Hechao's Blog

An Invalid bpf_context Access Bug

Posted on June 20, 2019

I got an interesting issue while debugging a BPF program. The BPF program couldn’t be loaded because of an “invalid memory access” error. Though the access was completely within the valid memory boundary, the verifier was still not happy about it. [Read More]

Tags: Linux BPF

My First Kernel Patch

Posted on June 12, 2019

Summary Recently, I submitted my first kernel patch. I will note down the basic workflow of submitting code to kernel in this post. This folder contains more detailed instructions on how to become a kernel developer. My note will only contain minimal steps for a noob like me. [Read More]

Tags: Linux Kernel

Set up OfflineIMAP + Mutt on Mac

Posted on June 11, 2019

Summary Recently, I submitted my first kernel patch. One of the lessons I learned is that kernel mail server doesn’t accept HTML content in emails. To make sure my email contains absolutely no HTML, I set up a text-based email client (Mutt + OfflineIMAP) on my laptop. This post is... [Read More]

Tags: Tool Mac

Use Map-in-Map in BPF programs via Libbpf

Posted on March 19, 2019

Introduction Among all BPF map types, two special ones, BPF_MAP_TYPE_ARRAY_OF_MAPS and BPF_MAP_TYPE_HASH_OF_MAPS, are more complex than others. As the names imply, they are “map-in-map”, meaning that the value of each entry is also a map. [Read More]

Tags: BPF libbpf

Always Use always_inline In BPF Programs

Posted on March 7, 2019

TL;DR Recently, I solved a tricky bug in a BPF program. Because inline instead of always_inline was used to declare a function, when the function body grew big, the compiler decided to not inline it. Then the BPF program was not able to be loaded due to some verification error.... [Read More]

Tags: BPF

How Is Ping Deduplexed?

Posted on September 27, 2018

Preface A few days back, one of my friends asked an interesting question - How is ping deduplexed by Linux kernel network stack? In other words, when a Linux machine receives the ping reply, how does it know which socket to send to? [Read More]

Tags: Network Linux Code Walkthrough Lab

Linux Bridge Port Flags

Posted on September 11, 2018

Overview Linux bridge can set flags on a port. The defined flags (until Linux 4.0) are: [Read More]

Tags: Linux Bridge Network Linux Code Walkthrough

The Spinning Tree Protocol

How it guarantees a loop-free topology

Posted on August 8, 2018

Overview We talked about what is a bridge (switch) and how it works here and here. However, we only saw the topology in which one bridge is used to merge LANs. In real life, redundancy is always necessary in order to provide high availability. For example, instead of using only... [Read More]

Tags: Bridge Network STP Algorithm

Rate Limiting Part 2

Guava Implementation

Posted on June 27, 2018

Overview In previous pos, we learned some rate limiting algorithms. In this article, we will see how rate limiting is implemented in Google Guava library. [Read More]

Tags: Algorithm Java Rate Limiting

Rate Limiting Part 1

Algorithms

Posted on June 25, 2018

Overview In general, rate limiting is used to control the consumption rate of a resource. For example, up to 10 requests can be served by the server per minute. Or up to 1MB data can be sent to the network per second. Rate limiting is necessary for protecting a system... [Read More]

Tags: Algorithm Java Rate Limiting