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Hello XDP!

Source Code

Full code for the example in this chapter is available here

Example Project

While there are myriad trace points to attach to and program types to write we should start somewhere simple.

XDP (eXpress Data Path) programs permit our eBPF program to make decisions about packets that have been received on the interface to which our program is attached. To keep things simple, we'll build a very simplistic firewall to permit or deny traffic.

eBPF Component

Permit All

We must first write the eBPF component of our program. This is a minimal generated XDP program that permits all traffic. The logic for this program is located in xdp-hello-ebpf/src/main.rs and currently looks like this:

xdp-hello-ebpf/src/main.rs
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#![no_std] // (1)
#![no_main] // (2)

use aya_bpf::{bindings::xdp_action, macros::xdp, programs::XdpContext};
use aya_log_ebpf::info;

#[xdp] // (4)
pub fn xdp_hello(ctx: XdpContext) -> u32 {
    // (5)
    match unsafe { try_xdp_hello(ctx) } {
        Ok(ret) => ret,
        Err(_) => xdp_action::XDP_ABORTED,
    }
}

unsafe fn try_xdp_hello(ctx: XdpContext) -> Result<u32, u32> {
    // (6)
    info!(&ctx, "received a packet");
    // (7)
    Ok(xdp_action::XDP_PASS)
}

#[panic_handler] // (3)
fn panic(_info: &core::panic::PanicInfo) -> ! {
    unsafe { core::hint::unreachable_unchecked() }
}
  1. #![no_std] is required since we cannot use the standard library.
  2. #![no_main] is required as we have no main function.
  3. The #[panic_handler] is required to keep the compiler happy, although it is never used since we cannot panic.
  4. This indicates that this function is an XDP program.
  5. Our main entry point defers to another function and performs error handling, returning XDP_ABORTED, which will drop the packet.
  6. Write a log entry every time a packet is received.
  7. This function returns a Result that permits all traffic.

Now we can compile this using cargo xtask build-ebpf.

Verifying The Program

Let's take a look at the compiled eBPF program:

$ llvm-objdump -S target/bpfel-unknown-none/debug/xdp-hello

target/bpfel-unknown-none/debug/xdp-hello:  file format elf64-bpf

Disassembly of section .text:

0000000000000000 <memset>:
       0:   15 03 06 00 00 00 00 00 if r3 == 0 goto +6 <LBB1_3>
       1:   b7 04 00 00 00 00 00 00 r4 = 0

0000000000000010 <LBB1_2>:
       2:   bf 15 00 00 00 00 00 00 r5 = r1
       3:   0f 45 00 00 00 00 00 00 r5 += r4
       4:   73 25 00 00 00 00 00 00 *(u8 *)(r5 + 0) = r2
       5:   07 04 00 00 01 00 00 00 r4 += 1
       6:   2d 43 fb ff 00 00 00 00 if r3 > r4 goto -5 <LBB1_2>

0000000000000038 <LBB1_3>:
       7:   95 00 00 00 00 00 00 00 exit

0000000000000040 <memcpy>:
       8:   15 03 09 00 00 00 00 00 if r3 == 0 goto +9 <LBB2_3>
       9:   b7 04 00 00 00 00 00 00 r4 = 0

0000000000000050 <LBB2_2>:
      10:   bf 15 00 00 00 00 00 00 r5 = r1
      11:   0f 45 00 00 00 00 00 00 r5 += r4
      12:   bf 20 00 00 00 00 00 00 r0 = r2
      13:   0f 40 00 00 00 00 00 00 r0 += r4
      14:   71 00 00 00 00 00 00 00 r0 = *(u8 *)(r0 + 0)
      15:   73 05 00 00 00 00 00 00 *(u8 *)(r5 + 0) = r0
      16:   07 04 00 00 01 00 00 00 r4 += 1
      17:   2d 43 f8 ff 00 00 00 00 if r3 > r4 goto -8 <LBB2_2>

0000000000000090 <LBB2_3>:
      18:   95 00 00 00 00 00 00 00 exit

Disassembly of section xdp/xdp_hello:

0000000000000000 <xdp_hello>:
       0:   bf 16 00 00 00 00 00 00 r6 = r1
       1:   b7 07 00 00 00 00 00 00 r7 = 0
       2:   63 7a fc ff 00 00 00 00 *(u32 *)(r10 - 4) = r7
       3:   bf a2 00 00 00 00 00 00 r2 = r10
:
     245:   18 03 00 00 ff ff ff ff 00 00 00 00 00 00 00 00 r3 = 4294967295 ll
     247:   bf 04 00 00 00 00 00 00 r4 = r0
     248:   b7 05 00 00 aa 00 00 00 r5 = 170
     249:   85 00 00 00 19 00 00 00 call 25

00000000000007d0 <LBB0_2>:
     250:   b7 00 00 00 02 00 00 00 r0 = 2
     251:   95 00 00 00 00 00 00 00 exit

The output was trimmed for brevity. We can see an xdp/xdp_hello section here. And in <LBB0_2>, r0 = 2 sets register 0 to 2, which is the value of the XDP_PASS action. exit ends the program.

Simple!

User-space Component

Now our eBPF program is complete and compiled, we need a user-space program to load it and attach it to a trace point. Fortunately, we have a generated program ready in xdp-hello/src/main.rs which is going to do that for us.

Starting Out

Let's look at the details of our generated user-space application:

xdp-hello/src/main.rs
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use anyhow::Context;
use aya::{
    include_bytes_aligned,
    programs::{Xdp, XdpFlags},
    Bpf,
};
use aya_log::BpfLogger;
use clap::Parser;
use log::info;
use tokio::signal; // (1)

#[derive(Debug, Parser)]
struct Opt {
    #[clap(short, long, default_value = "eth0")]
    iface: String, // (2)
}

#[tokio::main] // (3)
async fn main() -> Result<(), anyhow::Error> {
    let opt = Opt::parse();

    env_logger::init();

    // This will include your eBPF object file as raw bytes at compile-time and load it at
    // runtime. This approach is recommended for most real-world use cases. If you would
    // like to specify the eBPF program at runtime rather than at compile-time, you can
    // reach for `Bpf::load_file` instead.
    // (4)
    // (5)
    #[cfg(debug_assertions)]
    let mut bpf = Bpf::load(include_bytes_aligned!(
        "../../target/bpfel-unknown-none/debug/xdp-hello"
    ))?;
    #[cfg(not(debug_assertions))]
    let mut bpf = Bpf::load(include_bytes_aligned!(
        "../../target/bpfel-unknown-none/release/xdp-hello"
    ))?;
    BpfLogger::init(&mut bpf)?;
    // (6)
    let program: &mut Xdp = bpf.program_mut("xdp_hello").unwrap().try_into()?;
    program.load()?; // (7)
                     // (8)
    program.attach(&opt.iface, XdpFlags::default())
        .context("failed to attach the XDP program with default flags - try changing XdpFlags::default() to XdpFlags::SKB_MODE")?;

    info!("Waiting for Ctrl-C...");
    signal::ctrl_c().await?;
    info!("Exiting...");

    Ok(())
}
  1. tokio is the async library we're using, which provides our Ctrl-C handler. It will come in useful later as we expand the functionality of the initial program:
  2. Here we declare our CLI flags. Just --iface for now for passing the interface name
  3. Here's our main entry point
  4. include_bytes_aligned!() copies the contents of the BPF ELF object file at the compile time
  5. Bpf::load() reads the BPF ELF object file contents from the output of the previous command, creates any maps, performs BTF relocations
  6. We extract the XDP program
  7. And then load it in to the kernel
  8. Finally, we can attach it to an interface

Let's try it out!

$ cargo xtask run -- -h
    Finished dev [unoptimized + debuginfo] target(s) in 0.05s
     Running `target/debug/xtask run -- -h`
:
    Finished dev [optimized] target(s) in 0.90s
    Finished dev [unoptimized + debuginfo] target(s) in 0.60s
xdp-hello

USAGE:
    xdp-hello [OPTIONS]

OPTIONS:
    -h, --help             Print help information
    -i, --iface <IFACE>    [default: eth0]

Interface Name

This command assumes the interface is eth0 by default. If you wish to attach to an interface with another name, use RUST_LOG=info cargo xtask run -- --iface wlp2s0, where wlp2s0 is your interface.

$ RUST_LOG=info cargo xtask run
[2022-12-21T18:03:09Z INFO  xdp_hello] Waiting for Ctrl-C...
[2022-12-21T18:03:11Z INFO  xdp_hello] received a packet
[2022-12-21T18:03:11Z INFO  xdp_hello] received a packet
[2022-12-21T18:03:11Z INFO  xdp_hello] received a packet
[2022-12-21T18:03:11Z INFO  xdp_hello] received a packet
^C[2022-12-21T18:03:11Z INFO  xdp_hello] Exiting...

So everytime a packet was received on the interface, a log was printed!

Error Loading Program?

If you get an error loading the program, try changing XdpFlags::default() to XdpFlags::SKB_MODE

The Lifecycle of an eBPF Program

The program runs until CTRL+C is pressed and then exits. On exit, Aya takes care of detaching the program for us.

If you issue the sudo bpftool prog list command when xdp_hello is running you can verify that it is loaded:

958: xdp  name xdp_hello  tag 0137ce4fce70b467  gpl
    loaded_at 2022-06-23T13:55:28-0400  uid 0
    xlated 2016B  jited 1138B  memlock 4096B  map_ids 275,274,273
    pids xdp-hello(131677)

Running the command again once xdp_hello has exited will show that the program is no longer running.