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eBPF Explained

Scope

This document explains eBPF as a Linux capability you can actually reason about, instead of mystical conference smoke.

It covers:

  • what eBPF is
  • why it exists
  • verifier model
  • program types and attach points
  • maps
  • tracing, networking, and security uses
  • operational risks and limits

Reference anchors: - https://docs.kernel.org/bpf/ - https://docs.kernel.org/bpf/verifier.html - https://docs.kernel.org/userspace-api/ebpf/index.html - https://docs.kernel.org/userspace-api/ebpf/syscall.html

Big Picture

eBPF is a sandboxed in-kernel execution mechanism.

That means you can run constrained programs inside the kernel without writing and loading a traditional kernel module for many use cases.

That is why it is such a big deal.

It enables: - tracing - observability - packet processing - policy enforcement - security hooks - performance analysis

without always recompiling or patching the kernel.

What eBPF Is Not

It is not: - arbitrary native code execution in kernel space - a free-for-all scripting engine - a replacement for every module/driver - magic with no safety checks

The verifier exists precisely because unrestricted kernel execution would be insane.

Core Model

You typically have:

  1. eBPF program
  2. verifier checks safety
  3. program loaded into kernel
  4. attached to an event/hook
  5. maps used for state exchange
  6. userspace tool reads/writes maps or loads programs

This gives a split model: - kernel executes the fast/safe hook logic - userspace orchestrates and interprets results

Why eBPF Exists

Traditional kernel instrumentation/control options had problems: - patch kernel code - load full kernel modules - static tracing only - weaker flexibility

eBPF gives a safer and more dynamic extension point.

In plain English: it lets Linux grow runtime instrumentation and policy muscles without handing everybody a chainsaw.

Verifier

The verifier is central.

It checks things like: - control-flow sanity - bounded behavior - safe memory access - type/state tracking - helper call validity - register and stack correctness

If the verifier cannot prove the program is safe enough, it rejects it.

This is why eBPF programming often feels like negotiating with a paranoid compiler that distrusts your ancestry.

Maps

Maps are kernel objects used to store/share state.

Examples conceptually: - hash maps - arrays - per-CPU maps - ring buffers - LPM tries - program arrays for tail calls

Maps let programs: - count events - share state with userspace - maintain policy tables - publish telemetry efficiently

Without maps, eBPF would be much less useful.

Common Program Families

Tracing / observability

Attach to: - kprobes - uprobes - tracepoints - perf events

Use cases: - syscall tracing - latency measurement - profiling - scheduler insight - filesystem/network introspection

Networking

Attach to: - XDP - tc hooks - socket filters - cgroup networking hooks

Use cases: - packet filtering - load balancing - drops/redirects - traffic accounting

Security

Attach to: - LSM hooks - cgroup controls - policy events

Use cases: - audit - enforcement - visibility into suspicious behavior

XDP Intuition

XDP is very early packet processing, near the driver receive path. It is used for very fast packet decision-making: - drop - pass - redirect - transmit

This is why people use eBPF for high-performance network filtering and load-balancing work.

Tracing Intuition

eBPF tracing is one of the highest-value practical uses.

You can ask: - which syscalls are hot? - where is latency introduced? - which process is causing IO storms? - which kernel path is slow? - what is the distribution, not just the average?

That is gold for production debugging.

Relationship to Containers and Modern Ops

eBPF is heavily used in modern cloud-native stacks for: - networking - observability - security policy - service mesh-adjacent capabilities - low-overhead tracing

This is why it keeps showing up around Kubernetes, Cilium, performance tooling, and security products.

Limits and Risks

Verifier complexity

Useful programs may still be painful to get accepted.

Kernel-version nuances

Capabilities and helper availability vary.

Operational misuse

Just because you can attach deep hooks does not mean you should spray them everywhere.

Performance overhead

Usually good when done right, but not free.

Security model

Loading eBPF is privileged/sensitive for a reason.

Useful Userland Tools / Concepts

  • bpftool
  • BCC/BPFtrace style tooling
  • perf integration
  • tracing toolkits built on eBPF
  • Cilium ecosystem concepts

Interview-Level Things to Explain

You should be able to explain:

  • what eBPF is
  • why verifier exists
  • what maps are for
  • tracing vs networking use cases
  • why eBPF is safer than random kernel hacks, but not "safe by default forever"
  • why cloud-native systems care about it

Fast Mental Model

eBPF is a verified, sandboxed mechanism for running specialized programs inside the Linux kernel at well-defined hook points, with maps providing shared state and userspace orchestration.

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