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Adding tracing03-xdp-debug-print example
In this lesson we will show how to print message from eBPF program into tracefs buffer. Signed-off-by: Jiri Olsa <jolsa@kernel.org>
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‎tracing03-xdp-debug-print/Makefile‎

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# SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
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XDP_TARGETS := xdp_prog_kern
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USER_TARGETS := trace_read
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LLC ?= llc
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CLANG ?= clang
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CC := gcc
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LIBBPF_DIR = ../libbpf/src/
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COMMON_DIR = ../common/
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include $(COMMON_DIR)/common.mk
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COMMON_OBJS := $(COMMON_DIR)/common_params.o

‎tracing03-xdp-debug-print/README.org‎

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# -*- fill-column: 76; -*-
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#+TITLE: Tutorial: Tracing03 - debug print
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#+OPTIONS: ^:nil
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In this lesson we will show how to print message from eBPF program
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into tracefs buffer.
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* Table of Contents :TOC:
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- [[#bpf-trace-printk][eBPF trace printk helper]]
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- [[#tracefs-pipe-reader][The tracefs pipe reader]]
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- [[#assignments][Assignments]]
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- [[#assignment-1-setting-up-your-test-lab][Assignment 1: Setting up your test lab]]
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- [[#assignment-2-bpf-trace-printk][Assignment 2: Run debug code]]
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* eBPF trace printk helper
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The bpf_trace_print helper function is very useful when debugging or
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when there's need for immediate feedback from the eBPF program.
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It offers limited trace_printk capability and basically stores message
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into the tracefs buffer.
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The bpf_trace_printk interface is:
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#+begin_example sh
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/*
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* Only limited trace_printk() conversion specifiers allowed:
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* %d %i %u %x %ld %li %lu %lx %lld %lli %llu %llx %p %s
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*/
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BPF_CALL_5(bpf_trace_printk, char *, fmt, u32, fmt_size, u64, arg1,
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u64, arg2, u64, arg3)
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#+end_example
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Because the above interface requires to put the size of the format
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string it's more convenient to use following macro, which allows
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alone string argument to be passed:
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#+begin_example sh
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#define bpf_printk(fmt, ...) \
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({ \
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char ____fmt[] = fmt; \
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bpf_trace_printk(____fmt, sizeof(____fmt), \
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##__VA_ARGS__); \
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})
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SEC("xdp")
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int xdp_prog_simple(struct xdp_md *ctx)
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{
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bpf_printk("...");
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return XDP_PASS;
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}
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#+end_example
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* The tracefs pipe reader
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To retrieve the message printed by bpf_trace_printk, you can either
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read tracefs buffer directly:
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#+begin_example sh
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$ sudo cat /sys/kernel/debug/tracing/trace_pipe
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#+end_example
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Or you can use standard C file-reading/parsing code to get the data:
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#+begin_example sh
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stream = fopen(TRACEFS_PIPE, "r");
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...
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while ((nread = getline(&line, &len, stream)) != -1) {
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#+end_example
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for more details please check on trace_read.c file.
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* Assignments
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** Assignment 1: Setting up your test lab
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In this lesson we will use the setup of the previous lesson:
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Basic02 - loading a program by name [[https://github.com/xdp-project/xdp-tutorial/tree/master/basic02-prog-by-name#assignment-2-add-xdp_abort-program]]
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Setup the environment:
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#+begin_example sh
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$ sudo ../testenv/testenv.sh setup --name veth-basic02
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#+end_example
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Load XDP program from xdp_prog_kern.o that will print
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ethertnet header on every incomming packet:
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#+begin_example sh
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$ sudo ../basic02-prog-by-name/xdp_loader --dev veth-basic02 --force --progsec xdp
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#+end_example
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and make some packets:
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#+begin_example sh
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$ sudo ../testenv/testenv.sh enter --name veth-basic02
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# ping fc00:dead:cafe:1::1
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PING fc00:dead:cafe:1::1(fc00:dead:cafe:1::1) 56 data bytes
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#+end_example
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- Assignment 2: Run debug code
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#+begin_example sh
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bpf_printk("src: %llu, dst: %llu, proto: %u\n",
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ether_addr_to_u64(eth->h_source),
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ether_addr_to_u64(eth->h_dest),
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bpf_ntohs(eth->h_proto));
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#+end_example
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You can monitor the message either via tracefs:
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#+begin_example sh
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$ sudo cat /sys/kernel/debug/tracing/trace_pipe
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ping-28172 [001] ..s1 155229.100016: 0: src: 99726513069783, dst: 63819112930922, proto: 56710
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ping-28172 [001] ..s1 155230.124054: 0: src: 99726513069783, dst: 63819112930922, proto: 56710
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ping-28172 [001] ..s1 155231.148018: 0: src: 99726513069783, dst: 63819112930922, proto: 56710
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ping-28172 [001] ..s1 155232.172022: 0: src: 99726513069783, dst: 63819112930922, proto: 56710
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#+end_example
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or with the trace_read application:
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#+begin_example sh
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$ sudo ./trace_read
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src: 5a:b3:63:62:de:d7 dst: 3a:b:b:8e:5e:6a proto: 56710
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src: 5a:b3:63:62:de:d7 dst: 3a:b:b:8e:5e:6a proto: 56710
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src: 5a:b3:63:62:de:d7 dst: 3a:b:b:8e:5e:6a proto: 56710
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...
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#+end_example

‎tracing03-xdp-debug-print/trace_read.c‎

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/* SPDX-License-Identifier: GPL-2.0 */
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#define _GNU_SOURCE
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#include <stdio.h>
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#include <stdlib.h>
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#include <string.h>
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#include "../common/common_defines.h"
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#include <netinet/ether.h>
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#define TRACEFS_PIPE "/sys/kernel/debug/tracing/trace_pipe"
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#ifndef PATH_MAX
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#define PATH_MAX 4096
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#endif
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static void print_ether_addr(const char *type, char *str)
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{
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__u64 addr;
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if (1 != sscanf(str, "%llu", &addr))
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return;
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printf("%s: %s ", type, ether_ntoa((struct ether_addr *) &addr));
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}
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int main(int argc, char **argv)
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{
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FILE *stream;
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char *line = NULL;
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size_t len = 0;
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ssize_t nread;
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stream = fopen(TRACEFS_PIPE, "r");
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if (stream == NULL) {
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perror("fopen");
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exit(EXIT_FAILURE);
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}
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while ((nread = getline(&line, &len, stream)) != -1) {
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char *tok, *saveptr;
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unsigned int proto;
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tok = strtok_r(line, " ", &saveptr);
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while (tok) {
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if (!strncmp(tok, "src:", 4)) {
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tok = strtok_r(NULL, " ", &saveptr);
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print_ether_addr("src", tok);
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}
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if (!strncmp(tok, "dst:", 4)) {
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tok = strtok_r(NULL, " ", &saveptr);
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print_ether_addr("dst", tok);
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}
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if (!strncmp(tok, "proto:", 5)) {
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tok = strtok_r(NULL, " ", &saveptr);
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if (1 == sscanf(tok, "%u", &proto))
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printf("proto: %u", proto);
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}
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tok = strtok_r(NULL, " ", &saveptr);
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}
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printf("\n");
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}
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free(line);
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fclose(stream);
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return EXIT_OK;
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}

‎tracing03-xdp-debug-print/xdp_prog_kern.c‎

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/* SPDX-License-Identifier: GPL-2.0 */
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#include <linux/bpf.h>
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#include <linux/if_ether.h>
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#include <linux/if_packet.h>
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#include <linux/if_vlan.h>
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#include <linux/ip.h>
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#include <linux/in.h>
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#include <stdbool.h>
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#include "bpf_helpers.h"
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#include "bpf_endian.h"
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#define bpf_printk(fmt, ...) \
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({ \
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char ____fmt[] = fmt; \
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bpf_trace_printk(____fmt, sizeof(____fmt), \
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##__VA_ARGS__); \
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})
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/* to u64 in host order */
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static inline __u64 ether_addr_to_u64(const __u8 *addr)
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{
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__u64 u = 0;
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int i;
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for (i = ETH_ALEN; i >= 0; i--)
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u = u << 8 | addr[i];
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return u;
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}
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SEC("xdp")
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int xdp_prog_simple(struct xdp_md *ctx)
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{
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void *data = (void *)(long)ctx->data;
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void *data_end = (void *)(long)ctx->data_end;
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struct ethhdr *eth = data;
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__u64 offset = sizeof(*eth);
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if ((void *)eth + offset > data_end)
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return 0;
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bpf_printk("src: %llu, dst: %llu, proto: %u\n",
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ether_addr_to_u64(eth->h_source),
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ether_addr_to_u64(eth->h_dest),
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bpf_ntohs(eth->h_proto));
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return XDP_PASS;
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}
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char _license[] SEC("license") = "GPL";

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