Kernel: Anatomi Kernel Source

Sumber: http://www.linuxchix.org/content/courses/kernel_hacking/lesson6

Pada bagian ini, kita akan mempelajari secara umum berbagai bagian dari kernel, seperti,

letak pada source tree (pohon source code).
urutan mereka di jalankan / di eksekusi.
bagaimana cara mencari source code tertentu.

Dimana source code berada?

Mulai dari directory pada top-level di Linux source tree, biasanya (walaupun belum tentu selalu) ada di bawah /usr/src/linux-<version>. Kita tidak akan terlalu detail disini, karena source Linux selalu berubah, paling tidak informasi yang akan diberikan cukup untuk mencari driver atau function tertentu berada.

Makefile

File ini adalah top-level Makefile untuk seluruh source tree. File ini mendefinisikan banyak sekali variable dan rule, seperti flag kompilasi gcc dll.

Documentation/

Directory ini berisi banyak sekali informasi bermanfaat (walaupun kadang-kadang kadaluarsa) tentang cara konfigurasi kernel, menjalankan dengan ramdisk, dan banyak lagi. Informasi help yang berhubungan dengan berbagai pilihan konfigurasi tidak dapat ditemukan disini - walaupun mereka dapat di temukan pada file Kconfig di masing-masing directory source.

arch/

Semua code spesifik arsitektur berada di directory ini dan di directory include/asm-<arch>. Setiap arsitektur mempunyai directory di bawah directory ini. Contoh, code untuk komputer PowerPC dapat di temukan di arch/ppc. Kita dapat menemukan low-level memory management, penanganan interupsi, inisialisasi awal, dan banyak lagi di directory ini.

crypto/

Ini adalah API cryptographic untuk digunakan oleh kernel itu sendiri.

drivers/

Aturan umum, code untuk menjalankan device peripheral dapat di temukan di subdirectory dari directory ini. Ini termasuk, driver video, driver network card, driver SCSI low-level, dan berbagai hal sekitar itu. Contoh, kebanyakan driver network card dapat ditemukan di drivers/net. Beberapa higher level code digunakan untuk menempelkan semua tipe driver, yang mungkin tidak berada di directory yang sama seperti low level driver itu sendiri, menjadi satu kesatuan.

fs/

Baik code file sistem generik (dikenal sebagai VFS, atau Virtual File System) dan code untuk berbagai file sistem yang berbeda dapat di temukan pada directory ini. Root file sistem yang kita gunakan mungkin ext2; code untuk membaca format ext2 dapat di temukan dalam fs/ext2. Tidak semua file sistem dapat di compile dan di run, file system yang baru dan aneh adalah contoh yang baik bagi mereka yang ingin mencari proyek baru untuk kernel.

include/

Most of the header files included at the beginning of a .c file are found in this directory. Architecture specific include files are in asm-<arch>. Part of the kernel build process creates the symbolic link from asm to asm-<arch>, so that #include <asm/file.h> will get the proper file for that architecture without having to hard code it into the .c file. The other directories contain non-architecture specific header files. If a structure, constant, or variable is used in more than one .c file, it should be probably be in one of these header files.

init/

This directory contains the files main.c, version.c, and code for creating "early userspace". version.c defines the Linux version string. main.c can be thought of as the kernel "glue". We'll talk more about main.c in the next section. Early userspace provides functionality that needs to be available while a Linux kernel is coming up, but that doesn't need to be run inside the kernel itself.

ipc/

"IPC" singkatan dari "Inter-Process Communication". Ini berisi code untuk shared memory, semaphores, dan berbagai bentuk IPC.

kernel/

Generic kernel level code that doesn't fit anywhere else goes in here. The upper level system call code is here, along with the printk() code, the scheduler, signal handling code, and much more. The files have informative names, so you can type ls kernel/ and guess fairly accurately at what each file does.

lib/

Routines of generic usefulness to all kernel code are put in here. Common string operations, debugging routines, and command line parsing code are all in here.

mm/

High level memory management code is in this directory. Virtual memory (VM) is implemented through these routines, in conjunction with the low-level architecture specific routines usually found in arch/<arch>/mm/. Early boot memory management (needed before the memory subsystem is fully set up) is done here, as well as memory mapping of files, management of page caches, memory allocation, and swap out of pages in RAM (along with many other things).

net/

The high-level networking code is here. The low-level network drivers pass received packets up to and get packets to send from this level, which may pass the data to a user-level application, discard the data, or use it in-kernel, depending on the packet. The net/core directory contains code useful to most of the different network protocols, as do some of the files in the net/ directory itself. Specific network protocols are implemented in subdirectories of net/. For example, IP (version 4) code is found in the directory net/ipv4.

scripts/

This directory contains scripts that are useful in building the kernel, but does not include any code that is incorporated into the kernel itself. The various configuration tools keep their files in here, for example.

security/

Code for different Linux security models can be found here, such as NSA Security-Enhanced Linux and socket and network security hooks.

sound/

Drivers for sound cards and other sound related code is placed here.

usr/

This directory contains code that builds a cpio-format archive containing a root filesystem image, which will be used for early userspace.

Where does it all come together?

The central connecting point of the whole Linux kernel is the file init/main.c. Each architecture executes some low-level setup functions and then executes the function called start_kernel, which is found in init/main.c.

The order of execution of code looks something like this:

  Architecture-specific setup code (in arch/<arch>/*)
   |
   v
  The function start_kernel() (in init/main.c)
   |
   v
  The function init() (in init/main.c)
   |
   v
  The user level "init" program

In more detail, this is what happens:

   Architecture-specific set up code that does:
       Unzip and move the kernel code itself, if necessary
       Initialize the hardware
           This may include setting up low-level memory management
       Transfer control to the function start_kernel()
   start_kernel() does, among other things:
       Print out the kernel version and command line
       Start output to the console
       Enable interrupts
       Calibrate the delay loop
       Calls rest_init(), which does:
           Start a kernel thread to run the init() function
           Enter the idle loop
   init() does:
       Start the other processors (on SMP machines)
       Start the device subsystems
       Mount the root filesystem
       Free up unused kernel memory
       Run /sbin/init (or /etc/init, or...)

At this point, the userlevel init program is running, which will do things like start networking services and run getty (the login program) on your console(s).

You can figure out when a subsystem is initialized from start_kernel() or init() by putting in your own printk's and seeing when the printk's from that subsystem appear with regard to your own printk's. For example, if you wanted to find out when the ALSA sound system was initialized, put printk's at the beginning of start_kernel() and init() and look for where "Advanced Linux Sound Architecture [...]" is printed out relative to your printk's. (See Kernel Hacking Lesson #5 for help with using the printk() function.)

Finding things in the kernel source tree

So, you want to start working on, say, the USB driver. Where do you start looking for the USB code?

First, you can try a find command from the top-level kernel directory:

  $ find . -name \*usb\*

This command will print out every filename that has the string "usb" in the middle of it.

Another thing you might try is looking for a unique string. This unique string can be the output of a printk(), the name of a file in /proc, or any other unique string that might be found in the source code for that driver. For example, USB prints out the message:

  usb-ohci.c: USB OHCI at membase 0xcd030000, IRQ 27

So you might try using a recursive grep to find the part of that printk that is not a conversion character like %d:

  $ grep -r "USB OHCI at" .

Another way you might try to find the USB source code is by looking in /proc. If you type find /proc -name usb, you might find that there is a directory named /proc/bus/usb. You might be able to find a unique string to grep for by reading the entries in that directory.

If all else fails, try descending into individual directories and listing the files, or looking at the output of ls -lR. You may see a filename that looks related. But this should really be a last resort, and something to be tried only after you have run many different find and grep commands.

Once you've found the source code you are interested in, you can start reading it. Reading and understanding Linux kernel code is another lesson in itself. Just remember that the more you read kernel code, the easier it gets. Have fun exploring the kernel!

Referensi

http://www.linuxchix.org/content/courses/kernel_hacking/lesson6