Hello XDP!

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. The logic for this program is located in myapp-ebpf/src/main.rs and currently looks like this:

#![no_std]
#![no_main]

#[panic_handler]
fn panic(_info: &core::panic::PanicInfo) -> ! {
    unreachable!()
}

#![no_std] is required since we cannot use the standard library.
#![no_main] is required as we have no main function.
The #[panic_handler] is required to keep the compiler happy, although it is never used since we cannot panic.

Let's expand this by adding an XDP program that permits all traffic.

First we'll add some imports:

use aya_bpf::bindings::xdp_action;
use aya_bpf::cty::c_long;
use aya_bpf::macros::xdp;
use aya_bpf::programs::XdpContext;

Then our application logic:

#[xdp]
pub fn xdp_firewall(ctx: XdpContext) -> u32 {
    match unsafe { try_xdp_firewall(ctx) } {
        Ok(ret) => ret,
        Err(_) => xdp_action::XDP_ABORTED,
    }
}

unsafe fn try_xdp_firewall(_ctx: XdpContext) -> Result<u32, c_long> {
    Ok(xdp_action::XDP_PASS)
}

#[xdp] indicates that this function is an XDP program
The try_xdp_firewall function returns a Result that permits all traffic
The xdp_firewall program calls try_xdp_firewall and handles any errors by returning XDP_ABORTED, which will drop the packet and raise a tracepoint exception.

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/myapp

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


Disassembly of section xdp:

0000000000000000 <xdp_firewall>:
       0:       b7 00 00 00 02 00 00 00 r0 = 2
       1:       95 00 00 00 00 00 00 00 exit

We can see an xdp_firewall section here. r0 = 2 sets register 0 to 2, which is the value of the XDP_PASS action. exit ends the program.

Simple!

Completed Program

#![no_std]
#![no_main]

use aya_bpf::bindings::xdp_action;
use aya_bpf::cty::c_long;
use aya_bpf::macros::xdp;
use aya_bpf::programs::XdpContext;

#[panic_handler]
fn panic(_info: &core::panic::PanicInfo) -> ! {
    unreachable!()
}

#[xdp]
pub fn xdp_firewall(ctx: XdpContext) -> u32 {
    match unsafe { try_xdp_firewall(ctx) } {
        Ok(ret) => ret,
        Err(_) => xdp_action::XDP_ABORTED,
    }
}

unsafe fn try_xdp_firewall(_ctx: XdpContext) -> Result<u32, c_long> {
    Ok(xdp_action::XDP_PASS)
}

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 program ready in myapp/src/main.rs which is going to do that for us.

Starting Out

The generated application has the following content:

fn main() {
    if let Err(e) = try_main() {
        eprintln!("error: {:#}", e);
    }
}

fn try_main() -> Result<(), anyhow::Error> {
    Ok(())
}

Let's adapt it to load our program.

We'll need the following imports at the top of the file:

use aya::Bpf;
use aya::programs::{Xdp, XdpFlags};
use std::{
    convert::TryInto,
    env,
    thread,
    time::Duration,
};

Then we'll adapt the try_main function to load our program:

fn try_main() -> Result<(), anyhow::Error> {
    let path = match env::args().nth(1) {
        Some(iface) => iface,
        None => panic!("not path provided"),
    };
    let iface = match env::args().nth(2) {
        Some(iface) => iface,
        None => "eth0".to_string(),
    };
    let mut bpf = Bpf::load_file(&path)?;
    let probe: &mut Xdp = bpf.program_mut("xdp")?.try_into()?;
    probe.load()?;
    probe.attach(&iface, XdpFlags::default())?;
    for _i in 1..10 {
        thread::sleep(Duration::from_secs(1));
    };
    Ok(())
}

The program takes two positional arguments

The path to our eBPF application
The interface we wish to attach it to (defaults to eth0)

The line let mut bpf = Bpf::load_file(&path)?;:

Opens the file
Reads the ELF contents
Creates any maps
If your system supports BPF Type Format (BTF), it will read the current BTF description and performs any necessary relocations

Once our file is loaded, we can extract the XDP probe with let probe: &mut Xdp = bpf.program_mut("xdp")?.try_into()?; and then load it in to the kernel with probe.load().

Finally, we can attach it to an interface with probe.attach(&iface, XdpFlags::default())?;

Let's try it out!

cargo build
sudo ./target/debug/myapp ./target/bpfel-unknown-none/debug/myapp wlp2s0

That was uneventful. Did it work?

The Lifecycle of an eBPF Program

You'll notice that our program ends by sleeping for 10 seconds and you may even have wondered what this is for...

When you load an eBPF program or map in to the kernel, the kernel maintains a reference count. So, when our eBPF application is loaded, the kernel returns a file descriptor and the reference count is incremented. When our program terminates, the file descriptor is closed, the reference count is decremented and the memory (eventually) freed.

You can see this when by issuing the sudo bpftool prog list command when myapp is running:

84: xdp  tag 3b185187f1855c4c  gpl
        loaded_at 2021-08-05T13:35:06+0100  uid 0
        xlated 16B  jited 18B  memlock 4096B
        pids myapp(69184)

For our firewall to work once the user-space program has exited, we'll need to pin it to the BPF FS.

Building eBPF Programs With Aya