samples/bpf/tracex1_kern.c

/* Copyright (c) 2013-2015 PLUMgrid, http://plumgrid.com
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of version 2 of the GNU General Public
 * License as published by the Free Software Foundation.
 */
#include <linux/skbuff.h>
#include <linux/netdevice.h>
#include <uapi/linux/bpf.h>
#include <linux/version.h>
#include <bpf/bpf_helpers.h>
#include <bpf/bpf_tracing.h>

#define _(P)                                                                   \
	({                                                                     \
		typeof(P) val = 0;                                             \
		bpf_probe_read_kernel(&val, sizeof(val), &(P));                \
		val;                                                           \
	})

/* kprobe is NOT a stable ABI
 * kernel functions can be removed, renamed or completely change semantics.
 * Number of arguments and their positions can change, etc.
 * In such case this bpf+kprobe example will no longer be meaningful
 */
SEC("kprobe/__netif_receive_skb_core")
int bpf_prog1(struct pt_regs *ctx)
{
	/* attaches to kprobe __netif_receive_skb_core,
	 * looks for packets on loobpack device and prints them
	 */
	char devname[IFNAMSIZ];
	struct net_device *dev;
	struct sk_buff *skb;
	int len;

	/* non-portable! works for the given kernel only */
	bpf_probe_read_kernel(&skb, sizeof(skb), (void *)PT_REGS_PARM1(ctx));
	dev = _(skb->dev);
	len = _(skb->len);

	bpf_probe_read_kernel(devname, sizeof(devname), dev->name);

	if (devname[0] == 'l' && devname[1] == 'o') {
		char fmt[] = "skb %p len %d\n";
		/* using bpf_trace_printk() for DEBUG ONLY */
		bpf_trace_printk(fmt, sizeof(fmt), skb, len);
	}

	return 0;
}

char _license[] SEC("license") = "GPL";
u32 _version SEC("version") = LINUX_VERSION_CODE;