Ever wondered about the magic behind the Linux kernel’s symbols and tables? Let’s embark on this fun-filled journey to uncover the mystery! And don’t miss the Curiosity Corner for some intriguing tidbits!
📘 Understanding Symbols: Within the programming sphere, a symbol is like a label that points to either a data storage space (think of it as a drawer in a vast cabinet) or a sequence of commands to run (imagine a set of choreographed dance steps). In simple terms? They’re nicknames for memory addresses!
📘 Peeking into the Symbol Table: A symbol table is like the index of a massive book, crafted diligently by the compiler, enlisting all symbols in a program. And for our beloved Linux kernel? There’s a dedicated symbol table accompanying every built image. Take a quick peek at /boot/System.map-<linux_version>, and voila! A roadmap of kernel symbols awaits.
📘 The Art of Exporting Symbols: Imagine crafting a special dance move in a studio. It’s yours, and no one else knows it unless you show it off. Similarly, in the Linux kernel, a new function or variable stays within its home module unless you export it. Decide between EXPORT_SYMBOL or EXPORT_SYMBOL_GPL – it’s all about the licensing jazz!
📘 Choosing between EXPORT_SYMBOL and EXPORT_SYMBOL_GPL:
EXPORT_SYMBOL: Think of this as an open invitation – any kernel module can come and dance!EXPORT_SYMBOL_GPL: It’s an exclusive party. Only those modules grooving to the GPL license can join.
📘 System.map vs. /proc/kallsyms: It’s like having a treasure map vs. a dynamic GPS. System.map shows the treasures hidden since the beginning. On the other hand, /proc/kallsyms not only reveals those treasures but also updates with the new ones discovered!
Curiosity Corner:
🤔 Why EXPORT_SYMBOL_GPL over EXPORT_SYMBOL? To uphold the open-source spirit! This ensures the dance move (or symbol) is only showcased to modules that swing to the GPL rhythm, keeping proprietary ones from crashing the party.
🤔 How can one symbol grace both System.map and /proc/kallsyms? When a symbol is a kernel celebrity, part of the built-in modules! While System.map showcases the kernel’s hall of fame, /proc/kallsyms adds pop-up stars (dynamically loaded modules) too.
🤔 Drama with identical symbol names in two modules? Cue suspenseful music! If two modules strut out with the same dance move (or symbol name), the kernel’s bouncer stops the second one. A glaring spotlight (or error) flags the scene!
Symbols & Tables Made Easy:
Picture this: You’re in a grand library 📚:
- Symbols: They’re book titles, each narrating a unique story or concept.
- Symbol Table: It’s your trusty catalog, guiding where each tale rests.
- Exporting Symbols: Authored a new book? Add it to the library! Decide if it’s available to all (
EXPORT_SYMBOL) or exclusive club members (EXPORT_SYMBOL_GPL). - System.map vs. /proc/kallsyms: Two catalogs! One logs the timeless classics, and the other keeps tabs on new releases and limited editions.
For those who crave a deeper dive, here’s an exciting walkthrough with /proc/kallsyms – the kernel’s dynamic symbol guidebook. Just like browsing a library with a magnifying glass, combine the cat and grep commands and uncover the tales of symbols like print_jiffies.
Curiosity Corner: /proc/kallsyms Edition:
🤔 Significance of /proc/kallsyms? It’s our kernel’s comprehensive catalog! Spilling details of all symbols, whether timeless classics or dynamic newbies, aiding debugging escapades and kernel quests.
🤔 A missing print_jiffies in /proc/kallsyms means? It’s like a sought-after book missing from the library – perhaps it’s yet to be stocked, misplaced, or maybe it’s just a legend.
🤔 Any other treasure maps besides /proc/kallsyms? Absolutely! There’s System.map in /boot/, mapping the timeless tales. But remember, it won’t show you the new bestsellers!
Bringing the Puzzle Pieces Together:
Imagine a sprawling jigsaw puzzle 🧩. Each piece is a kernel function or variable. Now, /proc/kallsyms is your reference picture. Seeking print_jiffies? Check this picture with your trusty cat and grep magnifying glass. If it’s missing, maybe it’s time to hunt for the missing piece!
For those eager beavers wanting to dive even deeper, check out my detailed explorations on my GitHub
Happy coding and always stay curious!