In this lesson, you will learn how to bridge the gap between your kernel-space BPF programs and user-space controllers using BPF Maps.
The Bridge Between Worlds
When you run a BPF program in the kernel (like an XDP or TC filter), it operates in a restricted environment. It can inspect packets and update counters, but it cannot simply "print" to the terminal or communicate with a standard user-space application via traditional pipes. To allow your user-space controller to influence firewall policies or read statistics, we use BPF Maps.
Think of a BPF Map as a shared memory region—a key-value store—that exists in kernel space but is accessible to both the BPF program and your user-space control application. When the kernel program needs to send data (like a packet count) to your controller, it writes to the map; when the controller wants to update a filter rule, it modifies the same map.
How it Works in Practice
To share data, you define a map structure in your BPF code. For example, to track the number of packets dropped by your firewall, you might use an array map:
struct {
__uint(type, BPF_MAP_TYPE_ARRAY);
__uint(max_entries, 1);
__type(key, __u32);
__type(value, __u64);
} drop_counts SEC(".maps");
In the BPF program, you increment the counter:
__u32 key = 0;
__u64 *val = bpf_map_lookup_elem(&drop_counts, &key);
if (val) __sync_fetch_and_add(val, 1);
On the user-space side, your controller application uses standard system calls (like bpf() or high-level libraries like libbpf) to look up the map by its name, drop_counts. It can then read the current value of the counter or write new configuration entries into the map, allowing for dynamic, real-time adjustments to your firewall logic without reloading the BPF program.
By decoupling the data plane (packet processing) from the control plane (management), maps enable you to build complex, responsive network administration tools that operate at high performance.
Key Takeaway: BPF Maps serve as the essential communication channel between kernel-space BPF logic and user-space controllers, enabling real-time data reporting and dynamic configuration updates.