Kernel source tree
- Include - shared structures (ex task_struct), constants …
- Kernel - OS features : scheduler, signals, synchronization primitives (mutex) …
- MM - memory management : allocator, page map …
- Arch - processor architecture dependent code
- Net - network stack : protocols, sockets
- Drivers - device drivers, further subdivided by categories (usb, video …)
- Fs - filesystems (ext4, jffs2, fat …)
Built-in or loadable module
To avoid recompiling the kernel each time a new device is installed drivers can be loaded as kernel modules. The modules can be in the initial ram disk so that the kernel is not compiled with a potentially high number of fs modules (ext4, reiser …) => initrd allows a same build to run in several machine configurations
- Vmlinuz - compressed kernel (single monolithic ELF executable) to make it smaller all symbols are removed. Contains minimal set of drivers required.
To load a module into the kernel
- Load the ELF object to kernel space
- Link object to kernel symbols (the only symbol visible have been explicitly declare with a macro in the kernel code)
- Add any module symbol to the exported kernel symbol table (for other dependent module)
- Strip the object from useless ELF sections to minimize kernel memory space
- Call module entry function to setup interface, register IRQ, set queues …
- VFS allows loadable modules to provide an interface to user code without introducing a new sys call number (=> recompile)
