *ctx)
{
struct meta_info *meta;
void *data, *data_end;
int ret;
/* Reserve space in-front of data pointer for our meta info.
* (Notice drivers not supporting data_meta will fail here!)
*/
ret = bpf_xdp_adjust_meta(ctx, -(int)sizeof(*meta));
if (ret < 0)
return XDP_ABORTED;
/* Notice: Kernel-side verifier requires that loading of
* ctx->data MUST happen _after_ helper bpf_xdp_adjust_meta(),
* as pkt-data pointers are invalidated. Helpers that require
* this are determined/marked by bpf_helper_changes_pkt_data()
*/
data = (void *)(unsigned long)ctx->data;
/* Check data_meta have room for meta_info struct */
meta = (void *)(unsigned long)ctx->data_meta;
if (meta + 1 > data)
return XDP_ABORTED;
meta->mark = 42;
return XDP_PASS;
}
SEC("tc_mark")
int _tc_mark(struct __sk_buff *ctx)
{
void *data = (void *)(unsigned long)ctx->data;
void *data_end = (void *)(unsigned long)ctx->data_end;
void *data_meta = (void *)(unsigned long)ctx->data_meta;
struct meta_info *meta = data_meta;
/* Check XDP gave us some data_meta */
if (meta + 1 > data) {
ctx->mark = 41;
/* Skip "accept" if no data_meta is avail */
return TC_ACT_OK;
}
/* Hint: See func tc_cls_act_is_valid_access() for BPF_WRITE access */
ctx->mark = meta->mark; /* Transfer XDP-mark to SKB-mark */
return TC_ACT_OK;
}
/* Manually attaching these programs:
export DEV=ixgbe2
export FILE=xdp2skb_meta_kern.o
# via TC command
tc qdisc del dev $DEV clsact 2> /dev/null
tc qdisc add dev $DEV clsact
tc filter add dev $DEV ingress prio 1 handle 1 bpf da obj $FILE sec tc_mark
tc filter show dev $DEV ingress
# XDP via IP command:
ip link set dev $DEV xdp off
ip link set dev $DEV xdp obj $FILE sec xdp_mark
# Use iptable to "see" if SKBs are marked
iptables -I INPUT -p icmp -m mark --mark 41 # == 0x29
iptables -I INPUT -p icmp -m mark --mark 42 # == 0x2a
# Hint: catch XDP_ABORTED errors via
perf record -e xdp:*
perf script
*/