Logging Packets

In the previous chapter, our XDP application ran for 10 seconds and permitted some traffic. There was however no output on the console, so you just have to trust that it was working correctly. Let's expand this program to log the traffic that is being permitted

Getting Data to User-Space

To get data from kernel-space to user-space we use an eBPF map. There are numerous types of maps to chose from, but in this example we'll be using a PerfEventArray.

While we could go all out and extract data all the way up to L7, we'll constrain our firewall to L3, and to make things easier, IPv4 only. The data structure that we'll need to send information to user-space will need to hold an IPv4 address and an action for Permit/Deny, we'll encode both as a u32.

Let's go ahead and add that to myapp-common/src/lib.rs

#[repr(C)]
pub struct PacketLog {
    pub ipv4_address: u32,
    pub action: u32,
}

#[cfg(feature = "user")]
unsafe impl aya::Pod for PacketLog {}

💡 HINT: Struct Alignment

Structs must be aligned to 8 byte boundaries. You can do this manually, or alternatively you may use #[repr(packed)]. If you do not do this, the eBPF verifier will get upset and emit an invalid indirect read from stack error.

We implement the aya::Pod trait for our struct since it is Plain Old Data as can be safely converted to a byte-slice and back.

eBPF: Map Creation

Let's create a map called EVENTS in myapp-ebpf/src/main.rs

use aya_bpf::macros::map;
use aya_bpf::maps::PerfMap;
use myapp_common::PacketLog;

#[map(name = "EVENTS")]
static mut EVENTS: PerfMap<PacketLog> = PerfMap::<PacketLog>::with_max_entries(1024, 0);

When the eBPF program is loaded by Aya, the map will be created for us.

Userspace: Map Creation

After our call to probe.attach() we'll add the following code.

use aya::maps::AsyncPerfEventArray;

let mut perf_array = AsyncPerfEventArray::try_from(bpf.map_mut("EVENTS")?)?;

Our perf_array is a mutable reference to the map that was created after the XDP program was loaded by Aya.

Writing Data

Now we've got our maps set up, let's add some data!

Generating Bindings To vmlinux.h

To get useful data to add to our maps, we first need some useful data structures to populate with data from the XdpContext. We want to log the Source IP Address of incoming traffic, so we'll need to:

Read the Ethernet Header to determine if this is an IPv4 Packet
Read the Source IP Address from the IPv4 Header

The two structs in the kernel for this are ethhdr from uapi/linux/if_ether.h and iphdr from uapi/linux/ip.h. If I were to use bindgen to generate Rust bindings for those headers, I'd be tied to the kernel version of the system that I'm developing on. This is where aya-gen comes in to play. It can easily generate bindings for using the BTF information in /sys/kernel/btf/vmlinux.

Once the bindings are generated and checked in to our repository they shouldn't need to be regenerated again unless we need to add a new struct.

Lets use xtask to automate this so we can easily reproduce this file in future.

We'll add the following content to xtask/src/codegen.rs

use aya_gen::btf_types;
use std::{
    fs::File,
    io::Write,
    path::{Path, PathBuf},
};

pub fn generate() -> Result<(), anyhow::Error> {
    let dir = PathBuf::from("myapp-ebpf/src");
    let names: Vec<&str> = vec!["ethhdr", "iphdr"];
    let bindings = btf_types::generate(Path::new("/sys/kernel/btf/vmlinux"), &names, false)?;
    // Write the bindings to the $OUT_DIR/bindings.rs file.
    let mut out = File::create(dir.join("bindings.rs"))?;
    write!(out, "{}", bindings).expect("unable to write bindings to file");
    Ok(())
}

This will generate a file called myapp-ebpf/src/bindings.rs. If you've chosen an application name other than myapp you'll need to adjust the path appropriately.

Add a new dependencies to xtask/Cargo.toml:

[dependencies]
aya-gen = { git = "http://github.com/alessandrod/aya", branch = "main" }

And finally, we must register the command in xtask/src/main.rs:

mod build_ebpf;
mod codegen;

use std::process::exit;

use structopt::StructOpt;
#[derive(StructOpt)]
pub struct Options {
    #[structopt(subcommand)]
    command: Command,
}

#[derive(StructOpt)]
enum Command {
    BuildEbpf(build_ebpf::Options),
    Codegen,
}

fn main() {
    let opts = Options::from_args();

    use Command::*;
    let ret = match opts.command {
        BuildEbpf(opts) => build_ebpf::build(opts),
        Codegen => codegen::generate(),
    };

    if let Err(e) = ret {
        eprintln!("{:#}", e);
        exit(1);
    }
}

Once we've generated our file using cargo xtask codegen from the root of the project.

These can then be accessed from within myapp-ebpf/src/main.rs:

mod bindings;
use bindings::{ethhdr, iphdr};

Getting Packet Data From The Context

The XdpContext contains two fields, data and data_end. data is a pointer to the start of the data in kernel memory and data_end, a pointer to the end of the data in kernel memory. In order to access this data and ensure that the eBPF verifier is happy, we'll introduce a helper function:

#[inline(always)]
unsafe fn ptr_at<T>(ctx: &XdpContext, offset: usize) -> Result<*const T, ()> {
    let start = ctx.data();
    let end = ctx.data_end();
    let len = mem::size_of::<T>();

    if start + offset + len > end {
        return Err(());
    }

    Ok((start + offset) as *const T)
}

This function will ensure that before we access any data, we check that it's contained between data and data_end. It is marked as unsafe because when calling the function, you must ensure that there is a valid T at that location or there will be undefined behaviour.

Writing Data To The Map

With our helper function in place, we can:

Read the Ethertype field to check if we have an IPv4 packet.
Read the IPv4 Source Address from the IP header

First let's add another dependency on memoffset = "0.6" to myapp-ebpf/Cargo.toml, and then we'll change our try_xdp_firewall function to look like this:

use memoffset::offset_of;

fn try_xdp_firewall(ctx: XdpContext) -> Result<u32, ()> {
    let h_proto = u16::from_be(unsafe { *ptr_at(&ctx, offset_of!(ethhdr, h_proto))? });
    if h_proto != ETH_P_IP {
        return Ok(xdp_action::XDP_PASS)
    }
    let source = u32::from_be(unsafe { *ptr_at(&ctx, ETH_HDR_LEN + offset_of!(iphdr, saddr))? });

    let log_entry = PacketLog{
        ipv4_address: source,
        action: xdp_action::XDP_PASS,
    };
    unsafe { EVENTS.output(&ctx, &log_entry, 0); }
    Ok(xdp_action::XDP_PASS)
}

💡 HINT: Reading Fields Using offset_of!

As there is limited stack space, it's more memory efficient to use the offset_of! macro to read a single field from a struct, rather than reading the whole struct and accessing the field by name.

Once we have our IPv4 source address, we can create a PacketLog struct and output this to our PerfEventArray

Reading Data

Going Async

In order to read from the AsyncPerfEventArray, we have to call AsyncPerfEventArray::open() for each online CPU, then we have to poll the file descriptor for events. While this is do-able using PerfEventArray and mio or epoll, the code is much less easy to follow. Instead, we'll use tokio to make our user-space application async.

Let's add some dependencies to myapp/src/Cargo.toml:

[dependencies]
aya = { git = "https://github.com/alessandrod/aya", branch="main", features=["async_tokio"] }
myapp-common = { path = "../myapp-common", features=["userspace"] }
anyhow = "1.0.42"
bytes = "1"
tokio = { version = "1.9.0", features = ["full"] }

And adjust our myapp/src/main.rs to look like this:

use aya::{
    maps::perf::AsyncPerfEventArray,
    programs::{Xdp, XdpFlags},
    util::online_cpus,
    Bpf,
};
use bytes::BytesMut;
use std::{
    convert::{TryFrom, TryInto},
    env, fs, net,
};
use tokio::{signal, task};

use myapp_common::PacketLog;

#[tokio::main]
async fn 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 data = fs::read(path)?;
    let mut bpf = Bpf::load(&data, None)?;

    let probe: &mut Xdp = bpf.program_mut("xdp")?.try_into()?;
    probe.load()?;
    probe.attach(&iface, XdpFlags::default())?;

    let mut perf_array = AsyncPerfEventArray::try_from(bpf.map_mut("EVENTS")?)?;

    for cpu_id in online_cpus()? {
        let mut buf = perf_array.open(cpu_id, None)?;
        task::spawn(async move {
            let mut buffers = (0..10)
                .map(|_| BytesMut::with_capacity(1024))
                .collect::<Vec<_>>();

            loop {
                let events = buf.read_events(&mut buffers).await.unwrap();
                for i in 0..events.read {
                    let buf = &mut buffers[i];
                    let ptr = buf.as_ptr() as *const PacketLog;
                    let data = unsafe { ptr.read_unaligned() };
                    let src_addr = net::Ipv4Addr::from(data.ipv4_address);
                    println!("LOG: SRC {}, ACTION {}", src_addr, data.action);
                }
            }
        });
    }
    signal::ctrl_c().await.expect("failed to listen for event");
    Ok::<_, anyhow::Error>(())
}

This will now spawn a tokio::task to read each of the AsyncPerfEventArrayBuffers contained in out AsyncPerfEventArray. When we receive an event, we use read_unaligned to read our data into a PacketLog. We then use println! to log the event to the console. We no longer need to sleep, as we run until we receive the CTRL+C signal.

Running the program

$ cargo build
$ cargo xtask build-ebpf
$ sudo ./target/debug/myapp ./target/bpfel-unknown-none/debug/myapp wlp2s0
LOG: SRC 192.168.1.205, ACTION 2
LOG: SRC 192.168.1.21, ACTION 2
LOG: SRC 192.168.1.21, ACTION 2
LOG: SRC 18.168.253.132, ACTION 2
LOG: SRC 18.168.253.132, ACTION 2
LOG: SRC 18.168.253.132, ACTION 2
LOG: SRC 140.82.121.6, ACTION 2

Building eBPF Programs With Aya