eBPF

The Linux kernel used to be a place where you read the menu but couldn't enter the kitchen. You could observe what it did, request services through system calls, maybe load a kernel module if you were brave. But actually running your own code inside the kernel? That required modifying kernel source, recompiling, rebooting, and accepting that a bug could crash everything.

eBPF changes this. It hands you an apron—with strict rules about what knives you can use.

What eBPF Actually Is

Extended Berkeley Packet Filter (eBPF) lets you run custom programs inside the Linux kernel safely and efficiently, without changing kernel source code or loading traditional kernel modules. Your code runs in kernel space with kernel privileges, but under constant supervision.

The original Berkeley Packet Filter was a simple system for filtering network packets efficiently. Modern eBPF evolved far beyond packet filtering into a general-purpose kernel programming framework. You write programs (typically in C), compile them to eBPF bytecode, and the kernel loads them at runtime. No reboot. No kernel recompilation. Just load and run.

The magic is the verifier. Before your program executes a single instruction, the eBPF verifier analyzes it exhaustively. It checks for memory safety violations, ensures all loops terminate, verifies you won't access memory you shouldn't. Programs that don't pass verification are rejected—they never run. This makes eBPF fundamentally safer than kernel modules, which can crash the kernel if buggy.

Once verified, programs attach to hooks throughout the kernel: packet arrival, system calls, function entry and exit, tracepoints, and dozens of others. When those events occur, your code runs. You're inside the kernel, responding to events as they happen, with microsecond-level overhead.

Why This Matters

Consider what was previously required to process network packets at high speed. You needed kernel modifications or specialized hardware. To trace what the kernel was doing, you needed to recompile with debugging enabled or use heavyweight tools that slowed everything down. To enforce security policies at the kernel level, you were limited to what iptables could express.

eBPF makes all of this dynamic and programmable:

Networking: XDP (eXpress Data Path) processes packets the moment the network driver receives them, before they even enter the normal kernel network stack. This enables line-rate packet processing—filtering, load balancing, DDoS mitigation—at speeds that were previously impossible without specialized hardware. TC (Traffic Control) eBPF programs handle QoS and traffic shaping. Socket-level filtering provides per-connection policies with far better performance than iptables.

Observability: You can instrument any kernel function, capturing arguments, return values, and timing without modifying code. Monitor every system call to see exactly what applications are doing. Track every network connection with detailed flow information. Build flamegraphs showing exactly where CPU time goes. Tools like BCC and bpftrace make this accessible—you can write one-liners that reveal behavior that previously required specialized kernel builds.

Security: Runtime monitoring detects unexpected behavior by watching system calls, file access, and network connections in real time. Container security platforms observe container behavior and enforce policies preventing privilege escalation. Network filtering happens at kernel level with better performance than traditional approaches. Seccomp-BPF lets applications restrict their own system call capabilities.

The Verifier: Paranoid by Necessity

The verifier deserves special attention because it's both eBPF's greatest strength and its most frustrating constraint.

The verifier is genuinely paranoid—and has to be. It will reject your perfectly safe program because proving safety is mathematically undecidable. It cannot know in general whether your loop will terminate (that's the halting problem), so it requires bounded loops. It cannot know whether your pointer arithmetic is safe in all cases, so it tracks pointer provenance obsessively.

This means valid programs sometimes get rejected. You'll write something that's obviously correct, and the verifier will refuse it because it can't prove it's correct. Learning to write verifier-friendly code is part of eBPF development.

But this paranoia is why eBPF is production-safe. The verifier's conservatism means that if your program loads, it won't crash the kernel. It won't consume infinite resources. It won't corrupt memory. This guarantee is what makes running custom code in the kernel acceptable.

Maps: State and Communication

eBPF programs are event-driven—they run when something happens and then exit. But useful programs need state. Where do you store counters? How do you communicate with userspace?

eBPF maps solve this. They're data structures shared between eBPF programs and userspace applications:

• Hash maps store key-value pairs for lookups and counters
• Arrays provide fast indexed access
• Ring buffers efficiently stream events from kernel to userspace
• Per-CPU maps avoid locking for scalability

Maps let eBPF programs accumulate statistics across millions of events, maintain connection state for packet processing, and pass detailed event data to userspace applications for analysis.

Performance That Enables New Possibilities

eBPF programs run in kernel space, eliminating context switches between kernel and userspace. The bytecode is JIT-compiled to native machine code for maximum speed. Helper functions provide efficient access to kernel functionality.

The result: single-digit microsecond overhead per event. Processing millions of events per second is routine. This performance makes eBPF viable for critical paths in production systems—you can inspect every packet, trace every system call, without meaningful overhead.

This performance unlocks use cases that were previously impractical. Service meshes can replace sidecar proxies with eBPF for lower latency. Observability tools can capture everything without sampling. Security platforms can monitor runtime behavior comprehensively.

The Ecosystem

eBPF has spawned a rich ecosystem:

Cilium provides Kubernetes networking using eBPF for routing, load balancing, and network policy enforcement. Its Hubble component offers deep network observability. Cilium demonstrates what's possible when you rebuild networking from eBPF primitives.

Falco uses eBPF for runtime security monitoring, detecting unexpected behavior in containers and hosts. Rules define normal behavior; deviations trigger alerts. It shows how eBPF enables real-time security that was impractical with traditional approaches.

Development tools have matured significantly. libbpf provides a standard C library. CO-RE (Compile Once, Run Everywhere) lets compiled programs work across different kernel versions. bpftool provides CLI inspection and debugging. BCC and bpftrace offer high-level languages for rapid development.

Constraints

eBPF isn't unlimited:

• Program size limits exist (typically 1 million instructions after JIT)
• Verifier complexity makes some valid programs hard to express
• Newer eBPF capabilities require newer kernels
• Portability across distributions and kernel configurations can be challenging

These limitations are real but gradually being addressed as eBPF matures. And they're the price of safety—the constraints exist because running arbitrary code in the kernel is inherently dangerous, and eBPF has to make it safe.

Where This Goes

eBPF is expanding beyond Linux. The eBPF for Windows project is bringing these capabilities to Windows. More kernel subsystems are gaining eBPF hooks. Tooling continues improving, making eBPF more accessible to developers who aren't kernel experts.

eBPF is becoming fundamental infrastructure. If you're building networking, observability, or security tools in 2024, eBPF is likely part of your stack—or part of the stack you're competing against.

The kernel is no longer a walled garden. The door is open. The rules are strict. And what's being built inside is remarkable.