Writing an eBPF Program Using Ringbuf Map with libbpfgo

In this blog post, we will explore how to write an eBPF (extended Berkeley Packet Filter) program that utilizes a ringbuf map to transfer data. We will also learn how to process the data stored in the ringbuf map using libbpfgo, a Go library for interacting with eBPF programs.

Introduction to Ringbuf Map

A ringbuf map is a type of map provided by eBPF that allows efficient transfer of data between eBPF programs and user space. It is particularly useful for scenarios where you need to push data, such as packet samples, from an eBPF program to a daemon running in user space.

Writing the eBPF Code

To use the ringbuf map in an eBPF program, we follow these main steps:

Define a BPF_MAP_TYPE_RINGBUF map

We declare a ringbuf map with a specified maximum number of entries.

/* BPF ringbuf map */
struct {
        __uint(type, BPF_MAP_TYPE_RINGBUF);
        __uint(max_entries, 256 * 1024 /* 256 KB */);
} events SEC(".maps");

Reserve memory space and write data

Before writing data, we need to apply for memory space using the bpf_ringbuf_reserve function. It is important to ensure the application for memory space is successful before writing data; otherwise, the program execution may fail with an error.

SEC("kprobe/do_sys_openat2")
int kprobe__do_sys_openat2(struct pt_regs *ctx)
{
    struct event *e;

    e = bp*f_ringbuf_reserve(&events, sizeof(*e), 0);
    if (!e) {
        return 0;
    }

    e->pid = bpf_get_current_pid_tgid() >> 32;

    bpf_ringbuf_submit(e, 0);

    return 0;
}

In the above example, we reserve memory space for an event structure, set the pid field of the event, and submit it to the ringbuf map using the bpf_ringbuf_submit function.

Using libbpfgo to Process Data from the Ringbuf Map

To process the data stored in the ringbuf map using libbpfgo, we can follow these steps:

Initialize the ringbuf map data receiver

We use the InitRingBuf method provided by libbpfgo to initialize a ringbuf map data receiving instance. This method takes the name of the map and a channel where the data will be sent.

eventsChannel := make(chan []byte)
pb, err := bpfModule.InitRingBuf("events", eventsChannel)
if err != nil {
    panic(err)
}

Start the instance: We start the initialized instance using the Start method.

pb.Start()
defer func() {
    pb.Stop()
    pb.Close()
}()

Receive and decode data

We continuously receive data from the channel and decode it according to the expected format.

for {
    select {
    case e := <-eventsChannel:
        // decode data: u32 pid
        pid := binary.LittleEndian.Uint32(e[0:4])
        log.Printf("pid %d", pid)
    }
}

In the above code snippet, we receive a byte slice from the eventsChannel and decode it by extracting the pid field using the binary.LittleEndian.Uint32 function.

Conclusion

In this blog post, we explored how to write an eBPF program that utilizes a ringbuf map for data transfer