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Tutorial: Basic01 - loading your first BPF program

Welcome to the first step in this XDP tutorial.

The programming language for XDP is eBPF (Extended Berkeley Packet Filter) which we will just refer to as BPF. Thus, this tutorial will also be relevant for learning how to write other BPF programs; however, the main focus is on BPF programs that can be used in the XDP-hook. In this and the following couple of lessons we will be focusing on the basics to get up and running with BPF; the later lessons will then build on this to teach you how to do packet processing with XDP.

Since this is the first lesson, we will start out softly by not actually including any assignments. Instead, just read the text below and make sure you can load the program and that you understand what is going on.

Table of Contents

First step: setup dependencies

There are a number of setup dependencies, that are needed in order to compile the source code in this git repository. Please go read and complete the ../setup_dependencies.org guide if you haven’t already.

Then return here, and see if the next step compiles.

Compiling example code

If you completed the setup dependencies guide, then you should be able to simply run the make command, in this directory. (The Makefile and common.mk will try to be nice and detect if you didn’t complete the setup steps).

Simple XDP code

The very simple XDP code used in this step is located in xdp_pass_kern.c, and displayed below:

SEC("xdp")
int  xdp_prog_simple(struct xdp_md *ctx)
{
        return XDP_PASS;
}

Compiling process

The LLVM+clang compiler turns this restricted-C code into BPF-byte-code and stores it in an ELF object file, named xdp_pass_kern.o.

Looking into the BPF-ELF object

You can inspect the contents of the xdp_pass_kern.o file with different tools like readelf or llvm-objdump. As the Makefile enables the debug option -g (LLVM version >= 4.0), the llvm-objdump tool can annotate assembler output with the original C code:

Run: llvm-objdump -S xdp_pass_kern.o

xdp_pass_kern.o:	file format ELF64-BPF

Disassembly of section xdp:
xdp_prog_simple:
; {
       0:	b7 00 00 00 02 00 00 00 	r0 = 2
; return XDP_PASS;
       1:	95 00 00 00 00 00 00 00 	exit

If you don’t want to see the raw BPF instructions add: -no-show-raw-insn. The define/enum XDP_PASS has a value of 2, as can be seen in the dump. The section name “xdp” was defined by SEC("xdp"), and the xdp_prog_simple: is our C-function name.

Loading and the XDP hook

As you should understand by now, the BPF byte code is stored in an ELF file. To load this into the kernel, userspace needs an ELF loader to read the file and pass it into the kernel in the right format. The libbpf library provides both an ELF loader and several XDP helper functions. In this tutorial you will learn how to write C code using this library, which is where our libelf-devel dependency comes from.

The C code in xdp_pass_user.c (which gets compiled to the program xdp_pass_user) shows how to write a BPF loader specifically for our xdp_pass_kern.o ELF file. This loader attached the program in the ELF file as an XDP hook on a network device.

Loading via iproute2 ip

It does seem overkill to write a C program to simply load and attach a specific BPF-program. However, we still include this in the tutorial since it will help you integrate BPF into other Open Source projects.

As an alternative to writing a new loader, the standard iproute2 tool also contains a BPF ELF loader. However, this loader is not based on libbpf, which unfortunately makes it incompatible when starting to use BPF maps.

The iproute2 loader can be used with the standard ip tool; so in this case you can actually load our ELF-file xdp_pass_kern.o (where we named our ELF section “xdp”) like this:

ip link set dev lo xdpgeneric obj xdp_pass_kern.o sec xdp

Listing the device via ip link show also shows the XDP info:

$ ip link show dev lo
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 xdpgeneric qdisc noqueue state UNKNOWN mode DEFAULT group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    prog/xdp id 220 tag 3b185187f1855c4c jited

Removing the XDP program again from the device:

ip link set dev lo xdpgeneric off

Loading using xdp_pass_user

To load the program using our own loader, simply issue this command:

$ sudo ./xdp_pass_user --dev lo --skb-mode
Success: Loading XDP prog name:xdp_prog_simple(id:225) on device:lo(ifindex:1)

Loading it again will fail, as there is already a program loaded. This is because we use the xdp_flag XDP_FLAGS_UPDATE_IF_NOEXIST. This is good practice to avoid accidentally unloading an unrelated XDP program.

$ sudo ./xdp_pass_user --dev lo --skb-mode
ERR: dev:lo link set xdp fd failed (16): Device or resource busy
Hint: XDP already loaded on device use --force to swap/replace

As the hint suggest, the option --force can be used to replace the existing XDP program.

$ sudo ./xdp_pass_user --dev lo --skb-mode --force
Success: Loading XDP prog name:xdp_prog_simple(id:231) on device:lo(ifindex:1)

You can list XDP programs on the device using different commands, and verify that the program ID is the same:

  • ip link list dev lo
  • bpftool net list dev lo